conditions of success, a number of singular effects forced themselves
upon my attention. Meanwhile, Count Schaffgotsch also followed up the
subject. To a great extent we travelled over the same ground, neither
of us knowing how the other was engaged; but, so far as the experiments
then executed are common to us both, to Count Schaffgotsch belongs the
priority.

Let me here repeat his first observation. Within this tube, 11 inches
long, burns a small gas-flame, bright and silent. The note of the
tube has been ascertained, and now, standing at some distance from
the flame, I sound that note; the flame _quivers_. To obtain the
_extinction_ of the flame it is necessary to employ a burner with a
very narrow aperture, from which the gas issues under considerable
pressure. On gently sounding the note of the tube surrounding such a
flame, it quivers; but on throwing more power into the voice the flame
is extinguished.

The cause of the quivering of the flame will be best revealed by an
experiment with the siren. As the note of the siren approaches that of
the flame you hear beats, and at the same time you observe a dancing of
the flame synchronous with the beats. The jumps succeed each other more
slowly as unison is approached. They cease when the unison is perfect,
and they begin again as soon as the siren is urged beyond unison,
becoming more rapid as the discordance is increased. The cause of the
quiver observed by M. Schaffgotsch was revealed to me. The flame jumped
because the note of the tube surrounding it was nearly, but not quite,
in unison with the voice of the experimenter.

That the jumping of the flame proceeds in exact accordance with the
beats is well shown by a tuning-fork, which yields the same note as
the flame. Loading such a fork with a bit of wax, so as to throw it
slightly out of unison, and bringing it, when agitated, near the tube
in which the flame is singing, the beats and the leaps of the flame
occur at the same intervals. When the fork is placed over a resonant
jar, all of you can hear the beats, and see at the same time the
dancing of the flame. By changing the load upon the tuning-fork, or by
slightly altering the size of the flame, the rate at which the beats
succeed each other may be altered; but in all cases the jumps address
the eye at the moments when the beats address the ear.

While executing these experiments I noticed that, on raising my voice
to the proper pitch, a flame which had been burning silently in its
tube began to sing. The same observation had, without my knowledge,
been made a short time previously by Count Schaffgotsch. A tube, 12
inches long, is placed over this flame, which occupies a position about
an inch and a half from the lower end of the tube. When the proper note
is sounded the flame trembles, but it does not sing. When the tube
is lowered until the flame is three inches from its end, the song is
spontaneous. Between these two positions there is a third, at which, if
the flame be placed, it will burn silently; but if it be excited by
the voice it will sing, and continue to sing.

[Illustration: FIG. 118.]

Even when the back is turned toward the flame the sonorous pulses run
round the body, reach the tube, and call forth the song. A pitch-pipe,
or any other instrument which yields a note of the proper height,
produces the same effect. Mounting a series of tubes, capable of
emitting all the notes of the gamut, over suitable flames, with an
instrument sufficiently powerful, and from a distance of 20 or 30
yards, a musician, by running over the scale, might call forth all the
notes in succession, the whole series of flames finally joining in the
song.

When a silent flame, capable of being excited in the manner here
described, is looked at in a moving mirror, it produces there a
continuous band of light. Nothing can be more beautiful than the sudden
breaking up of this band into a string of richly-luminous pearls at the
instant the voice is pitched to the proper note.

One singing flame may be caused to effect the musical ignition of
another. Before you are two small flames, _f′_ and _f_, Fig. 118 (p.
273), the tube over _f′_ being 10-1/2 inches, that over _f_ 12 inches
long. The shorter tube is clasped by a paper slider, _s_. The flame
_f′_ is now singing, but the flame _f_, in the longer tube, is silent.
I raise the paper slider which surrounds _f′_, so as to lengthen the
tube, and on approaching the pitch of the tube surrounding _f_, that
flame sings. The experiment may be varied by making _f_ the singing
flame and _f′_ the silent one at starting. Raising the telescopic
slider, a point is soon attained where the flame _f′_ commences its
song. In this way one flame may excite another through considerable
distances. It is also possible to silence the singing flame by the
proper management of the voice.

SENSITIVE NAKED FLAMES

§ 7. _Discovery of Sensitive Flames by Le Conte_

We have hitherto dealt with flames surrounded by resonant tubes; and
none of these flames, if naked, would respond in any way to such noise
or music as could be here applied. Still it is possible to make naked
flames thus sympathetic. This action of musical sounds upon naked
flames was first observed by Prof. Le Conte at a musical party in the
United States. His observation is thus described: “Soon after the
music commenced, I observed that the flame exhibited pulsations which
were _exactly synchronous_ with the audible beats. This phenomenon
was very striking to every one in the room, and especially so when the
strong notes of the violoncello came in. It was exceedingly interesting
to observe how perfectly even the _trills_ of this instrument were
reflected on the sheet of flame. _A deaf man might have seen the
harmony._ As the evening advanced, and the diminished consumption of
gas in the city _increased the pressure_, the phenomenon became more
conspicuous. The _jumping_ of the flame gradually increased, became
somewhat irregular, and, finally, it began to flare continuously,
emitting the characteristic sound, indicating the escape of a greater
amount of gas than could be properly consumed. I then ascertained,
by experiment, that the phenomenon _did not_ take place unless the
discharge of gas was so regulated that the flame approximated to
the condition of _flaring_. I likewise determined, by experiment,
that the effects _were not_ produced by jarring or shaking the floor
and walls of the room by means of repeated concussions. Hence it is
obvious that the pulsations of the flame _were not_ owing to _indirect_
vibrations propagated through the medium of the walls of the room to
the burning-apparatus, but must have been produced by the _direct_
influence of aërial sonorous pulses on the burning jet.”[52]

The significant remark, that the jumping of the flame was not observed
until it was near flaring, suggests the means of repeating the
experiments of Dr. Le Conte; while a more intimate knowledge of the
conditions of success enables us to vary and exalt them in a striking
degree. Before you burns a bright candle-flame, but no sound that can
be produced here has any effect upon it. Though sonorous waves of great
power be sent through the air, the candle-flame remains insensible.

[Illustration: FIG. 119.]

[Illustration: FIG. 120.]

But by proper precautions even a candle-flame may be rendered
sensitive. Urging from a small blow-pipe a narrow stream of air through
such a flame, an incipient flutter is produced. The flame then jumps
visibly to the sound of a whistle, or to a chirrup. The experiment may
be so arranged that, when the whistle sounds, the flame shall be either
restored almost to its pristine brightness, or that the small amount of
light it still possesses shall disappear.

The blow-pipe flame of our laboratory is totally unaffected by the
sound of the whistle as long as no air is urged through it. By properly
tempering the force of the blast, a flame is obtained of the shape
shown in Fig. 119. On sounding the whistle the erect portion of the
flame drops down, and while the sound continues the flame maintains
the form shown in Fig. 120.

§ 8. _Experiments on Fish-tail and Bat’s-wing Flames_

We now pass on to a thin sheet of flame, issuing from a common
fish-tail burner, Fig. 121. You might sing to this flame, varying the
pitch of your voice; no shiver of the flame would be visible. You might
employ pitch-pipes, tuning-forks, bells, and trumpets, with a like
absence of all effect. A barely perceptible motion of the interior of
the flame may be noticed when a shrill whistle is blown close to it.
But by turning the cock more fully on, the flame is brought to the
verge of flaring. And now, when the whistle is blown, the flame thrusts
suddenly out seven quivering tongues, Fig. 122. The moment the sound
ceases, the tongues disappear, and the flame becomes quiescent.

[Illustration: FIG. 121.]

[Illustration: FIG. 122.]

Passing from a fish-tail to a bat’s-wing burner, we obtain a broad,
steady flame, Fig. 123. It is quite insensible to the loudest
sound which would be tolerable here. The flame is fed from a small
gas-holder.[53] Increasing gradually the pressure, a slight flutter of
the edge of the flame at length answers to the sound of the whistle.
Turning on the gas until the flame is on the point of roaring, and
blowing the whistle, it roars, and suddenly assumes the form shown in
Fig. 124.

When a distant anvil is struck with a hammer, the flame instantly
responds by thrusting forth its tongues.

[Illustration: FIG. 123.]

[Illustration: FIG. 124.]

An essential condition to entire success in these experiments disclosed
itself in the following manner: I was operating on two fish-tail
flames, one of which jumped to a whistle while the other did not. The
gas of the non-sensitive flame was turned off, additional pressure
being thereby thrown upon the other flame. It flared, and its cock
was turned so as to lower the flame; but it now proved non-sensitive,
however close it might be brought to the point of flaring. The narrow
orifice of the half-turned cock interfered with the action of the
sound. When the gas was turned fully on, the flame being lowered by
opening the cock of the other burner, it became again sensitive. Up
to this time a great number of burners had been tried, but with many
of them the action was _nil_. Acting, however, upon the hint conveyed
by this observation, the cocks which fed the flames were more widely
opened, and our most refractory burners thus rendered sensitive.

In this way the observation of Dr. Le Conte is easily and strikingly
illustrated; in our subsequent, and far more delicate, experiments the
precaution just referred to is still more essential.

§ 9. _Experiments on Flames from Circular Apertures_

A long flame may be shortened and a short one lengthened, according
to circumstances, by sonorous vibrations. The flame shown in Fig. 125
is long, straight, and smoky; that in Fig. 126 is short, forked, and
brilliant. On sounding the whistle, the long flame becomes short,
forked, and brilliant, as in Fig. 127; while the forked flame becomes
long and smoky, as in Fig. 128. As regards, therefore, their response
to the sound of the whistle, one of these flames is the complement of
the other.

In Fig. 129 is represented another smoky flame which, when the whistle
sounds, breaks up into the form shown in Fig. 130.

When a brilliant sensitive flame illuminates an otherwise dark room,
in which a suitable bell is caused to strike, a series of periodic
quenchings of the light by the sound occurs. Every stroke of the bell
is accompanied by a momentary darkening of the room.

The foregoing experiments illustrate the lengthening and shortening of
flames by sonorous vibrations. They may also produce _rotation_. From
some of our homemade burners issue flat flames, about ten inches high,
and three inches across at their widest part. When the whistle sounds,
the plane of each flame turns ninety degrees round, and continues in
its new position as long as the sound continues.

[Illustration: FIG. 125.]

[Illustration: FIG. 126.]

[Illustration: FIG. 127.]

[Illustration: FIG. 128.]

[Illustration: FIG. 129.]

[Illustration: FIG. 130.]

A flame of admirable steadiness and brilliancy now burns before you.
It issues from a single circular orifice in a common iron nipple. This
burner, which requires great pressure to make its flame flare, has
been specially chosen for the purpose of enabling you to observe, with
distinctness, the gradual change from apathy to sensitiveness. The
flame, now 4 inches high, is quite indifferent to sound. On increasing
the pressure its height becomes 6 inches; but it is still indifferent.
When its length is 12 inches, a barely perceptible quiver responds to
the whistle. When 16 or 17 inches high, it jumps briskly the moment an
anvil is tapped or the whistle sounded. When the flame is 20 inches
long you observe a quivering at intervals, which announces that it is
near roaring. A slight increase of pressure causes it to roar, and
shorten at the same time to 8 inches.

Diminishing the pressure a little, the flame is again 20 inches long,
but it is on the point of roaring and shortening. Like the singing
flames which were started by the voice, it stands on the brink of
a precipice. The proper note pushes it over. It shortens when the
whistle sounds, exactly as it did when the pressure is in excess. The
action reminds one of the story of the Swiss muleteers, who are said
to tie up their bells at certain places lest the tinkle should bring
an avalanche down. The snow must be very delicately poised before this
could occur. It probably never did occur, but our flame illustrates the
principle. We bring it to the verge of falling, and the sonorous pulses
precipitate what was already imminent. This is the simple philosophy of
all these sensitive flames.

When the flame flares, the gas in the orifice of the burner is in a
state of vibration; conversely, when the gas in the orifice is thrown
into vibration, the flame, if sufficiently near the flaring point,
will flare. Thus the sonorous vibrations, by acting on the gas in the
passage of the burner, become equivalent to an augmentation of pressure
in the holder. In fact, we have here revealed to us the physical
cause of flaring through excess of pressure, which, common as it
is, has never been hitherto explained. The gas encounters friction
in the orifice of the burner, which, when the force of transfer is
sufficiently great, throws the issuing stream into the state of
vibration that produces flaring. It is because the flaring is thus
caused that an infinitesimal amount of energy in the form of vibrations
of the proper period can produce an effect equivalent to a considerable
increase of pressure.

§ 10. _Seat of Sensitiveness_

[Illustration: FIG. 131.]

That the external vibrations act upon the gas in the orifice of the
burner, and not first upon the burner itself, the tube leading to it,
or the flame above it, is thus proved. A glass funnel R, Fig. 131,
is attached to a tube 3 feet long and half an inch in diameter. A
sensitive flame _b_ is placed at the open end T of the tube, while a
small high-pitched reed is placed in the funnel at R. When the sound
is converged upon the root of the flame, as in Fig. 131, the action is
violent; when converged on a point half an inch above the burner, as in
Fig. 132, or at half an inch below the burner, as in Fig. 133, there is
no action. The glass tube may be dispensed with and the funnel alone
employed, if care be taken to screen off all sound, save that which
passes through the shank of the funnel.[54]

§ 11. _Influence of Pitch_

[Illustration: FIG. 132.]

[Illustration: FIG. 133.]

All sounds are not equally effective on the flame; waves of special
periods are required to produce the maximum effect. The effectual
periods are those which synchronize with the waves produced by the
friction of the gas itself against the sides of its orifice. With some
of these flames a low deep whistle is more effective than a shrill
one. With others the exciting tremors must be very rapid, and the
sound consequently shrill. Not one of these four tuning-forks, which
vibrate 256 times, 320 times, 384 times, and 512 times respectively
in a second, has any effect upon the flame from our iron nipple.
But, besides their fundamental tones, these forks, as you know, can
be caused to yield a series of overtones of very high pitch. The
vibrations of this series are 1,600, 2,000, 2,400, and 3,200 per
second, respectively. The flame jumps in response to each of these
sounds; the response to that of the highest pitch being the most prompt
and energetic of all.

To the tap of a hammer upon a board the flame responds; but to the tap
of the same hammer upon an anvil the response is much more brisk and
animated. The reason is, that the clang of the anvil is rich in the
higher tones to which the flame is most sensitive. The powerful tone
obtained when our inverted bell is reinforced by its resonant tube has
no power over this flame. But when a halfpenny is brought into contact
with the vibrating surface the flame instantly shortens, flutters, and
roars. I send an assistant with a smaller bell, worked by clockwork, to
the most distant part of the gallery. He there detaches the hammer; the
strokes follow each other in rhythmic succession, and at every stroke
the flame falls from a height of 20 to a height of 8 inches, roaring as
it falls.

[Illustration: FIG. 134.]

[Illustration: FIG. 135.]

The rapidity with which sound is propagated through air is well
illustrated by these experiments. There is no sensible interval between
the stroke of the bell and the ducking of the flame.

When the sound acting on the flame is of very short duration a curious
and instructive effect is observed. The sides of the flame half-way
down, and lower, are seen suddenly fringed by luminous tongues, the
central flame remaining apparently undisturbed in both height and
thickness. The flame in its normal state is shown in Fig. 134, and
with its fringes in Fig. 135. The effect is due to the retention of
the impression upon the retina. The flame actually falls as low as
the fringes, but its recovery is so quick that to the eye it does not
appear to shorten at all.[55]

§ 12. _The Vowel-flame_

A flame of astonishing sensitiveness now burns before you. It issues
from the single orifice of a steatite burner, and reaches a height of
24 inches. The slightest tap on a distant anvil reduces its height to 7
inches. When a bunch of keys is shaken the flame is violently agitated,
and emits a loud roar. The dropping of a sixpence into a hand already
containing coin, at a distance of 20 yards, knocks the flame down. It
is not possible to walk across the floor without agitating the flame.
The creaking of boots sets it in violent commotion. The crumpling, or
tearing of paper, or the rustle of a silk dress, does the same. It is
startled by the patter of a rain-drop. I hold a watch near the flame:
nobody hears its ticks; but you all see their effect upon the flame.
At every tick it falls and roars. The winding up of the watch also
produces tumult. The twitter of a distant sparrow shakes the flame; the
note of a cricket would do the same. A chirrup from a distance of 30
yards causes it to fall and roar. I repeat a passage from Spenser:

    “Her ivory forehead full of bounty brave,
      Like a broad table did itself dispread;
    For love his lofty triumphs to engrave,
      And write the battles of his great godhead.
    All truth and goodness might therein be read,
      For there their dwelling was, and when she spake,
    Sweet words, like dropping honey she did shed;
      And through the pearls and rubies softly brake
  A silver sound, which heavenly music seemed to make.”

The flame selects from the sounds those to which it can respond. It
notices some by the slightest nod, to others it bows more distinctly,
to some its obeisance is very profound, while to many sounds it turns
an entirely deaf ear.

[Illustration: FIG. 136.]

In Fig. 136, this wonderful flame is represented. On chirruping to
it, or on shaking a bunch of keys within a few yards of it, it falls
to the size shown in Fig. 137, the whole length, _a b_, of the flame
being suddenly abolished. The light at the same time is practically
destroyed, a pale and almost non-luminous residue of it alone
remaining. These figures are taken from photographs of the flame.

[Illustration: FIG. 137.]

To distinguish it from the others I have called this the “vowel-flame,”
because the different vowel-sounds affect it differently. A loud and
sonorous U does not move the flame; on changing the sound to O, the
flame quivers; when E is sounded, the flame is strongly affected. I
utter the words _boot_, _boat_, and _beat_, in succession. To the first
there is no response; to the second, the flame starts; by the third, is
thrown into greater commotion; the sound _Ah!_ is still more powerful.
Did we not know the constitution of vowel-sounds this deportment would
be an insoluble enigma. As it is, however, the flame illustrates the
theory of vowel-sounds. It is most sensitive to sounds of high pitch;
hence we should infer that the sound _Ah!_ contains higher notes than
the sound E; that E contains higher notes than O; and O higher notes
than U. I need not say that this agrees perfectly with the analysis of
Helmholtz.

[Illustration: FIG. 138.]

This flame is peculiarly sensitive to the utterance of the letter S.
A hiss contains the elements that most forcibly affect the flame.
The gas issues from its burner with a hiss, and an external sound of
this character is therefore exceedingly effective. From a metal box
containing compressed air I allow a puff to escape; the flame instantly
ducks down not by any transfer of _air_ from the box to the flame, for
the distance between both utterly excludes this idea—it is the _sound_
that affects the flame. From the most distant part of the gallery my
assistant permits the compressed air to issue in puffs from the box; at
every puff the flame suddenly falls. The hiss of the issuing air at the
one orifice precipitates the tumult of the flame at the other.

When a musical-box is placed on the table, and permitted to play, the
flame behaves like a sentient and motor creature—bowing slightly to
some tones, and courtesying deeply to others.

§ 13. _Mr. Philip Harry’s Sensitive Flame_

Mr. Philip Barry has discovered a new and very effective form of
sensitive flame, which he thus describes in a letter to myself: “It is
the most sensitive of all the flames that I am acquainted with, though
from its smaller size it is not so striking as your vowel-flame. It
possesses the advantage that the ordinary pressure in the gas-mains is
quite sufficient to produce it. The method of producing it consists in
igniting the gas (ordinary coal-gas) not at the burner but some inches
above it, by interposing between the burner and the flame a piece of
wire-gauze.”

I give a sketch of the arrangement adopted in Fig. 138. The space
between the burner and gauze was 2 inches. The gauze was about 7 inches
square, resting on the ring of a retort-stand. It had 32 meshes to the
lineal inch. The burner was Sugg’s steatite pinhole burner, the same as
used for the vowel-flame.

The flame is a slender cone about four inches high, the upper portion
giving a bright-yellow light, the base being a non-luminous blue flame.
At the least noise this flame roars, sinking down to the surface of the
gauze, becoming at the same time invisible. It is very active in its
responses, and, being rather a noisy flame, its sympathy is apparent to
the ear as well as the eye.

“To the vowel-sounds it does not appear to answer so discriminately as
the vowel-flame. It is extremely sensitive to A, very slightly to E,
more so to I, entirely non-sensitive to O, but slightly sensitive to U.

“It dances in the most perfect manner to a small musical snuff-box,
and is highly sensitive to most of the sonorous vibrations which affect
the vowel-flames.”

§ 14. _Sensitive Smoke-jets_

It is not to the flame, as such, that we owe the extraordinary
phenomena which have been just described. Effects substantially the
same are obtained when a jet of unignited gas, of carbonic acid,
hydrogen, or even air itself, issues from an orifice under proper
pressure. None of these gases, however, can be seen in its passage
through air, and, therefore, we must associate with them some substance
which, while sharing their motions, will reveal them to the eye. The
method employed from time to time in this place of rendering aërial
vortices visible is well known to many of you. By tapping a membrane
which closes the mouth of a large funnel filled with smoke, we
obtain beautiful smoke-rings, which reveal the motion of the air. By
associating smoke with our gas-jets, in the present instance, we can
also trace their course, and, when this is done, the unignited gas
proves as sensitive as the flames. The smoke-jets jump, shorten, split
into forks, or lengthen into columns, when the proper notes are sounded.

Underneath this gas-holder are placed two small basins, the one
containing hydrochloric acid, and the other ammonia. Fumes of
sal-ammoniac are thus copiously formed, and mingle with the gas
contained in the holder. We may, as already stated, operate with
coal-gas, carbonic acid, air, or hydrogen; each of them yields good
effects. From our excellent steatite burner now issues a thin column of
smoke. On sounding the whistle, which was so effective with the flames,
it is found ineffective. When, moreover, the highest notes of a series
of Pandean pipes are sounded, they are also ineffective. Nor will the
lowest notes answer. But when a certain pipe, which stands about the
middle of the series, is sounded, the smoke-column falls, forming a
short stem with a thick, bushy head. It is also pressed down, as if by
a vertical wind, by a knock upon the table. At every tap it drops. A
stroke on an anvil, on the contrary, produces little or no effect. In
fact, the notes here effective are of a much lower pitch than those
which were most efficient in the case of the flames.

[Illustration: FIG. 139.]

The amount of shrinkage exhibited by some of these smoke-columns, in
proportion to their length, is far greater than that of the flames. A
tap on the table causes a smoke-jet eighteen inches high to shorten
to a bushy bouquet, with a stem not more than an inch in height. The
smoke-column, moreover, responds to the voice. A cough knocks it down;
and it dances to the tune of a musical-box. Some notes cause the mere
top of the smoke-column to gather itself up into a bunch; at other
notes the bunch is formed midway down; while notes of more suitable
pitch cause the column to contract itself to a cumulus not much more
than an inch above the end of the burner. Various forms of the dancing
smoke-jet are shown in Fig. 139. As the music continues, the action of
the smoke-column consists of a series of rapid leaps from one of these
forms to another.

In a perfectly still atmosphere these slender smoke-columns rise
sometimes to a height of nearly two feet, apparently vanishing into
air at the summit. When this is the case, our most sensitive flames
fall far behind them in delicacy; and though less striking than the
flames, the smoke-wreaths are often more graceful. Not only special
words, but every word, and even every syllable, of the foregoing stanza
from Spenser, tumbles a really sensitive smoke-jet into confusion. To
produce such effects, a perfectly tranquil atmosphere is necessary.
Flame-experiments, in fact, are possible in an atmosphere where
smoke-jets are utterly unmanageable.[56]

§ 15. _Constitution of Liquid Veins: Sensitive Water-jets_

[Illustration: FIG. 140.]

[Illustration: FIG. 141.]

[Illustration: FIG. 142.]

We have thus far confined our attention to jets of ignited and
unignited coal-gas—of carbonic acid, hydrogen, and air. We will now
turn to jets of water. And here a series of experiments, remarkable
for their beauty, has long existed, which claim relationship to those
just described. These are the experiments of Felix Savart on liquid
veins. If the bottom of a vessel containing water be pierced by a
circular orifice, the descending liquid vein will exhibit two parts
unmistakably distinct. The part of the vein nearest the orifice is
steady and limpid, presenting the appearance of a solid glass rod.
It decreases in diameter as it descends, reaches a point of maximum
contraction, from which point downward it appears turbid and unsteady.
The course of the vein, moreover, is marked by periodic swellings and
contractions. Savart has represented these appearances as in Fig. 140.
The part _a n_ nearest the orifice is limpid and steady, while all the
part below _n_ is in a state of quivering motion. This lower part of
the vein appears continuous to the eye; but the finger can be sometimes
passed through it without being wetted. This, of course, could not be
the case if the vein were really continuous. The upper portion of the
vein, moreover, intercepts vision; the lower portion, even when the
liquid is mercury, does not. In fact, the vein resolves itself, at
_n_, into liquid spherules, its apparent continuity being due to the
retention of the impressions made by the falling drops upon the retina.
If, while looking at the disturbed portion of the vein, the head be
suddenly lowered, the descending column will be resolved for a moment
into separate drops. Perhaps the simplest way of reducing the vein to
its constituent spherules is to illuminate the vein, in a dark room, by
a succession of electric flashes. Every flash reveals the drops, as if
they were perfectly motionless in the air.

Could the appearance of the vein illuminated by a single flash be
rendered permanent, it would be that represented in Fig. 141. And here
we find revealed the cause of those swellings and contractions which
the disturbed portion of the vein exhibits. The drops, as they descend,
are continually changing their forms. When first detached from the
end of the limpid portion of the vein, the drop is a spheroid with
its longest axis vertical. But a liquid cannot retain this shape, if
abandoned to the forces of its own molecules. The spheroid seeks to
become a sphere—the longer diameter, therefore, shortens; but, like a
pendulum which seeks to return to its position of rest, the contraction
of the vertical diameter goes too far, and the drop becomes a flattened
spheroid. Now, the contractions of the jet are formed at those places
where the longest axis of the drop is vertical, while the swellings
appear where the longest axis is horizontal. It will be noticed that
between every two of the larger drops is a third one of much smaller
dimensions. According to Savart, their appearance is invariable.

I wish to make the constitution of a liquid vein evident to you by a
simple but beautiful experiment. The condensing lens has been removed
from our electric lamp, the light being permitted to pass through a
vertical slit directly from the carbon-points. The slice of light
thus obtained is so divergent that it illuminates, from top to bottom,
a liquid vein several feet long, and placed at some distance from
the lamp. Immediately in front of the camera is a large disk of zinc
with six radial slits, about ten inches long and an inch wide. By the
rotation of the disk the light is caused to fall in flashes upon the
jet; and, when the suitable speed of rotation has been attained, the
vein is resolved into its constituent spherules. Receiving the shadow
of the vein upon a white screen, its constitution is rendered clearly
visible to all here present.

This breaking-up of a liquid vein into drops has been a subject of
frequent experiment and much discussion. Savart traced the pulsations
to the orifice, but he did not think that they were produced by
friction. They are powerfully influenced by sonorous vibrations. In the
midst of a large city it is hardly possible to obtain the requisite
tranquillity for the full development of the continuous, portion of
the vein; still, Savart was so far able to withdraw his vein from the
influence of such irregular vibrations that its limpid portion became
elongated to the extent shown in Fig. 142. It will be understood that
Fig. 141 represents a vein exposed to the irregular vibrations of the
city of Paris, while Fig. 142 represents one produced under precisely
the same conditions, but withdrawn from those vibrations.

The drops into which the vein finally resolves itself are incipient
even in its limpid portion, announcing themselves there as annular
protuberances, which become more and more pronounced, until finally
they separate. Their birthplace is near the orifice itself, and under
even moderate pressure they succeed each other with sufficient
rapidity to produce a feeble musical note. By permitting the drops to
fall upon a membrane, the pitch of this note may be fixed; and now we
come to the point which connects the phenomena of liquid veins with
those of sensitive flames and smoke-jets. If a note in unison with that
of the vein be sounded near it, the limpid portion instantly shortens;
the pitch may vary to some extent, and still cause a shortening;
but the unisonant note is the most effectual. Savart’s experiments
on vertically-descending veins have been recently repeated in our
laboratory with striking effect. From a distance of thirty yards the
limpid portion of the vein has been shortened by the sound of an
organ-pipe of the proper pitch and of moderate intensity.

I have also recently gone carefully, not merely by reading, but by
experiment, over Plateau’s account of the resolution of a liquid
vein into drops. In his researches on the figures of equilibrium of
bodies withdrawn from the action of gravity, he finds that a liquid
cylinder is stable as long as its length does not exceed three times
its diameter; or, more accurately, as long as the ratio between them
does not exceed that of the diameter of a circle to its circumference,
or 3.1416. If this be a little exceeded the cylinder begins to narrow
at some point or other of its length; nips itself together, breaks,
and forms immediately two spheres. If the ratio of the length of the
cylinder to its diameter greatly exceed 3.1416, then, instead of
breaking up into two spheres, it breaks up into several.

A liquid cylinder may be obtained by introducing olive-oil into a
mixture of alcohol and water, of the same density as the oil. The
latter forms a sphere. Two disks of smaller diameter than the sphere
are brought into contact with it, and then drawn apart; the oil clings
to the disks, and the sphere is transformed into a cylinder. If the
quantity of oil be insufficient to produce the maximum length of
cylinder, more may be added by a pipette. In making this experiment it
will be noticed that, when the proper length is exceeded, the nipped
portion of the cylinder elongates, and exists for a moment as a very
thin liquid cylinder uniting the two incipient spheres; and that, when
rupture occurs, the thin cylinder, which has also exceeded _its_ proper
length, breaks so as to form a small spherule between the two larger
ones. This is a point of considerable significance in relation to our
present question.

Now, Plateau contends that the play of the molecular forces in a liquid
cylinder is not suspended by its motion of translation. The first
portion of a vein of water quitting an orifice is a cylinder, to which
the laws which he has established regarding motionless cylinders apply.
The moment the descending vein exceeds the proper length it begins to
pinch itself so as to form drops; but urged forward as it is by the
pressure above it, and by its own gravity, in the time required for the
rounding of the drop it reaches a certain distance from the orifice.
At this distance, the pressure remaining constant, and the vein being
withdrawn from external disturbance, rupture invariably occurs. And the
rupture is accompanied by the phenomenon which has just been called
significant. Between every two succeeding large drops a small spherule
is formed, as shown in Fig. 141.

Permitting a vein of oil to fall from an orifice, not through the
air, but through a mixture of alcohol and water of the proper density,
the continuous portion of the vein, its resolution into drops, and the
formation of the small spherule between each liberated drop and the
end of the liquid cylinder which it has just quitted, may be watched
with the utmost deliberation. The effect of this and other experiments
upon the mind will be to produce the conviction that the very beautiful
explanation offered by Plateau is also the true one. The various laws
established experimentally by Savart all follow immediately from
Plateau’s theory.

In a small paper published more than twenty years ago I drew attention
to the fact that when a descending vein intersects a liquid surface
above the point of rupture, if the pressure be not too great, it enters
the liquid _silently_; but when the surface intersects the vein below
the point of rupture a rattle is immediately heard, and bubbles are
copiously produced. In the former case, not only is there no violent
dashing aside of the liquid, but round the base of the vein, and in
opposition to its motion, the liquid collects in a heap, by its surface
tension or capillary attraction. This experiment can be combined with
two other observations of Savart’s, in a beautiful and instructive
manner. In addition to the shortening of the continuous portion by
sound, Savart found that, when he permitted his membrane to intersect
the vein at one of its protuberances, the sound was louder than when
the intersection occurred at the contracted portion.

I permitted a vein to descend, under scarcely any pressure, from a tube
three-quarters of an inch in diameter, and to enter silently a basin of
water placed nearly 20 inches below the orifice. On sounding vigorously
a Ut_{2} tuning-fork the pellucid jet was instantly broken, and as
many as three of its swellings were seen above the surface. The rattle
of air-bubbles was instantly heard, and the basin was seen to be filled
with them. The sound was allowed slowly to die out; the continuous
portion of the vein lengthened, and a series of alternations in the
production of the bubbles was observed. When the swellings of the vein
cut the surface of the water, the bubbles were copious and loud; when
the contracted portions crossed the surface, the bubbles were scanty
and scarcely audible.

Removing the basin, placing an iron tray in its place, and exciting the
fork, the vein, which at first struck silently upon the tray, commenced
a rattle which rose and sank with the dying out of the sound, according
as the swellings or contractions of the jet impinged upon the surface.
This is a simple and beautiful experiment.

[Illustration: FIG. 143.]

[Illustration: FIG. 144.]

[Illustration: FIG. 145.]

Savart also caused his vein to issue horizontally and at various
inclinations to the horizon, and found that, in certain cases,
sonorous vibrations were competent to cause a jet to divide into two
or three branches. In these experiments the liquid was permitted to
issue through an orifice in a thin plate. Instead of this, however,
we will resort to our favorite steatite burner; for with water also
it asserts the same mastery over its fellows that it exhibited with
flames and smoke-jets. It will, moreover, reveal to us some entirely
novel results. By means of an India-rubber tube the burner is connected
with the water-pipes of the Institution, and, by pointing it obliquely
upward, we obtain a fine parabolic jet (Fig. 143). At a certain
distance from the orifice, the vein resolves itself into beautiful
spherules, whose motions are not rapid enough to make the vein appear
continuous. At the vertex of the parabola the spray of pearls is more
than an inch in width, and, further on, the drops are still more widely
scattered. On sweeping a fiddle-bow across a tuning-fork which executes
512 vibrations in a second, the scattered drops, as if drawn together
by their mutual attractions, instantly close up, and form an apparently
continuous liquid arch several feet in height and span (shown in Fig.
144). As long as the proper note is maintained the vein looks like a
frozen band, so motionless does it appear. On stopping the fork the
arch is shaken asunder, and we have the same play of liquid pearls as
before. Every sweep of the bow, however, causes the drops to fall into
a common line of march.

A pitch-pipe, or an organ-pipe yielding the note of this tuning-fork,
also powerfully controls the vein. The voice does the same. On pitching
it to a note of moderate intensity, it causes the wandering drops to
gather themselves together. At a distance of twenty yards, the voice
is, to all appearance, as powerful in curbing the vein, and causing its
drops to close up, as it is when close to the issuing jet.

The effect of “beats” upon the vein is also beautiful and instructive.
They may be produced either by organ-pipes or by tuning-forks. When two
forks vibrate, the one 512 times and the other 508 times in a second,
you will learn in our next lecture that they produce four beats in a
second. When the forks are sounded the beats are heard, and the liquid
vein is seen to gather up its pearls, and scatter them in synchronism
with the beats. The sensitiveness of this vein is astounding; it
rivals that of the ear itself. Placing the two tuning-forks on a
distant table, and permitting the beats to die gradually out, the
vein continues its rhythm almost as long as hearing is possible.
A more sensitive vein might actually prove superior to the ear—a
very surprising result, considering the marvellous delicacy of this
organ.[57]

By introducing a Leyden-jar into the circuit of a powerful
induction-coil, a series of dense and dazzling flashes of light, each
of momentary duration, is obtained. Every such flash in a darkened room
renders the drops distinct, each drop being transformed into a little
star of intense brilliancy. If the vein be then acted on by a sound
of the proper pitch, it instantly gathers its drops together into a
necklace of inimitable beauty.

In these experiments the whole vein gathers itself into a single arched
band when the proper note is sounded; but, by varying the conditions,
it may be caused to divide into two or more such bands, as shown in
Fig. 145. Drawings, however, are ineffectual here; for the wonder of
these experiments depends mainly on the sudden transition of the vein
from one state to the other. In the _motion_ dwells the surprise, and
this no drawing can render.[58]

SUMMARY OF CHAPTER VI

When a gas-flame is placed in a tube, the air in passing over the flame
is thrown into vibration, musical sounds being the consequence.

Making allowance for the high temperature of the column of air
associated with the flame, the pitch of the note is that of an open
organ-pipe of the length of the tube surrounding the flame.

The vibrations of the flame, while the sound continues, consist of a
series of periodic extinctions, total or partial, between every two of
which the flame partially recovers its brightness.

The periodicity of the phenomenon may be demonstrated by means of a
concave mirror which forms an image of the vibrating flame upon a
screen. When the image is sharply defined, the rotation of the mirror
reduces the single image to a series of separate images of the flame.
The dark spaces between the images correspond to the extinctions of
the flame, while the images themselves correspond to its periods of
recovery.

Besides the fundamental note of the associated tube, the flame can also
be caused to excite the higher overtones of the tube. The successive
divisions of the column of air are those of an open organ-pipe when its
harmonic tones are sounded.

On sounding a note nearly in unison with a tube containing a silent
flame, the flame jumps; and if the position of the flame in the tube be
rightly chosen, the extraneous sound will cause the flame to sing.

While the flame is singing, a note nearly in unison with its own
produces beats, and the flame is seen to jump in synchronism with the
beats. The jumping is also observed when the position of the flame
within its tube is not such as to enable it to sing.

NAKED FLAMES

When the pressure of the gas which feeds a naked flame is augmented,
the flame, up to a certain point, increases in size. But if the
pressure be too great, the flame roars or flares.

The roaring or flaring of the flame is caused by the state of vibration
into which the gas is thrown in the orifice of the burner, when the
pressure which urges it through the orifice is excessive.

If the vibrations in the orifice of the burner be super-induced by an
extraneous sound, the flame will flare under a pressure less than that
which, of itself, would produce flaring.

The gas under excessive pressure has vibrations of a definite period
impressed upon it as it passes through the burner. To operate with
a maximum effect upon the flame the external sound must contain
vibrations synchronous with those of the issuing gas.

When such a sound is chosen, and when the flame is brought sufficiently
near its flaring-point, it furnishes an acoustic reagent of unexampled
delicacy.

At a distance of 30 yards, for example, the chirrup of a house-sparrow
would be competent to throw the flame into commotion.

It is not to the flame, as such, that we are to ascribe these effects.
Effects substantially similar are produced when we employ jets of
unignited coal-gas, carbonic acid, hydrogen, or air. These jets may be
rendered visible by smoke, and the smoke jets show a sensitiveness to
sonorous vibrations even greater than that of the flames.

When a brilliant sensitive flame illuminates an otherwise dark room,
in which a suitable bell is caused to strike, a series of periodic
quenchings of the light by the sound occurs. Every stroke of the bell
is accompanied by a momentary darkening of the room.

A jet of water descending from a circular orifice is composed of two
distinct portions, the one pellucid and calm; the other in commotion.
When properly analyzed the former portion is found continuous; the
latter being a succession of drops.

If these drops be received upon a membrane, a musical sound is
produced. When an extraneous sound of this particular pitch is produced
in the neighborhood of the vein, the continuous portion is seen to
shorten.

The continuous portion of the vein presents a series of swellings and
contractions, in the former of which the drops are flattened, and in
the latter elongated. The sound produced by the flattened drops on
striking the membrane is louder than that produced by the elongated
ones.

Above its point of rupture a vein of water may be caused to enter water
_silently_; but on sounding a suitable note, the rattle of bubbles
is immediately heard; the discontinuous part of the vein rises above
the surface, and as the sound dies out the successive swellings and
contractions produce alternations of the quantity and sound of the
bubbles.

In veins propelled obliquely, the scattered water-drops may be called
together by a suitable sound, so as to form an apparently continuous
liquid arch.

Liquid veins may be analyzed by the electric spark, or by a succession
of flashes illuminating the veins.

CHAPTER VII

_PART I_

RESEARCHES ON THE ACOUSTIC TRANSPARENCY OF THE ATMOSPHERE IN RELATION
TO THE QUESTION OF FOG-SIGNALLING

  Introduction—Instruments and Observations—Contradictory
  Results from the 19th of May to the 1st of July
  inclusive—Solution of Contradictions—Aërial Reflection
  and its Causes—Aërial Echoes—Acoustic Clouds—Experimental
  Demonstration of Stoppage of Sound by Aërial Reflection

§ 1. _Introduction_

We are now fully equipped for the investigation of an important
practical problem. The cloud produced by the puff of a locomotive can
quench the rays of the noonday sun; it is not, therefore, surprising
that in dense fogs our most powerful coast-lights, including even the
electric light, should become useless to the mariner.

Disastrous shipwrecks are the consequence. During the last ten years
no less than two hundred and seventy-three vessels have been reported
as totally lost on our own coasts in fog or thick weather. The loss,
I believe, has been far greater on the American seaboard, where trade
is more eager, and fogs more frequent, than they are here. No wonder,
then, that earnest efforts should be made to find a substitute for
light in sound-signals, powerful enough to give warning and guidance to
mariners while still at a safe distance from the shore.

Such signals have been established to some extent upon our own coasts,
and to a still greater extent along the coasts of Canada and the United
States. But the evidence as to their value and performance is of the
most conflicting character, and no investigation sufficiently thorough
to clear up the uncertainty has hitherto been made. In fact, while the
_velocity_ of sound has formed the subject of refined and repeated
experiment by the ablest philosophers, the publication of Dr. Derham’s
celebrated paper in the “Philosophical Transactions” for 1708 marks the
latest systematic inquiry into the causes which affect the _intensity_
of sound in the atmosphere.

Jointly with the Elder Brethren of the Trinity House, and as their
scientific adviser, I have recently had the honor of conducting an
inquiry designed to fill the blank here indicated.

One or two brief references will suffice to show the state of the
question when this investigation began. “Derham,” says Sir John
Herschel, “found that fogs and falling rain, but more especially snow,
tend powerfully to obstruct the propagation of sound, and that the same
effect was produced by a coating of fresh-fallen snow on the ground,
though when glazed and hardened at the surface by freezing it had no
such influence.”[59]

In a very clear and able letter, addressed to the President of the
Board of Trade in 1863,[60] Dr. Robinson, of Armagh, thus summarizes
our knowledge of fog-signals: “Nearly all that is known about
fog-signals is to be found in the ‘Report on Lights and Beacons’; and
of it much is little better than conjecture. Its substance is as
follows:

“‘Light is scarcely available for this purpose. Blue lights are used in
the Hooghly; but it is not stated at what distance they are visible in
fog; their glare may be seen further than their flame.[61] It might,
however, be desirable to ascertain how far the electric light, or its
flash, can be traced.[62]

“‘Sound is the only known means really effective; but about it
testimonies are conflicting, and there is scarcely one fact relating to
its use as a signal which can be considered as established. Even the
most important of all, the distance at which it ceases to be heard, is
undecided.

“‘Up to the present time all signal-sounds have been made in air,
though this medium has grave disadvantages: its own currents interfere
with the sound-waves, so that a gun or bell which is heard several
miles _down_ the wind is inaudible more than a few furlongs _up_ it. A
still greater evil is that it is least effective when most needed; for
fog is a powerful damper of sound.’”

Dr. Robinson here expresses the universally-prevalent opinion, and he
then assigns the theoretic cause. “Fog,” he says, “is a mixture of air
and globules of water, and at each of the innumerable surfaces where
these two touch, a portion of the vibration is reflected and lost.[63]
... Snow produces a similar effect, and one still more injurious.”

Reflection being thus considered to take place at the surfaces of
the suspended particles, it followed that the greater the number of
particles, or, in other words, the denser the fog, the more injurious
would be its action upon sound. Hence optic transparency came to be
considered a measure of acoustic transparency. On this point Dr.
Robinson, in the letter referred to, expresses himself thus: “At the
outset, it is obvious that, to make experiments _comparable_, we
must have some measure of the fog’s power of stopping sound, without
attending to which the most anomalous results may be expected. It
seems probable that this will bear some simple relation to its opacity
to light, and that the distance at which a given object, as a flag
or pole, disappears may be taken as the measure.” “Still, clear air”
was regarded in this letter as the best vehicle of sound, the alleged
action of fogs, rain, and snow being ascribed to their rendering the
atmosphere “a discontinuous medium.”

Prior to the investigation now to be described, the views here
enunciated were those universally entertained. That sound is unable
to penetrate fogs was taken to be “a matter of common observation.”
The bells and horns of ships were affirmed “not to be heard so far
in fogs as in clear weather.” In the fogs of London the noise of the
carriage-wheels was reported to be so much diminished that “they seem
to be at a distance where really close by.” My knowledge does not
inform me of the existence of any other source for these opinions
regarding the deadening power of fog than the paper of Derham,
published one hundred and sixty-seven years ago. In consequence of
their _à priori_ probability, his conclusions seem to have been
transmitted unquestioned from generation to generation of scientific
men.

§ 2. _Instruments and Observations_

On the 19th of May, 1873, this inquiry began. The South Foreland,
near Dover, was chosen as the signal-station, steam-power having
been already established there to work two powerful magneto-electro
lights. The observations for the most part were made afloat, one of
the yachts of the Trinity Corporation being usually employed for this
purpose. Two stations had been established, the one at the top, the
other at the bottom, of the South Foreland Cliff; and at each of them
trumpets, air-whistles, and steam-whistles of great size were mounted.
The whistles first employed were of English manufacture. To these was
afterward added a large United States whistle, and also a Canadian
whistle, of great reputed power.

On the 8th of October another instrument, which has played a specially
important part in these observations, was introduced. This was a
steam-siren, constructed and patented by Mr. Brown of New York, and
introduced by Prof. Henry into the lighthouse system of the United
States. As an example of international courtesy worthy of imitation, I
refer with pleasure to the fact that when informed by Major Elliot of
the United States Army that our experiments had begun, the Lighthouse
Board at Washington, of their own spontaneous kindness, forwarded to
us for trial a very noble instrument of this description, which was
immediately mounted at the South Foreland.

In the steam-siren, as in the ordinary one, described in Chapter II., a
fixed disk and a rotating disk are employed, but radial slits are used
instead of circular apertures. One disk is fixed vertically across the
throat of a conical trumpet 16-1/2 feet long, 5 inches in diameter
where the disk crosses it, and gradually opening out till at the other
extremity it reaches a diameter of 2 feet 3 inches. Behind the fixed
disk is the rotating one, which is driven by separate mechanism. The
trumpet is connected with a boiler. In our experiments steam of 70
lbs. pressure was for the most part employed. Just as in the ordinary
siren, when the radial slits of the two disks coincide, and then only,
a strong puff of steam escapes. Sound-waves of great intensity are
thus sent through the air, the pitch of the note depending on the
velocity of rotation. (A drawing of the steam-siren constitutes our
frontispiece.)

To the siren, trumpets, and whistles were added three guns—an
18-pounder, a 5-1/2-inch howitzer, and a 13-inch mortar. In our summer
experiments all three were fired; but the howitzer having shown itself
superior to the other guns it was chosen in our autumn experiments
as not only a fair but a favorable representative of this form of
signal. The charges fired were for the most part those now employed
at Holyhead, Lundy Island, and the Kish light-vessel; namely, 3 lbs.
of powder. Gongs and bells were not included in this inquiry, because
previous observations had clearly proved their inferiority to the
trumpets and whistles.

On the 19th of May the instruments tested were:

On the top of the cliff:

_a._ Two brass trumpets or horns, 11 feet 2 inches long, 2 inches in
diameter at the mouth-piece, and opening out at the other end to a
diameter of 22-1/2 inches. They were provided with vibrating steel
reeds 9 inches long, 2 inches wide, and 1/4 inch thick, and were
sounded by air of 18 lbs. pressure.

_b._ A whistle, shaped like that of a locomotive, 6 inches in diameter,
also sounded by air of 18 lbs. pressure.

_c._ A steam-whistle, 12 inches in diameter, attached to a boiler, and
sounded by steam of 64 lbs. pressure.

At the bottom of the cliff:

_d._ Two trumpets or horns, of the same size and arrangement as those
above, and sounded by air of the same pressure. They were mounted
vertically on the reservoir of compressed air; but within about two
feet of their extremities they were bent at a right angle, so as to
present their mouths to the sea.

_e._ A 6-inch air-whistle, similar to the one above, and sounded by the
same means.

The upper instruments were 235 feet above high-water mark, the lower
ones 40 feet. A vertical distance of 195 feet, therefore, separated the
instruments. A shaft, provided with a series of twelve ladders, led
from the one to the other.

Numerous comparative experiments made at the outset gave a slight
advantage to the upper instruments. They, therefore, were for the most
part employed throughout the subsequent inquiry.

Our first observations were a preliminary discipline rather than an
organized effort at discovery. On May 19th the maximum distance reached
by the sound was about three and a half miles.[64] The wind, however,
was high and the sea rough, so that local noises interfered to some
extent with our appreciation of the sound.

Mariners express the strength of the wind by a series of numbers
extending from 0 = calm to 12 = a hurricane, a little practice in
common producing a remarkable unanimity between different observers as
regards the force of the wind. Its force on May 19th was 6, and it blew
at right angles to the direction of the sound.

The same instruments on May 20th covered a greater range of sound;
but not much greater, though the disturbance due to local noises was
absent. At 4 miles’ distance in the axes of the horns they were barely
heard, the air at the time being calm, the sea smooth, and all other
circumstances exactly those which have been hitherto regarded as most
favorable to the transmission of sound. We crept a little further
away, and by stretched attention managed to hear at intervals, at a
distance of 6 miles, the faintest hum of the horns. A little further
out we again halted; but though local noises were absent, and though we
listened intently, we heard nothing.

This position, clearly beyond the range of whistles and trumpets, was
expressly chosen with the view of making what might be considered a
decisive comparative experiment between horns and guns as instruments
for fog-signalling. The distinct report of the 12 o’clock gun fired at
Dover on the 19th suggested this comparison, and through the prompt
courtesy of General Sir A. Horsford we were enabled to carry it out.
At 12.30 precisely the puff of an 18-pounder, with a 3-lb. charge, was
seen at Dover Castle, which was about a mile further off than the South
Foreland. Thirty-six seconds afterward the loud report of the gun was
heard, its complete superiority over the trumpets being thus, to all
appearance, demonstrated.

We clinched this observation by steaming out to a distance of 8-1/2
miles, where the report of a second gun was well heard by all of us.
At a distance of 10 miles the report of a third gun was heard by some,
and at 9·7 miles the report of a fourth gun was heard by all.

The result seemed perfectly decisive. Applying the law of inverse
squares, the sound of the gun at a distance of 6 miles from the
Foreland must have had more than two and a half times the intensity
of the sound of the trumpets. It would not have been rash under the
circumstances to have reported without qualification the superiority
of the gun as a fog-signal. No single experiment is, to my knowledge,
on record to prove that a sound once predominant would not be always
predominant, or that the atmosphere on different days would show
preferences to different sounds. On many subsequent occasions, however,
the sound of the horns proved distinctly superior to that of the
gun. This _selective_ power of the atmosphere revealed itself more
strikingly in our autumn experiments than in our summer ones; and it
was sometimes illustrated within a few hours of the same day: of two
sounds, for example, one might have the greatest range at 10 A.M., and
the other the greatest range at 2 P.M.

In the experiments on May 19th and 20th the superiority of the trumpets
over the whistles was decided; and indeed, with few exceptions, this
superiority was maintained throughout the inquiry. But there were
exceptions. On June 2d, for example, the whistles rose in several
instances to full equality with, and on rare occasions subsequently
even surpassed, the horns. The sounds were varied from day to day,
and various shiftings of the horns and reeds were resorted to, with a
view of bringing out their maximum power. On the date last mentioned
a single horn was sounded, two were sounded, and three were sounded
together; but the utmost range of the loudest sound, even with the
paddles stopped, did not exceed 6 miles. With the view of concentrating
their power, the axes of the horns had been pointed in the same
direction, and, unless stated to the contrary, this in all subsequent
experiments was the case.

On June 3d the three guns already referred to were permanently mounted
at the South Foreland. They were ably served by gunners from Dover
Castle.

On the same day dense clouds quite covered the firmament, some of them
particularly black and threatening, but a marked advance was observed
in the transmissive power of the air. At a distance of 6 miles the
horn-sounds were not quite quenched by the paddle-noises; at 8 miles
the whistles were heard, and the horns better heard; while at 9 miles,
with the paddles stopped, the horn-sounds alone were fairly audible.
During the day’s observations a remarkable and instructive phenomenon
was observed. Over us rapidly passed a torrential shower of rain,
which, according to Derham, is a potent damper of sound. We could,
however, notice no subsidence of intensity as the shower passed. It
is even probable that, had our minds been free from bias, we should
have noticed an augmentation of the sound, such as occurred with the
greatest distinctness on various subsequent occasions during violent
rain.

The influence of “beats” was tried on June 3d, by throwing the horns
slightly out of unison; but though the beats rendered the sound
characteristic, they did not seem to augment the range. At a distance
from the station curious fluctuations of intensity were noticed. Not
only did the different blasts vary in strength, but sudden swellings
and fallings off, even of the same blast, were observed. This was not
due to any variation on the part of the instruments, but purely to the
changes of the medium traversed by the sound. What these changes were
shall be indicated subsequently.

The range of our best horns on June 10th was 8-3/4 miles. The guns at
this distance were very feeble. That the loudness of the sound depends
on the shape of the gun was proved by the fact that thus far the
howitzer, with a 3-lb. charge, proved more effective than the other
guns.

On June 25th a gradual improvement in the transmissive power of the
air was observed from morning to evening; but at the last the maximum
range was only moderate. The fluctuations in the strength of the sound
were remarkable, sometimes sinking to inaudibility and then rising to
loudness. A similar effect, due to a similar cause, is often noticed
with church-bells. The acoustic transparency of the air was still
further augmented on the 26th: at a distance of 9-1/4 miles from the
station the whistles and horns were plainly heard against a wind with a
force of 4; while on the 25th, with a favoring wind, the maximum range
was only 6-1/2 miles. Plainly, therefore, something else than the wind
must be influential in determining the range of the sound.

On Tuesday, July 1st, observations were made on the decay of the sound
at various angular distances from the axis of the horn. As might be
expected, the sound in the axis was loudest, the decay being gradual on
both sides. In the case of the gun, however, the direction of pointing
has very little influence.

The day was acoustically clear; at a distance of 10 miles the horn
yielded a plain sound, while the American whistle seemed to surpass
the horn. Dense haze at this time quite hid the Foreland. At 10-1/2
miles occasional blasts of the horn came to us, but after a time all
sound ceased to be audible; it seemed as if the air, after having been
exceedingly transparent, had become gradually more opaque to the sound.

At 4.45 P.M. we took the master of the Varne light-ship on board the
“Irene.” He and his company had heard the sound at intervals during the
day, although he was dead to windward and distant 12-3/4 miles from the
source of sound.

Here a word of reflection on our observations may be fitly introduced.
It is, as already shown, an opinion entertained in high quarters that
the waves of sound are reflected at the limiting surfaces of the minute
particles which constitute haze and fog, the alleged waste of sound in
fog being thus explained. If, however, this be an efficient practical
cause of the stoppage of sound, and if clear calm air be, as alleged,
the best vehicle, it would be impossible to understand how to-day, in
a thick haze, the sound reached a distance of 12-3/4 miles, while on
May 20th, in a calm and hazeless atmosphere, the maximum range was only
from 5 to 6 miles. Such facts foreshadow a revolution in our notions
regarding the action of haze and fogs upon sound.

An interval of 12 hours sufficed to change in a surprising degree the
acoustic transparency of the air. On the 1st of July the sound had a
range of nearly 13 miles; on the 2d the range did not exceed 4 miles.

§ 3. _Contradictory Results_

Thus far the investigation proceeded with hardly a gleam of a principle
to connect the inconstant results. The distance reached by the sound
on the 19th of May was 3-1/2 miles; on the 20th it was 5-1/2 miles; on
the 2d of June 6 miles; on the 3d more than 9 miles; on the 10th it
was also 9 miles; on the 25th it fell to 6-1/2 miles; on the 26th it
rose again to more than 9-1/4 miles; on the 1st of July, as we have
just seen, it reached 12-3/4, whereas on the 2d the range shrunk to 4
miles. None of the meteorological agents observed could be singled out
as the cause of these fluctuations. The wind exerts an acknowledged
power over sound, but it could not account for these phenomena. On the
25th of June, for example, when the range was only 6-1/2 miles, the
wind was favorable; on the 26th, when the range exceeded 9-1/4 miles,
it was opposed to the sound. Nor could the varying optical clearness
of the atmosphere be invoked as an explanation; for on July 1st, when
the range was 12-3/4 miles, a thick haze hid the white cliffs of the
Foreland, while on many other days, when the acoustic range was not
half so great, the atmosphere was optically clear. Up to July 3d all
remained enigmatical; but on this date observations were made which
seemed to me to displace surmise and perplexity by the clearer light of
physical demonstration.

§ 4. _Solution of Contradictions_

On July 3d we first steamed to a point 2·9 miles S.W. by W. of the
signal-station. No sounds, not even the guns, were heard at this
distance. At 2 miles they were equally inaudible. But this being a
position at which the sounds, though strong in the axis of the horn,
invariably subsided, we steamed to the exact bearing from which our
observations had been made on July 1st. At 2.15 P.M., and at a distance
of 3-3/4 miles from the station, with calm, clear air and a smooth sea,
the horns and whistle (American) were sounded, but they were inaudible.
Surprised at this result, I signalled for the guns. They were all
fired, but, though the smoke seemed at hand, no sound whatever reached
us. On July 1st, in this bearing, the observed range of both horns and
guns was 10-1/2 miles, while on the bearing of the Varne light-vessel
it was nearly 13 miles. We steamed in to 3 miles, paused, and listened
with all attention; but neither horn nor whistle was heard. The guns
were again signalled for; five of them were fired in succession, but
not one of them was heard. We steamed on in the same bearing to 2
miles, and had the guns fired pointblank at us. The howitzer and the
mortar, with 3-lb. charges, yielded a feeble thud, while the 18-pounder
was wholly unheard. Applying the law of inverse squares, it follows
that, with the air and sea, according to accepted notions, in a far
worse condition, the sound at 2 miles’ distance on July 1st must have
had more than forty times the intensity which it possessed at the same
distance at 3 P.M. on the 3d.

“On smooth water,” says Sir John Herschel, “sound is propagated with
remarkable clearness and strength.” Here was the condition; still,
with the Foreland so close to us, the sea so smooth, and the air
so transparent, it was difficult to realize that the guns had been
fired or the trumpets blown at all. What could be the reason? Had
the sound been converted by internal friction into heat? or had
it been wasted in partial reflections at the limiting surfaces of
non-homogeneous masses of air? I ventured, two or three years ago, to
say something regarding the function of the Imagination in Science,
and, notwithstanding the care then taken, to define and illustrate
its real province, some persons, among whom were one or two able men,
deemed me loose and illogical. They misunderstood me. The faculty to
which I referred was that power of visualizing processes in space,
and the relations of space itself, which must be possessed by all
great physicists and geometers. Looking, for example, at two pieces
of polished steel, we have not a sense, or the rudiment of a sense,
to distinguish the inner condition of the one from that of the other.
And yet they may differ materially, for one may be a magnet, the other
not. What enabled Ampère to surround the atoms of such a magnet with
channels in which electric currents ceaselessly run, and to deduce from
these pictured currents all the phenomena of ordinary magnetism? What
enabled Faraday to visualize his lines of force, and make his mental
picture a guide to discoveries which have rendered his name immortal?
Assuredly it was the disciplined imagination. Figure the observers on
the deck of the “Irene,” with the invisible air stretching between
them and the South Foreland, knowing that it contained something which
stifled the sound, but not knowing what that something is. Their senses
are not of the least use to them; nor could all the philosophical
instruments in the world render them any assistance. They could not, in
fact, take a single step toward the solution without the formation of a
mental image—in other words, without the exercise of the imagination.

Sulphur, in homogeneous crystals, is exceedingly transparent to radiant
heat, whereas the ordinary brimstone of commerce is highly impervious
to it—the reason being that the brimstone does not possess the
molecular continuity of the crystal, but is a mere aggregate of minute
grains not in perfect optical contact with each other. Where this is
the case, a portion of the heat is always reflected on entering and on
quitting a grain; hence, when the grains are minute and numerous, this
reflection is so often repeated that the heat is entirely wasted before
it can plunge to any depth into the substance. The same remark applies
to snow, foam, clouds, and common salt, indeed, to all transparent
substances in powder; they are all impervious to light, not through the
immediate absorption or extinction of the light, but through repeated
internal reflection.

Humboldt, in his observations at the Falls of the Orinoco, is known to
have applied these principles to sound. He found the noise of the falls
far louder by night than by day, though in that region the night is far
noisier than the day. The plain between him and the falls consisted
of spaces of grass and rock intermingled. In the heat of the day he
found the temperature of the rock to be considerably higher than that
of the grass. Over every heated rock, he concluded, rose a column of
air rarefied by the heat; its place being supplied by the descent of
heavier air. He ascribed the deadening of the sound to the reflections
which it endured at the limiting surfaces of the rarer and denser
air. This philosophical explanation made it generally known that a
non-homogeneous atmosphere is unfavorable to the transmission of sound.

But what on July 3d, not with the variously-heated plain of Antures,
but with a calm sea as a basis for the atmosphere, could so destroy its
homogeneity as to enable it to quench in so short a distance so vast a
body of sound? My course of thought at the time was thus determined:
As I stood upon the deck of the “Irene” pondering the question, I
became conscious of the exceeding power of the sun beating against
my back and heating the objects near me. Beams of equal power were
falling on the sea, and must have produced copious evaporation. That
the vapor generated should so rise and mingle with the air as to form
an absolutely homogeneous medium, was in the highest degree improbable.
It would be sure, I thought, to rise in invisible streams, breaking
through the superincumbent air now at one point, now at another, thus
rendering the air _flocculent_ with wreaths and striæ, charged in
different degrees with the buoyant vapor. At the limiting surfaces of
these spaces, though invisible, we should have the conditions necessary
to the production of partial echoes and the consequent waste of sound.
Ascending and descending air-currents, of different temperatures, as
far as they existed, would also contribute to the effect.

Curiously enough, the conditions necessary for the testing of this
explanation immediately set in. At 3.15 P.M. a solitary cloud threw
itself athwart the sun, and shaded the entire space between us and the
South Foreland. The heating of the water and the production of vapor-
and air-currents were checked by the interposition of this screen;
hence the probability of suddenly-improved transmission. To test
this inference, the steamer was immediately turned and urged back to
our last position of inaudibility. The sounds, as I expected, were
distinctly though faintly heard. This was at 3 miles’ distance. At
3-3/4 miles, the guns were fired, both pointblank and elevated. The
faintest pop was all that we heard; but we did hear a pop, whereas
we had previously heard nothing, either here or three-quarters of a
mile nearer. We steamed out to 4-1/4 miles, where the sounds were
for a moment faintly heard; but they fell away as we waited; and
though the greatest quietness reigned on board, and though the sea
was without a ripple, we could hear nothing. We could plainly see the
steam-puffs which announced the beginning and the end of a series of
trumpet-blasts, but the blasts themselves were quite inaudible.

It was now 4 P.M., and my intention at first was to halt at this
distance, which was beyond the sound-range, but not far beyond it,
and see whether the lowering of the sun would not restore the power
of the atmosphere to transmit the sound. But after waiting a little
the anchoring of a boat was suggested, so as to liberate the steamer
for other work; and though loth to lose the anticipated revival of the
sounds myself, I agreed to this arrangement. Two men were placed in the
boat and requested to give all attention, so as to hear the sound if
possible. With perfect stillness around them they heard nothing. They
were then instructed to hoist a signal if they should hear the sounds,
and to keep it hoisted as long as the sounds continued.

At 4.45 we quitted them and steamed toward the South Sand Head
light-ship. Precisely 15 minutes after we had separated from them the
flag was hoisted; the sound had at length succeeded in piercing the
body of air between the boat and the shore.

We continued our journey to the light-ship, went on board, heard the
report of the lightsmen, and returned to our anchored boat. We then
learned that when the flag was hoisted the horn-sounds were heard, that
they were succeeded after a little time by the whistle-sounds, and that
both increased in intensity as the evening advanced. On our arrival, of
course, we heard the sounds ourselves.

We pushed the test further by steaming further out. At 5-3/4 miles we
halted and heard the sounds: at 6 miles we heard them distinctly, but
so feebly that we thought we had reached the limit of the sound-range;
but while we waited the sounds rose in power. We steamed to the Varne
buoy, which is 7-3/4 miles from the signal-station, and heard the
sounds there better than at 6 miles’ distance. We continued our course
outward to 10 miles, halted there for a brief interval, but heard
nothing.

Steaming, however, on to the Varne light-ship, which is situated at
the other end of the Varne shoal, we hailed the master, and were
informed by him that up to 5 P.M. nothing had been heard, but that at
that hour the sounds began to be audible. He described one of them as
“very gross, resembling the bellowing of a bull,” which very accurately
characterizes the sound of the large American steam-whistle. At the
Varne light-ship, therefore, the sounds had been heard toward the close
of the day; though it is 12-3/4 miles from the signal-station. I think
it probable that, at a point 2 miles from the Foreland, the sound at 5
P.M. possessed fifty times the intensity which it possessed at 2 P.M.
To such undreamed-of fluctuations is the atmosphere liable. On our
return to Dover Bay, at 10 P.M., we heard the sounds, not only distinct
but loud, where nothing could be heard in the morning.

§ 5. _Other Remarkable Instances of Acoustic Opacity_

In his excellent lecture entitled “Wirkungen aus der Ferne,” Dove has
collected some striking cases of the interception of sound. The Duke of
Argyll has also favored me with some highly-interesting illustrations;
but nothing of this description that I have read equals in point of
interest the following account of the battle of Gaines’s Farm, for
which I am indebted to the Rector of the University of Virginia:

  “LYNCHBURG, VIRGINIA, _March 19, 1874_.

  “SIR—I have just read with great interest your lecture of
  January 16th, on the acoustic transparency and opacity of the
  atmosphere. The remarkable observations you mention induce me
  to state to you a fact which I have occasionally mentioned,
  but always, where I am not well known, with the apprehension
  that my veracity would be questioned. It made a strong
  impression on me at the time, but was an insoluble mystery
  until your discourse gave me a possible solution.

  “On the afternoon of June 28, 1862, I rode, in company
  with General G. W. Randolph, then Secretary of War of the
  Confederate States, to Price’s house, about nine miles from
  Richmond; the evening before General Lee had begun his attack
  on McClellan’s army, by crossing the Chickahominy about four
  miles above Price’s, and driving in McClellan’s right wing.
  The battle of Gaines’s Farm was fought the afternoon to
  which I refer. The valley of the Chickahominy is about one
  mile and a half wide from hilltop to hilltop. Price’s is on
  one hilltop, that nearest to Richmond; Gaines’s farm, just
  opposite, is on the other, reaching back in a plateau to Cold
  Harbor.

  “Looking across the valley I saw a good deal of the battle,
  Lee’s right resting in the valley, the Federal left wing the
  same. My line of vision was nearly in the line of the lines of
  battle. I saw the advance of the Confederates, their repulse
  two or three times, and in the gray of the evening the final
  retreat of the Federal forces.

  “I distinctly saw the musket-fire of both lines, the smoke,
  individual discharges, the flash of the guns. I saw batteries
  of artillery on both sides come into action and fire rapidly.
  Several field-batteries on each side were plainly in sight.
  Many more were hid by the timber which bounded the range of
  vision.

  “Yet looking for nearly two hours, from about 5 to 7 P.M. on
  a midsummer afternoon, at a battle in which at least 50,000
  men were actually engaged, and doubtless at least 100 pieces
  of field-artillery, through an atmosphere optically as limpid
  as possible, _not a single sound of the battle_ was audible to
  General Randolph and myself. I remarked it to him at the time
  as astonishing.

  “Between me and the battle was the deep broad valley of the
  Chickahominy, partly a swamp shaded from the declining sun by
  the hills and forest in the west (my side). Part of the valley
  on each side of the swamp was cleared; some in cultivation,
  some not. Here were conditions capable of providing several
  belts of air, varying in the amount of watery vapor (and
  probably in temperature), arranged like laminæ at right angles
  to the acoustic waves as they came from the battlefield to me.

  “Respectfully,

  “Your obedient servant,

  “R. G. H. KEAN.

  “PROF. JOHN TYNDALL.”

I learn from a subsequent letter that during the battle the air was
still.—J. T.

§ 6. _Echoes from Invisible Acoustic Clouds_

But both the argument and the phenomena have a complementary side,
which we have now to consider. A stratum of air less than 3 miles thick
on a calm day has been proved competent to stifle both the cannonade
and the horn-sounds employed at the South Foreland; while, according
to the foregoing explanation, this result was due to the reflection of
the sound from invisible _acoustic clouds_ which filled the atmosphere
on a day of perfect _optical_ transparency. But, granting this, it is
incredible that so great a body of sound could utterly disappear in so
short a distance without rendering some account of itself. Supposing,
then, instead of placing ourselves behind the acoustic cloud, we were
to place ourselves in front of it, might we not, in accordance with the
law of conservation, expect to receive by reflection the sound which
had failed to reach us by transmission? The case would then be strictly
analogous to the reflection of light from an ordinary cloud to an
observer between it and the sun.

My first care in the early part of the day in question was to assure
myself that our inability to hear the sound did not arise from any
derangement of the instruments on shore. Accompanied by the private
secretary of the Deputy Master of the Trinity House, at 1 P.M. I was
rowed to the shore, and landed at the base of the South Foreland
Cliff. The body of air which had already shown such extraordinary
power to intercept the sound, and which manifested this power still
more impressively later in the day, was now in front of us. On it
the sonorous waves impinged, and from it they were sent back with
astonishing intensity. The instruments, hidden from view, were on the
summit of a cliff 235 feet above us, the sea was smooth and clear of
ships, the atmosphere was without a cloud, and there was no object
in sight which could possibly produce the observed effect. From the
perfectly transparent air the echoes came, at first with a strength
apparently little less than that of the direct sound, and then dying
away. A remark made by my talented companion in his notebook at the
time shows how the phenomenon affected him: “Beyond saying that the
echoes seemed to come from the expanse of ocean, it did not appear
possible to indicate any more definite point of reflection.” Indeed
no such point was to be seen; the echoes reached us, as if by magic,
from the invisible acoustic clouds with which the optically transparent
atmosphere was filled. The existence of such clouds in all weathers,
whether optically cloudy or serene, is one of the most important points
established by this inquiry.

Here, in my opinion, we have the key to many of the mysteries and
discrepancies of evidence which beset this question. The foregoing
observations show that there is no need to doubt either the veracity
or the ability of the conflicting witnesses, for the variations of
the atmosphere are more than sufficient to account for theirs. The
mistake, indeed, hitherto has been, not in reporting incorrectly, but
in neglecting the monotonous operation of repeating the observations
during a sufficient time. I shall have occasion to remark subsequently
on the mischief likely to arise from giving instructions to mariners
founded on observations of this incomplete character.

It required, however, long pondering and repeated observation before
this conclusion took firm root in my mind; for it was opposed to
the results of great observers, and to the statements of celebrated
writers. In science as elsewhere, a mind of any depth which accepts
a doctrine undoubtingly, discards it unwillingly. The question of
aërial echoes has a historic interest. While cloud-echoes have been
accepted as demonstrated by observation, it has been hitherto held as
established that audible echoes never occur in optically clear air. We
owe this opinion to the admirable report of Arago on the experiments
made to determine the velocity of sound at Montlhéry and Villejuif in
1822.[65] Arago’s account of the phenomenon observed by him and his
colleagues is as follows: “Before ending this note we will only add
that the shots fired at Montlhéry were accompanied by a rumbling like
that of thunder, which lasted from 20 to 25 seconds. Nothing of this
kind occurred at Villejuif. Once we heard two distinct reports, a
second apart, of the Montlhéry cannon. In two other cases the report
of the same gun was followed by a prolonged rumbling. These phenomena
never occurred without clouds. Under a clear sky the sounds were
single and instantaneous. May we not, therefore, conclude that the
multiple reports of the Montlhéry gun heard at Villejuif were echoes
from the clouds, and may we not accept this fact as favorable to the
explanation given by certain physicists of the rolling of thunder?”

This explanation of the Montlhéry echoes is an inference from
observations made at Villejuif. The inference requires qualification.
Some hundreds of cannon-shots have been fired at the South Foreland,
many of them when the heavens were completely free from clouds, and
never in a single case has a _roulement_ similar to that noticed at
Montlhéry been absent. It follows, moreover, so hot upon the direct
sound as to present hardly a sensible breach of continuity between the
sound and the echo. This could not be the case if the clouds were its
origin. A reflecting cloud, at the distance of a mile; would leave
a silent interval of nearly ten seconds between sound and echo; and
had such an interval been observed at Montlhéry, it could hardly have
escaped record by the philosophers stationed there; but they have not
recorded it.

I think both the fact and the inference need reconsideration. For
our observations prove to demonstration that air of perfect visual
transparency is competent to produce echoes of great intensity and
long duration. The subject is worthy of additional illustration. On
the 8th of October, as already stated, the siren was established at
the South Foreland. I visited the station on that day, and listened
to its echoes. They were far more powerful than those of the horn.
Like the others, they were perfectly continuous, and faded, as if into
distance, gradually away. The direct sound seemed rendered complex and
multitudinous by its echoes, which resembled a band of trumpeters,
first responding close at hand, and then retreating rapidly toward the
coast of France. The siren echoes on that day had 11 seconds’, those
of the horn 8 seconds’, duration.

In the case of the siren, moreover, the reinforcement of the direct
sound by its echo was distinct. About a second after the commencement
of the siren-blast the echo struck in as a new sound. This first echo,
therefore, must have been flung back by a body of air not more than 600
or 700 feet in thickness. The few detached clouds visible at the time
were many miles away, and could clearly have had nothing to do with the
effect.

On the 10th of October I was again at the Foreland listening to the
echoes, with results similar to those just described. On the 15th I had
an opportunity of remarking something new concerning them at Dungeness,
where a horn similar to, but not so powerful as, those at the South
Foreland, has been mounted. It rotates automatically through an arc of
210°, halting at four different points on the arc and emitting a blast
of 6 seconds’ duration, these blasts being separated from each other by
intervals of silence of 20 seconds.

The new point observed was this: as the horn rotated the echoes were
always returned along the line in which the axis of the horn pointed.
Standing either behind or in front of the lighthouse tower, or closing
the eyes so as to exclude all knowledge of the position of the horn,
the direction of its axis when sounded could always be inferred from
the direction in which the aërial echoes reached the shore. Not only,
therefore, is knowledge of _direction_ given by a sound, but it may
also be given by the aërial echoes of the sound.

On the 17th of October, at about 5 P.M., the air being perfectly free
from clouds, we rowed toward the Foreland, landed, and passed over the
seaweed to the base of the cliff. As I reached the base the position of
the “Galatea” was such that an echo of astonishing intensity was sent
back from her side; it came as if from an independent source of sound
established on board the steamer. This echo ceased suddenly, leaving
the aërial echoes to die gradually into silence.

At the base of the cliff a series of concurrent observations made the
duration of the aërial siren-echoes from 13 to 14 seconds.

Lying on the shingle under a projecting roof of chalk, the somewhat
enfeebled diffracted sound reached me, and I was able to hear
with great distinctness, about a second after the starting of the
siren-blast, the echoes striking in and reinforcing the direct sound.
The first rush of echoed sound was very powerful, and it came, as
usual, from a stratum of air 600 or 700 feet in thickness. On again
testing the duration of the echoes, it was found to be from 14 to 15
seconds. The perfect clearness of the afternoon caused me to choose
it for the examination of the echoes. It is worth remarking that this
was our day of longest echoes, and it was also our day of greatest
acoustic transparency, this association suggesting that the duration
of the echo is a measure of the atmospheric _depths_ from which it
comes. On no day, it is to be remembered, was the atmosphere free from
invisible acoustic clouds; and on this day, and when their presence
did not prevent the direct sound from reaching to a distance of 15 or
16 nautical miles, they were able to send us echoes of 15 seconds’
duration.

On various occasions, when fully three miles from the shore, the
Foreland bearing north, we have had the distinct echoes of the siren
sent back to us from the cloudless _southern_ air.

To sum up this question of aërial echoes. The siren sounded three
blasts a minute, each of 5 seconds’ duration. From the number of days
and the number of hours per day during which the instrument was in
action we can infer the number of blasts. They reached nearly twenty
thousand. The blasts of the horns exceeded this number, while hundreds
of shots were fired from the guns. Whatever might be the state of the
weather, cloudy or serene, stormy or calm, the aërial echoes, though
varying in strength and duration from day to day, were never absent;
and on many days, “under a perfectly clear sky,” they reached, in the
case of the siren, an astonishing intensity. It is doubtless to these
air-echoes, and not to cloud-echoes, that the rolling of thunder is to
be ascribed.

§ 7. _Experimental Demonstration of Reflection from Gases_

Thus far we have dealt in inference merely, for the interception
of sound through aërial reflection has never been experimentally
demonstrated; and, indeed, according to Arago’s observation, which
has hitherto held undisputed possession of the scientific field, it
does not sensibly exist. But the strength of science consists in
verification, and I was anxious to submit the question of aërial
reflection to an experimental test. The knowledge gained in the last
lecture enables us to apply such a test; but, as in most similar cases,
it was not the simplest combinations that were first adopted. Two gases
of different densities were to be chosen, and I chose carbonic acid and
coal-gas. With the aid of my skillful assistant, Mr. John Cottrell,
a tunnel was formed, across which five-and-twenty layers of carbonic
acid were permitted to fall, and five-and-twenty alternate layers of
coal-gas to rise. Sound was sent through this tunnel, making fifty
passages from medium to medium in its course. These, I thought, would
waste in aërial echoes a sensible portion of sound.

To indicate this waste an objective test was found in one of the
sensitive flames described in the last chapter. Acquainted with it, we
are prepared to understand a drawing and description of the apparatus
first employed in the demonstration of aërial reflection. The following
clear account of the apparatus was given by a writer in “Nature,”
February 5, 1874:

“A tunnel _t t′_ (Fig. 146), 2 inches square, 4 feet 8 inches long,
open at both ends, and having a glass front, runs through the
box _a b c d_. The spaces above and below are divided into cells
opening into the tunnel by transverse orifices exactly corresponding
vertically. Each alternate cell of the upper series—the 1st, 3d,
5th, etc.—communicates by a bent tube (_e e e_) with a common upper
reservoir (_g_), its counterpart cell in the lower series having a
free outlet into the air. In like manner the 2d, 4th, 6th, etc., of
the lower series of cells are connected by bent tubes (_n n n_) with
the lower reservoir (_i_), each having its direct passage into the
air through the cell immediately above it. The gas-distributors (_g_
and _i_) are filled from both ends at the same time, the upper with
carbonic-acid gas, the lower with coal-gas, by branches from their
respective supply-pipes (_f_ and _h_). A well-padded box (P) open to
the end of the tunnel forms a little cavern, whence the sound-waves are
sent forth by an electric bell (dotted in the figure). A few feet
from the other end of the tunnel, and in a direct line with it, is a
sensitive flame (_k_), provided with a funnel as sound-collector, and
guarded from chance currents by a shade.

[Illustration: FIG. 146.]

“The bell was set ringing. The flame, with quick response to each
blow of the hammer, emitted a sort of musical roar, shortening and
lengthening as the successive sound-pulses reached it. The gases were
then admitted. Twenty-five flat jets of coal-gas ascended from the
tubes below, and twenty-five cascades of carbonic acid fell from the
tubes above. That which was a homogeneous medium had now fifty limiting
surfaces, from each of which a portion of the sound was thrown back.
In a few moments these successive reflections became so effective that
no sound having sufficient power to affect the flame could pierce the
clear, optically-transparent, but acoustically-opaque, atmosphere in
the tunnel. So long as the gases continued to flow the flame remained
perfectly tranquil. When the supply was cut off, the gases rapidly
diffused into the air. The atmosphere of the tunnel became again
homogeneous, and therefore acoustically transparent, and the flame
responded to each sound-pulse as before.”

Not only do gases of different densities act thus upon sound, but
atmospheric air in layers of different temperatures does the same.
Across a tunnel resembling _t t′_, Fig. 146, sixty-six platinum wires
were stretched, all of them being in metallic connection. The bell, in
its padded box, was placed at one end of the tunnel, and the sensitive
flame _k_, near its flaring-point, at the other. When the bell rang
the flame flared. A current from a strong voltaic battery being sent
through the platinum wires, they became heated: layers of warm air rose
from them through the tunnel, and immediately the agitation of the
flame was stilled. On stopping the current, the agitation recommenced.
In this experiment the platinum wires had not reached a red heat.
Employing half the number and the same battery, they were raised to
a red heat, the action in this case upon the sound-waves being also
energetic. Employing one-third of the number of wires, and the same
strength of battery, the wires were raised to a white heat. Here also
the flame was immediately rendered tranquil by the stoppage of the
sound.

§ 8. _Reflection from Vapors_

But not only do gases of different densities, and air of different
temperatures, act thus upon sound, but air saturated, in different
degrees, with the vapors of volatile liquids can be shown by experiment
to produce the same effect. Into the path pursued by the carbonic acid
in our first experiment a flask, which I have frequently employed to
charge air with vapor, was introduced. Through a volatile liquid,
partially filling the flask, air was forced into the tunnel _t t_′,
which was thus divided into spaces of air saturated with the vapor, and
other spaces in their ordinary condition. The action of such a medium
upon the sound-waves issuing from the bell is very energetic, instantly
reducing the violently-agitated flame to stillness and steadiness.
The removal of the heterogeneous medium instantly restores the noisy
flaring of the flame.

A few illustrations of the action of non-homogeneous atmospheres,
produced by the saturation of layers of air with the vapors of volatile
liquids, may follow here:

_Bisulphide of Carbon._—Flame very sensitive, and noisily responsive to
the sound. The action of the non-homogeneous atmosphere was prompt and
strong, stilling the agitated flame.

_Chloroform._—Flame still very sensitive; action similar to the last.

_Iodide of Methyl._—Action prompt and energetic.

_Amylene._—Very fine action; a short and violently-agitated flame was
immediately rendered tall and quiescent.

_Sulphuric Ether._—Action prompt and energetic.

The vapor of water at ordinary temperatures is so small in quantity and
so attenuated that it requires special precautions to bring out its
action. But with such precautions it was found competent to reduce to
quiescence the sensitive flame.

As the skill and knowledge of the experimenter augment he is often
able to simplify his experimental combinations. Thus, in the present
instance, by the suitable arrangement of the source of sound and the
sensitive flame, it was found that not only twenty-five layers, but
three or four layers of coal-gas and carbonic acid sufficed to still
the agitated flame. Nay, with improved manipulation, the action of a
single layer of either gas was rendered perfectly sensible. So also as
regards heated layers of air, not only were sixty-six or twenty-two
heated platinum wires found sufficient, but the heated air from two or
three candle-flames, or even from a single flame, or a heated poker,
was found perfectly competent to stop the flame’s agitation. The same
remark applies to vapors. Three or four heated layers of air, saturated
with the vapor of a volatile liquid, stilled the flame; and, by
improved manipulation, the action of a single saturated layer could be
rendered sensible. In all these cases, moreover, a small, high-pitched
reed might be substituted for the bell.

My assistant has devised the simple apparatus sketched in Fig. 147, for
showing reflection by gases, vapors, and heated air. At the end A of
the square pipe A B is a small vibrating reed of high pitch, the sound
of which violently agitates the sensitive flame _f_. To the horizontal
tube _g g′_ are attached four small burners, and above them four
chimneys, through which the heated gases from the flames can ascend
into A B. When the coverings of the chimneys are removed and the gas is
ignited, the air within A B is rendered rapidly non-homogeneous, and
immediately stills the agitated flame.

[Illustration: FIG. 147.]

The pipe A B may be turned upside down, an orifice seen between A and
B fitting on to the stand which supports the tube. The conduit _t_
leads into a shallow rectangular box, which communicates by a series of
transverse apertures with A B. When air, saturated with the vapor of a
volatile liquid, is forced through these apertures, the atmosphere in
A B is immediately rendered heterogeneous, the agitated flame being as
rapidly stilled.

[Illustration: FIG. 148.]

In the experiments at the South Foreland, not only was it proved that
the acoustic clouds stopped the sound; but, in the proper position, the
sounds which had been refused transmission were received by reflection.
I wished very much to render this echoed sound evident experimentally;
and stated to my assistant that we ought to be able to accomplish this.
Mr. Cottrell met my desire by the following beautiful experiment, which
has been thus described before the Royal Society:

A vibrating reed B (Fig. 148) was placed so as to send sound-waves
through a tin tube, 38 inches long, and 1-3/4 inch diameter, in the
direction B A, the action of the sound being rendered manifest by its
causing a sensitive flame placed at F′ to become violently agitated.

“The invisible heated layer immediately above the luminous portion
of an ignited coal-gas flame issuing from an ordinary bat’s-wing
burner was allowed to stream upward across the end A of the tin tube.
A portion of the sound issuing from the tube was reflected at the
limiting surfaces of the heated layer; the part transmitted being now
only competent to slightly agitate the sensitive flame at F′.

“The heated layer was then placed at such an angle that the reflected
portion of the sound was sent through a second tin tube, A F (of the
same dimensions as B A). Its action was rendered visible by causing a
second sensitive flame placed at the end of the tube at F to become
violently affected. This _echo_ continued active as long as the heated
layer intervened; but upon its withdrawal the sensitive flame placed
at F′, receiving the whole of the direct pulse, became again violently
agitated, and at the same moment the sensitive flame at F, ceasing to
be affected by the echo, resumed its former tranquillity.

“Exactly the same action takes place when the luminous portion of a
gas-flame is made the reflecting layer; but in the experiments above
described the invisible layer above the flame only was used. By proper
adjustment of the pressure of the gas the flame at F′ can be rendered
so moderately sensitive to the direct sound-wave that the portion
transmitted through the reflecting layer shall be incompetent to affect
the flame. Then by the introduction and withdrawal of the bat’s-wing
flame the two sensitive flames can be rendered alternately quiescent
and strongly agitated.

“An illustration is here afforded of the perfect analogy between
light and sound; for if a beam of light be projected from B to F′,
and a plate of glass be introduced at A in the exact position of the
reflecting layer of gas, the beam will be divided, one portion being
reflected in the direction A F, and the other portion transmitted
through the glass toward F′, exactly as the sound-wave is divided into
a reflected and transmitted portion by the layer of heated gas or
flame.”

Thus far, therefore, we have placed our subject in the firm grasp of
experiment; nor shall we find this test failing us further on.

_PART II_

INVESTIGATION OF THE CAUSES WHICH HAVE HITHERTO BEEN SUPPOSED EFFECTIVE
IN PREVENTING THE TRANSMISSION OF SOUND THROUGH THE ATMOSPHERE

  Action of Hail and Rain—Action of Snow—Action of Fog;
  Observations in London—Experiments on Artificial
  Fogs—Observations on Fogs at the South Foreland—Action of
  Wind—Atmospheric Selection—Influence of Sound-Shadow

§ 1. _Action of Hail and Rain_

In the first part of this chapter it was demonstrated that the optic
transparency and acoustic transparency of our atmosphere were by no
means necessarily coincident; that on days of marvellous optical
clearness the atmosphere may be filled with impervious acoustic clouds,
while days optically turbid may be acoustically clear. We have now
to consider, in detail, the influence of various agents which have
hitherto been considered potent in reference to the transmission of
sound through the atmosphere.

Derham, and after him all other writers, considered that falling rain
tended powerfully to obstruct sound. An observation on June 3d has
been already referred to as tending to throw doubt on this conclusion.
Two other crucial instances will suffice to show its untenability. On
the morning of October 8th, at 7.45 A.M., a thunderstorm accompanied by
heavy rain broke over Dover. But the clouds subsequently cleared away,
and the sun shone strongly on the sea. For a time the optical clearness
of the atmosphere was extraordinary, but it was acoustically opaque.
At 2.30 P.M. a densely-black scowl again overspread the heavens to the
W.S.W. The distance being 6 miles, and all hushed on board, the horn
was heard very feebly, the siren more distinctly, while the howitzer
was better than either, though not much superior to the siren.

A squall approached us from the west. In the Alps or elsewhere I have
rarely seen the heavens blacker. Vast cumuli floated to the N.E. and
S.E.; vast streamers of rain descended in the W.N.W.; huge scrolls of
cloud hung in the N.; but spaces of blue were to be seen to the N.N.E.

At 7 miles’ distance the siren and horn were both feeble, while the gun
sent us a very faint report. A dense shower now enveloped the Foreland.

The rain at length reached us, falling heavily all the way between
us and the Foreland; but the sound, instead of being deadened, rose
perceptibly in power. Hail was now added to the rain, and the shower
reached a tropical violence, the hailstones floating thickly on the
flooded deck. In the midst of this furious squall both the horns and
the siren were distinctly heard; and as the shower lightened, thus
lessening the local pattering, the sounds so rose in power that we
heard them at a distance of 7-1/2 miles distinctly louder than they
had been heard through the rainless atmosphere at 5 miles.

At 4 P.M. the rain had ceased and the sun shone clearly through the
calm air. At 9 miles’ distance the horn was heard feebly, the siren
clearly, while the howitzer sent us a loud report. All the sounds were
better heard at this distance than they had previously been at 5-1/2
miles; from which, by the law of inverse squares, it follows that
the intensity of the sound at 5-1/2 miles’ distance must have been
augmented at least threefold by the descent of the rain.

On the 23d of October our steamer had forsaken us for shelter, and I
sought to turn the weather to account by making other observations on
both sides of the fog-signal station. Mr. Douglass, the chief engineer
of the Trinity House, was good enough to undertake the observations
N.E. of the Foreland; while Mr. Ayers, the assistant engineer, walked
in the other direction. At 12.50 P.M. the wind blew a gale, and broke
into a thunderstorm with violent rain. Inside and outside the Cornhill
Coast-guard Station, a mile from the instruments in the direction of
Dover, Mr. Ayers heard the sound of the siren through the storm; and
after the rain had ceased, all sounds were heard distinctly louder
than before. Mr. Douglass had sent a fly before him to Kingsdown, and
the driver had been waiting for fifteen minutes before he arrived.
During this time no sound had been heard, though 40 blasts had
been blown in the interval; nor had the coast-guard man on duty, a
practiced observer, heard any of them throughout the day. During the
thunderstorm, and while the rain was actually falling with a violence
which Mr. Douglass describes as perfectly torrential, the sounds
became audible and were heard by all.

To rain, in short, I have never been able to trace the slightest
deadening influence upon sound. The reputed barrier offered by “thick
weather” to the passage of sound was one of the causes which tended to
produce hesitation in establishing sound-signals on our coasts. It is
to be hoped that the removal of this error may redound to the advantage
of coming generations of seafaring men.

§ 2. _Action of Snow_

Falling snow, according to Derham, is the most serious obstacle of all
to the transmission of sound. We did not extend our observations at the
South Foreland into snowy weather; but a previous observation of my own
bears directly upon this point. On Christmas night, 1859, I arrived at
Chamouni, through snow so deep as to obliterate the road-fences, and to
render the labor of reaching the village arduous in the extreme. On the
26th and 27th it fell heavily. On the 27th, during a lull in the storm,
I reached the Montavert, sometimes breast deep in snow. On the 28th,
with great difficulty, two lines of stakes were set out across the
glacier, with the view of determining its winter motion. On the 29th
the entry in my journal, written in the morning, is: “Snow, heavy snow;
it must have descended through the entire night, the quantity freshly
fallen is so great.”

Under these circumstances I planted my theodolite beside the Mer de
Glace, having waded to my position through snow, which, being dry,
reached nearly to my breast. Assistants were sent across the glacier
with instructions to measure the displacement of a transverse line of
stakes planted previously in the snow. A storm drifted up the valley,
darkening the air as it approached. It reached us, the snow falling
more heavily than I had ever seen it elsewhere. It soon formed a heap
on the theodolite, and thickly covered my own clothes. Here, then,
was a combination of snow in the air, and of soft fresh snow on the
ground, such as Derham could hardly have enjoyed; still through such
an atmosphere I was able to make my instructions audible quite across
the glacier, the distance being half a mile, while the experiment was
rendered reciprocal by one of my assistants making his voice audible to
me.

§ 3. _Passage of Sound through Textile Fabrics, and through Artificial
Showers_

The flakes here were so thick that it was only at intervals that I
was able to pick up the retreating forms of the men. Still the air
through which the flakes fell was continuous. Did the flakes merely
yield passively to the sonorous waves, swinging like the particles
of air themselves to and fro as the sound-waves passed them? Or did
the waves bend by diffraction round the flakes, and emerge from them
without sensible loss? Experiment will aid us here by showing the
astonishing facility with which sound makes its way among obstacles,
and passes through tissues, so long as the continuity of the air in
their interstices is preserved.

A piece of millboard or of glass, a plank of wood, or the hand, placed
across the open end _t′_ of the tunnel _a b c d_, Fig. 146 (page 334),
intercepts the sound of the bell, placed in the padded box P, and
stills the sensitive flame _k_.

An ordinary cambric pocket-handkerchief, on the other hand, placed
across the tunnel-end produced hardly an appreciable effect upon the
sound. Through two layers of the handkerchief the flame was strongly
agitated; through four layers it was still agitated; while through six
layers, though nearly stilled, it was not entirely so.

Dipping the same handkerchief into water, and stretching a single
wetted layer across the tunnel-end, it stilled the flame as effectually
as the millboard or the wood. Hence the conclusion that the sound-waves
in the first instance passed through the interstices of the cambric.

Through a single layer of thin silk the sound passed without sensible
interruption; through six layers the flame was strongly agitated; while
through twelve layers the agitation was quite perceptible.

A single layer of this silk, when wetted, stilled the flame.

A layer of soft lint produced but little effect upon the sound; a layer
of thick flannel was almost equally ineffectual. Through four layers of
flannel the flame was perceptibly agitated. Through a single layer of
green baize the sound passed almost as freely as through air; through
four layers of the baize the action was still sensible. Through a layer
of close hard felt, half an inch thick, the sound-waves passed with
sufficient energy to sensibly agitate the flame. Through 200 layers of
cotton-net the sound passed freely. I did not witness these effects
without astonishment.

A single layer of thin oiled silk stopped the sound and stilled the
flame. A leaf of common note-paper, or a five-pound note, also stopped
the sound.

The sensitive flame is not absolutely necessary to these experiments.
Let a ticking watch be hung six inches from the ear, a cambric
handkerchief dropped between it and the ear scarcely sensibly affects
the ticking; a sheet of oil-skin or an intensely heated gas-column cuts
it almost wholly off.

But though oiled silk, foreign post, or a banknote, can stop the
sound, a film sufficiently thin to yield freely to the aërial pulses
transmits it. A thick soap-film produces an obvious effect upon the
sensitive flame; a very thin one does not. The augmentation of the
transmitted sound may be observed simultaneously with the generation
and brightening of the colors which indicate the increasing thinness of
the film. A very thin collodion-film acts in the same way.

Acquainted with the foregoing facts regarding the passage of sound
through cambric, silk, lint, flannel, baize, felt, and cotton-net,
you are prepared for the statement that the sound-waves pass without
sensible impediment through heavy artificial showers of rain, hail,
and snow. Water-drops, seeds, sand, bran, and flocculi of various
kinds, have been employed to form such showers; through all of these,
as through the actual rain and hail already described, and through the
snow on the Mer de Glace, the sound passes without sensible obstruction.

§ 4. _Action of Fog. Observations in London_

But the mariner’s greatest enemy, fog, is still to be dealt with;
and here for a long time the proper conditions of experiment were
absent. Up to the end of November we had had frequent days of haze,
sufficiently thick to obscure the white cliffs of the Foreland, but
no real fog. Still those cases furnished demonstrative evidence that
the notions entertained regarding the reflection of sound by suspended
particles were wrong; for on many days of the thickest haze the sound
covered twice the range attained on other days of perfect optical
transparency. Such instances dissolved the association hitherto assumed
to exist between acoustic transparency and optic transparency, but they
left the action of dense fogs undetermined.

On December 9th a memorable fog settled down on London. I addressed a
telegram to the Trinity House suggesting some gun observations. With
characteristic promptness came the reply that they would be made in the
afternoon at Blackwall. I went to Greenwich in the hope of hearing the
guns across the river; but the delay of the train by the fog rendered
my arrival too late. Over the river the fog was very dense, and through
it came various sounds with great distinctness. The signal-bell of an
unseen barge rang clearly out at intervals, and I could plainly hear
the hammering at Cubitt’s Town, half a mile away, on the opposite side
of the river. No deadening of the sound by the fog was apparent.

Through this fog and various local noises, Captain Atkins and Mr.
Edwards heard the report of a 12-pounder carronade with a 1-lb. charge
distinctly better than the 18-pounder with a 3-lb. charge, an optically
clear atmosphere, and all noises absent, on July 3d.

Anxious to turn to the best account a phenomenon for which we had
waited so long, I tried to grapple with the problem by experiments on
a small scale. On the 10th, I stationed my assistant with a whistle
and organ-pipe on the walk below the southwest end of the bridge
dividing Hyde Park from Kensington Gardens. From the eastern end of
the Serpentine I heard distinctly both the whistle and the pipe, which
produced 380 waves a second. On changing places with my assistant, I
heard for a time the distinct blast of the whistle only. The deeper
note of the organ-pipe at length reached me, rising sometimes to great
distinctness, and sometimes falling to inaudibility. The whistle showed
the same intermittence as to period, but in an opposite sense; for,
when the whistle was faint, the pipe was strong, and _vice versa_. To
obtain the fundamental note of the pipe, it had to be blown gently, and
on the whole the whistle proved the most efficient in piercing the fog.

An extraordinary amount of sound filled the air during these
experiments. The resonant roar of the Bayswater and Knightsbridge
roads; the clangor of the great bell of Westminster; the
railway-whistles, which were frequently blown, and the fog-signals
exploded at the various metropolitan stations, were all heard with
extraordinary intensity. This could by no means be reconciled with the
statements so categorically made regarding the acoustic impenetrability
of a London fog.

On the 11th of December, the fog being denser than before, I heard
every blast of the whistle, and occasional blasts of the pipe, over
the distance between the bridge and the eastern end of the Serpentine.
On joining my assistant at the bridge, the loud concussion of a gun
was heard by both of us. A police-inspector affirmed that it came
from Woolwich, and that he had heard several shots about 2 P.M. and
previously. The fact, if a fact, was of the highest importance; so I
immediately telegraphed to Woolwich for information. Prof. Abel kindly
furnished me with the following particulars:

“The firing took place at 1.40 P.M. The guns proved were of
comparatively small size—64-pounders, with 10-lb. charges of powder.

“The concussion experienced at my house and office, about
three-quarters of a mile from the butt, was decidedly more severe than
that experienced when the heaviest guns are proved with charges of
110 to 120 lbs. of powder. There was a dense fog here at the time of
firing.”

These were the guns heard by the police-inspector; on subsequent
inquiry it was ascertained that two guns were fired about 3 P.M. These
were the guns heard by myself.

Prof. Abel also communicated to me the following fact: “Our workman’s
bell at the Arsenal Gate, which is of moderate size and anything but
clear in tone, is pretty distinctly heard by Prof. Bloxam _only_ when
the wind is _northeast_. During the whole of last week the bell was
heard with great distinctness, the wind being _southwesterly_ (opposed
to the sound). The distance of the bell from Bloxam’s house is about
three-quarters of a mile as the crow flies.”

Assuredly no question of science ever stood so much in need of revision
as this of the transmission of sound through the atmosphere. Slowly,
but surely, we mastered the question; and the further we advanced, the
more plainly it appeared that our reputed knowledge regarding it was
erroneous from beginning to end.

On the morning of the 12th the fog attained its maximum density. It
was not possible to read at my window, which fronted the open western
sky. At 10.30 I sent an assistant to the bridge, and listened for his
whistle and pipe at the eastern end of the Serpentine. The whistle
rose to a shrillness far surpassing anything previously heard, but it
sank sometimes almost to inaudibility; proving that, though the air
was on the whole highly homogeneous, acoustic clouds still drifted
through the fog. A second pipe, which was quite inaudible yesterday,
was plainly heard this morning. We were able to discourse across the
Serpentine to-day with much greater ease than yesterday.

During our summer observations I had once or twice been able to fix the
position of the Foreland in thick haze by the direction of the sound.
To-day my assistant, hidden by the fog, walked up to the Watermen’s
Boathouse sounding his whistle; and I walked along the opposite side of
the Serpentine, clearly appreciating for a time that the line joining
us was oblique to the axis of the river. Coming to a point which seemed
to be exactly abreast of him, I marked it; and on the following day,
when the fog had cleared away, the marked position was found to be
perfectly exact. When undisturbed by echoes, the ear, with a little
practice, becomes capable of fixing with great precision the direction
of a sound.

On reaching the Serpentine this morning, a peal of bells, which then
began to ring, seemed so close at hand that it required some reflection
to convince me that they were ringing to the north of Hyde Park. The
sounds fluctuated wonderfully in power. Prior to the striking of
eleven by the great bell of Westminster, a nearer bell struck with
loud clangor. The first five strokes of the Westminster bell were
afterward heard, one of them being extremely loud; but the last six
strokes were inaudible. An assistant was stationed to attend to the 12
o’clock bells. The clock which had struck so loudly at 11 was unheard
at 12, while of the Westminster bell eight strokes out of twelve were
inaudible. To such astonishing changes is the atmosphere liable.

At 7 P.M. the Westminster bell, striking seven, was not at all heard
from the Serpentine, while the nearer bell already alluded to was heard
distinctly. The fog had cleared away, and the lamps on the bridge
could be seen from the eastern end of the Serpentine burning brightly;
but, instead of the sound sharing the improvement of the light, what
might be properly called an acoustic fog took the place of its optical
predecessor. Several series of the whistle and organ-pipe were sounded
in succession; one series only of the whistle-sounds was heard, all
the others being quite inaudible. Three series of the organ-pipe were
heard, but very faintly. On reversing the positions and sounding as
before, nothing whatever was heard.

At 8 o’clock the chimes and hour-bell of the Westminster clock were
both very loud. The “acoustic fog” had shifted its position, or
temporarily melted away.

Extraordinary fluctuations were also observed in the case of the
church-bells heard in the morning: in a few seconds they would sink
from a loudly ringing-peal into utter silence, from which they would
rapidly return to loud-tongued audibility. The intermittent drifting of
fog over the sun’s disk (by which his light is at times obscured, at
times revealed) is the optical analogue of these effects. As regards
such changes, the acoustic deportment of the atmosphere is a true
transcript of its optical deportment.

At 9 P.M. three strokes only of the Westminster clock were heard; the
others were inaudible. The air had relapsed in part into its condition
at 7 P.M., when all the strokes were unheard. The quiet of the park
this evening, as contrasted with the resonant roar which filled the air
on the two preceding days, was very remarkable. The sound, in fact, was
stifled in the optically clear but acoustically flocculent atmosphere.

On the 13th, the fog being displaced by thin haze, I went again to
the Serpentine. The carriage-sounds were damped to an extraordinary
degree. The roar of the Knightsbridge and Bayswater roads had subsided,
the tread of troops which passed us a little way off was unheard,
while at 11 A.M. both the chimes and the hour-bell of the Westminster
clock were stifled. Subjectively considered, all was favorable to
auditory impressions; but the very cause that damped the local noises
extinguished our experimental sounds. The voice across the Serpentine
to-day, with my assistant plainly visible in front of me, was
distinctly feebler than it had been when each of us was hidden from the
other in the densest fog.

Placing the source of sound at the eastern end of the Serpentine I
walked along its edge from the bridge toward the end. The distance
between these two points is about 1,000 paces. After 500 of them had
been stepped, the sound was not so distinct as it had been at the
bridge on the day of densest fog; hence, by the law of inverse squares,
the optical cleansing of the air through the melting away of the fog
had so darkened it acoustically that a sound generated at the eastern
end of the Serpentine was lowered to one-fourth of its intensity at a
point midway between the end and the bridge.

To these demonstrative observations one or two subsequent ones may
be added. On several of the moist and warm days, at the beginning of
1874, I stood at noon beside the railing of St. James’s Park, near
Buckingham Palace, three-quarters of a mile from the clock-tower, which
was clearly visible. Not a single stroke of “Big Ben” was heard. On
January 19th fog and drizzling rain obscured the tower; still from the
same position I not only heard the strokes of the great bell, but also
the chimes of the quarter-bells.

During the exceedingly dense and “dripping” fog of January 22d, from
the same railings, I heard every stroke of the bell. At the end of the
Serpentine, when the fog was densest, the Westminster bell was heard
striking loudly eleven. Toward evening this fog began to melt away,
and at 6 o’clock I went to the end of the Serpentine to observe the
effect of the optical clearing upon the sound. Not one of the strokes
reached me. At 9 o’clock and at 10 o’clock my assistant was in the same
position, and on both occasions he failed to hear a single stroke of
the bell. It was a case precisely similar to that of December 13th,
when the dissolution of the fog was accompanied by a decided acoustic
thickening of the air.[66]

§ 5. _Observations at the South Foreland_

Satisfactory, and indeed conclusive, as these results seemed, I desired
exceedingly to confirm them by experiments with the instruments
actually employed at the South Foreland. On the 10th of February I had
the gratification of receiving the following note and inclosure from
the Deputy Master of Trinity House:

  “MY DEAR TYNDALL—The inclosed will show how accurately
  your views have been verified, and I send them on at once
  without waiting for the details. I think you will be glad
  to have them, and as soon as I get the report it shall be
  sent to you. I made up my mind ten days ago that there
  would be a chance in the light foggy-disposed weather at
  home, and therefore sent the ‘Argus’ off at an hour’s
  notice, and requested the Fog Committee to keep one
  member on board. On Friday I was so satisfied that the
  fog would occur that I sent Edwards down to record the
  observations.

  “Very truly yours,

  “FRED. ARROW.”

The inclosure referred to was notes from Captain Atkins and Mr.