is usual, from the west or south-west. These observations were
sufficiently numerous to form a synoptic chart at a considerable
height above Europe for comparison with the usual chart drawn from the
surface observations.

Besides the general work of the International Committee, special
investigations have been undertaken by the French, who formed an
Aerostatic Commission in Paris. The services of the eminent
physicists, MM. Cailletet and Violle, have been enlisted, while a
generous patron has been found in Prince Roland Bonaparte. The
apparatus of M. Cailletet to bring down samples of air from the high
regions may now be described. When the balloon has reached its
greatest height a cock of special construction, turned by clockwork,
opens and allows the air to enter a reservoir in which a vacuum
exists, and then the cock is automatically and hermetically closed. As
it is known that the balloon reaches its extreme height in about an
hour and a quarter, the time of opening the cock is so regulated, the
closing taking place a little later by its further rotation. In order
to protect the moving parts from the extreme cold, a receptacle filled
with fused acetate of soda is placed in the box containing the motor,
so that, notwithstanding the intense cold of the high regions, this
salt in assuming a crystalline state gives out enough heat during
several hours at least. During the ascent of an _AÈrophile_ air was
collected at 50,000 feet, which when analyzed by M. M¸ntz showed what
was supposed, viz. that at this altitude the composition of the air
does not differ much from that of the lower air. The slight excess of
carbonic acid found in the upper air might be due to the oxidation of
the grease used on the cock, and the smaller quantity of oxygen, as
compared with normal air, might be caused by the absorption of this
gas by the grease or even the absorption by the tinned sides of the
copper reservoir. By eliminating all possible sources of error in
future ascensions, M. M¸ntz thinks that it can be proved whether there
are real differences in the air at different heights, for the methods
of analysis are to-day accurate enough to show such differences if
they exist. But since it is probable that in the regions which can be
explored by the _ballons-sondes_, the air undergoes the same mixing
that renders the lower air nearly uniform, only the smallest
variations in its composition would naturally be found, requiring
minute precautions against errors. This is no doubt why previous
measures agreed in showing the invariable composition of the air at
lower altitudes.

Another important contribution of M. Cailletet is an apparatus for
measuring the height of the balloon by photography in order to verify
Laplace's formula connecting the barometric pressure with the
altitude. The idea was to replace the observers on the ground, who
sometimes made the trigonometrical measurements of the balloon
described, by a photographic apparatus carried by the balloon itself,
and which at frequent intervals should photograph automatically the
ground over which it passed, at the same time that an aneroid
barometer was photographed on the same sheet. The apparatus is hung
vertically below the balloon; in the lower portion of the box is an
objective which photographs the ground, and in the upper portion is a
second objective which photographs the face of an aneroid barometer
placed at the proper distance above. A clock-movement makes exposures
every two minutes, and a sensitive film unrolled between the
objectives receives the images on each side. If there are known, the
focal length of the objective, the distance of two points on the
ground, and the distance of two points on the photograph, a simple
proportion permits the height of the balloon to be determined at that
time, and consequently, from the barometric record, the law connecting
the pressure with height can be deduced. The apparatus was
successfully used in the voyage of a large balloon with observers, and
the accuracy of the determination of height was found to be within
1/250. If the apparatus is to be used at great heights it would be
necessary to protect the barometer and the camera from the very low
temperatures. Besides the use for which it was designed, this
apparatus may serve to trace the route of a balloon and to determine
the horizontal velocity at the different points of its path.

The exploration of the high atmosphere by _ballons-sondes_, which can
aid so many investigations, has been utilized by M. Violle to obtain
actinometric measures, that is, to determine the amount of heat given
by the sun, or what is called the "solar constant." This has been done
on mountains with varying results, due to the changing amount of
atmospheric absorption. In regions traversed by the balloon where the
pressure of the air is reduced to a few inches of mercury, where there
is a complete absence of water-vapour, and at heights to which
terrestrial dust does not extend, the measure of the quantity of heat
sent by the sun towards the earth is freed from almost all the errors
which we encounter on its surface. The actinometer of M. Violle is, in
principle, a sphere of copper, blackened externally, and having inside
a thermometric apparatus which registers some distance away. Under the
action of the solar rays the sphere is heated, and assumes equilibrium
when the loss by radiation and by contact with the air compensates for
the gain by the absorption of the direct heat. While at low levels the
atmosphere also contributes to heat the sphere, at great heights the
sun shines from an almost black sky and alone heats the sphere. Since
the balloon follows the wind the apparatus is protected from air
currents which would otherwise introduce errors. Each twenty minutes a
screen cuts off the solar rays from the sphere so that it cools to
the temperature of the air, which is also recorded. On account of its
weight this apparatus has not yet been carried by a _ballon-sonde_,
but it has operated successfully in a balloon with observers.

M. Teisserenc de Bort, who is actively engaged in exploring the air
from a meteorological standpoint, has constructed a very sensitive
thermometer made of a blade of German silver set in a frame of
nickel-steel that does not expand with heat. This may be ventilated by
a fan, and, while extremely sensitive to changes of temperature, it is
not affected by shocks, and consequently is well adapted for use in
_ballons-sondes_ that pass rapidly through air-strata of varying
temperature.

From this review of the development of the _ballons-sondes_ it is
evident that they offer possibilities of obtaining data in the high
atmosphere, perhaps up to fifteen miles or more, which, though subject
to inaccuracies, are of great interest to the physicist and
astronomer. The meteorologist is chiefly concerned with that portion
of the atmosphere which lies within two or three miles of the earth,
and he requires, moreover, accurate measurements for his conclusions.
The new and most satisfactory way of obtaining these data is by kites,
and the remaining chapters will treat of this method of exploring the
atmosphere and the results.

                               CHAPTER V

   KITES--HISTORY AND APPLICATION TO METEOROLOGICAL PURPOSES AT
   BLUE HILL AND ELSEWHERE

Kites are supposed to have been invented four hundred years before the
Christian era by Archytas, and at Smyrna the flying of kites remains a
national sport to this day. We are told that two hundred years later,
a Chinese general, Han Sin, employed kites as a means of communication
with the garrison of a besieged town, and there is a legend about
their use in Japan to dislodge and carry away a golden ornament from a
tower. Whatever may be the truth of these stories, we know that
kite-flying in the Malay Archipelago, in China, and in Japan, has been
a pastime for all classes during centuries, and that the Asiatic
people have always been the expert kite-fliers of the world.

Kites with tails seem to have been introduced into England about two
hundred and fifty years ago, and Isaac Newton when a school-boy made
some improvements in them. Notwithstanding the fact that generations
of boys have flown kites and so eminent a mathematician as Euler
investigated their theory, until recently kites remained toys unsuited
for practical purposes. Since the tailless kite has become a familiar
object, it has been said facetiously that kites lost their tails by
the same process of evolution which deprived man of his caudal
appendage; but as kites without tails have been flown in Asia for
centuries, the truth is that the tailed kites were the ones first
brought to Europe as playthings. To-day in Holland we see boys flying
the English bow-kite and the common kite with crossed sticks, both of
which require tails, and by the side of them tailless kites imported
from the Dutch colonies in Java. Fig. 7 represents a kite from the
east coast of Java, drawn from a model in a museum at Amsterdam, and
also a drawing of a Chinese bird-kite in the National Museum at
Washington. Like most of the oriental kites, they are made flat, but
when exposed to the wind the extremities of the wings, which have a
frame of split bamboo, bend backward, securing in this way the
stability which in our common flat kite is gained by the action of the
tail in lowering the centre of gravity and in maintaining the
inclination to the wind.

   [Illustration: FIG. 7.--Oriental tailless Kites.]

From historical researches that have been stimulated by the recent
practical applications of kites, it appears that their first use for
scientific purposes was in 1749, when Dr. Alexander Wilson of Glasgow,
and his pupil, Thomas Melvill, used kites to lift thermometers. Their
kites, from four to seven feet in height, and covered with paper, were
fastened behind one another, each kite taking up as much line as could
be supported, thereby allowing its companion to soar to an elevation
proportionally higher. It is related that "the uppermost one ascended
to an amazing height, disappearing at times among the white summer
clouds, whilst all the rest, in a series, formed with it in the air
below such a lofty scale, and that too affected by such regular and
conspiring motions, as at once changed a boyish pastime into a
spectacle which greatly interested every beholder.... To obtain the
information they wanted they contrived that thermometers, properly
secured, and having bushy tassels of paper tied to them, should be let
fall at stated periods from some of the higher kites, which was
accomplished by the gradual singeing of a match-line." How the
thermometers were prevented from changing their readings while falling
to the ground is not explained. The account concludes: "When engaged
in these experiments, though now and then they communicated
immediately with the clouds, yet, as this happened always in fine
weather, no symptoms whatever of an electrical nature came under their
observation. The sublime analysis of the thunderbolt, and of the
electricity of the atmosphere, lay yet entirely undiscovered, and was
reserved two years longer for the sagacity of the celebrated Dr.
Franklin." Hence it seems that Franklin's famous experiment of
collecting the electricity of a thunder-cloud by means of a kite,
performed at Philadelphia in 1752, was not its first scientific
application, and therefore America can claim only the later and most
remarkable development of this means of exploring the air.

About 1837 there existed in Philadelphia an organization called the
Franklin Kite Club that flew kites for recreation. Espy, the eminent
meteorologist, was a member, and he states "that on those days when
columnar clouds form rapidly and numerously the kite was frequently
carried upward nearly perpendicularly by columns of ascending air," a
phenomenon which is often observed to-day. Espy calculated the height
at which clouds should form by the cooling of the air to its
dew-point, and then employed kites to verify his calculations of the
heights of the clouds. It will be remembered that both these methods
are utilized in the measurements of cloud-heights at Blue Hill. Kites
were employed to get temperatures a hundred or more feet above the
Arctic ocean early in the present century, and in 1847 W. R. Birt flew
a kite at Kew Observatory, with which he hoped to obtain measures of
temperature, humidity, wind velocity, etc. This kite, hexagonal in
shape, required three divergent strings attached to the ground to keep
it steady, and the instruments were to be hoisted up to the kite by a
pulley.

Perhaps the first person to soar aloft on a kite was a lady, who, more
than fifty years ago, was lifted some hundred feet by a great kite
constructed by George Pocock, an Englishman, to serve as an aÎrial
observatory in warfare, and also to drag carriages along the ground.
It was proposed afterwards to make use of kites in shipwrecks to take
persons or life-lines ashore, and in 1865 Sir George Nares invented a
storm-kite, so called, with a tail made up of hollow cones. This form
of tail, subsequently used for both kites and balloons, is very
efficient, since it offers increasing resistance as the wind becomes
stronger.

In 1882 Mr. Douglas Archibald in England revived the use of kites for
meteorological observations, and outlined a comprehensive scheme of
exploring the air with kites which included almost all that has been
done since, but his actual work, performed during the next three
years, was limited to ascertaining the increase of wind velocity with
height. To do this, he attached registering anemometers at four
different points on the kite-wire, but since the total wind movements
only were registered from the time the anemometers left the ground
until they returned, it was impossible to obtain simultaneous records
near the ground and at the kite, as is done to-day. Still, Archibald
got differential measurements of the velocity of the wind up to the
height of 1200 feet. The kites he employed were diamond-shaped,
covered with silk, and were flown tandem, with the hollow cones,
already mentioned, attached to the tails. Although copper and iron
wire had been used for flying kites many years before, yet Archibald
was the first to substitute steel pianoforte wire for the string,
thereby increasing the strength while diminishing the weight, size,
and cost of the line. Mr. Archibald in 1887 took the first photograph
from a kite, a method which MM. Batut and Wenz developed in France,
and Messrs. Eddy and Woglom in the United States.

The subsequent progress of kite-flying for meteorological purposes has
taken place in the last-named country, and may be chronologically
stated as follows: in 1885 Mr. Alexander McAdie (later of the U. S.
Weather Bureau) repeated Franklin's kite experiment on Blue Hill, with
the addition of an electrometer; in 1891, and again in 1892, he
measured simultaneously the electric potential at the base of Blue
Hill, on the hill, and with kites as collectors several hundred feet
above the hill-top, about the same time that Dr. Weber, in Breslau,
Germany, was making a more extensive use of kites for the same
purpose. It was no doubt William A. Eddy of Bayonne, N. J., who turned
the attention of American scientific men to kite-flying, and created
the widespread interest in kites which exists to-day. About 1890 Mr.
Eddy lifted thermometers with an ordinary kite, but soon afterwards
devised a tailless kite, resembling the Java kite except that the
horizontal cross-piece is nearer the top of the vertical stick, and
its ends are bent backward in a bow and connected by a cord. This kite
starts upward on being held in the wind at the end of a taut line, and
continues to rise until the increasing wind-pressure on the portion
above the cross-stick balances the pressure on the larger lower
portion. The kite is kept from falling to one side by the looseness of
the covering on either side of the backbone, and if there is more
material on one side than on the other, or if the covering is too
tight to form pockets in the wind, the kite requires a tail.[1]

   [1] A tail will prevent any kite from turning over, or "diving,"
       because its weight keeps the lower end down while the
       pressure of the wind on the tail also pulls the lower end
       backward and maintains the necessary angle of the kite to the
       wind, the most efficient angle being about 22 degrees. Bending
       back the ends of the cross-stick gives stability to a kite
       because, when, on account of the eddies in the wind, a stronger
       pressure is exerted on one side of the kite, this side is
       driven backward, thereby presenting less effective surface to
       the wind, while as the other side comes forward more nearly at
       right angles to the wind, it receives greater pressure than
       before. In this way the equilibrium about the central stick is
       automatically maintained, the required inclination to the wind
       being secured by the greater surface presented to the wind
       below the point of attachment of the bridle.

In 1891 Mr. Eddy lifted a minimum thermometer by several of these
kites flown tandem, and proposed to obtain in this way data to
forecast the weather. In the _Proceedings of the Aeronautical
Conference_, held in connection with the Chicago Exposition, Prof. M.
W. Harrington, then Chief of the U. S. Weather Bureau, quoted Mr.
Eddy's estimate of the cost of exploring the air by means of kites
flown in series, and advocated their use.

Up to this time it does not appear that self-recording
instruments--that is to say, those which make continuous graphic
records--had been raised by kites. In the days of the early
experimenters such instruments were too heavy and cumbersome to be
lifted by the more or less unmanageable kites, but within the past few
years M. Richard of Paris has made the simple and light recording
instruments described in connection with balloons, which can be
attached to kites. In this way it is possible to obtain simultaneous
records at the kite and at a station on the ground, and from them to
study the differences of temperature and humidity, and this seems to
have been done first at Blue Hill Observatory. In August 1894 Mr. Eddy
brought his kites to Blue Hill and with them lifted a Richard
thermograph, which had been partly reconstructed of aluminium by Mr.
Fergusson so that it weighed but 1-1/4 lbs., to the height of 1500
feet, and so the earliest automatic record of temperature was obtained
by a kite. During the next summer, Mr. Eddy assisted again in the
experiments at Blue Hill, and secured photographs of the Observatory
and the hill by a camera carried between his kites to the height of a
hundred feet or more.

Now that the possibility of lifting self-recording meteorological
instruments to considerable heights had been demonstrated, an
investigation of the thermal and hygrometric conditions of the free
air was undertaken by the staff of the Blue Hill Observatory, who had
already made an investigation of the movements of the clouds by the
methods described in the second chapter.

The development of the kite and its accessory apparatus, and the
acquisition of the knowledge how to use them, required much time, and
resulted in the damage or loss of many kites. Two meteorographs, as
the combination of two or more self-recording instruments is called,
were dropped from a great height and no trace of them was found. When,
however, by the breaking of the line both kites and instrument are
carried away, the kites act as a parachute and bear the instrument
gently to the ground, where both are usually recovered uninjured; to
facilitate their return should they fall at a distance, the name and
address are marked on each. It would be tedious to relate the ups and
downs of scientific kite-flying at Blue Hill before the wind was
successfully harnessed to the service of science, and the kites were
prevented from kicking over the traces, or from breaking away, so only
a brief account of the progress of the work will be given, and then
the methods at present used will be described. At first the Eddy, or
Malay kites, as they are also called, covered with paper or with
varnished cloth, were coupled tandem to secure greater safety and
lifting power. The principle of attaching kites at several points on
the line was early adopted at Blue Hill, for although it can be
demonstrated theoretically that a greater height is possible by
concentrating all the pull at the end of the line, yet in the case of
a line which is not infinitely strong the best results are got by
distributing the pull, and in this way, too, kites can be added as the
wind conditions aloft warrant. To obviate the frequent breaking of the
bowed cross-piece, Mr. Fergusson made it in two pieces, each being
held in a metal socket on the central stick, the two pieces forming a
dihedral angle towards the wind. It had the advantage also of being
readily taken apart for transportation. This kite, shown in Fig. 8,
flew at a high angle above the horizon and through a considerable
range of wind velocity, but it could not be kept permanently in
balance, or made to adjust itself to great variations in wind
velocity, and therefore it was discarded.

The first meteorograph, a combined recording thermometer and barometer
(from which the height can be calculated), was constructed by Mr.
Fergusson in August 1895, and three months later he united a recording
anemometer to the thermometer, which was probably the first apparatus
of this kind to be attached to kites. A meteorograph, recording the
atmospheric pressure, air temperature, and relative humidity, was
ordered from M. Richard of Paris in 1895, like one already carried by
French aeronauts, except that, since for kites lightness is
all-essential, M. Richard constructed this triple-recorder for the
first time of aluminium, and hereby reduced its weight to 2-4/5 lbs.

   [Illustration: FIG. 8.--Eddy tailless Kite.]

   [Illustration: FIG. 9.--Hargrave Kite.]

One of these meteorographs was hung to a ring at the point of
attachment of the two kite-lines to the main line, a method which was
used until recently. In August 1895, besides the Eddy kites, there was
first used the cellular or box kite, invented by Lawrence Hargrave of
Sydney, Australia, which bears no resemblance to the conventional
forms of kites and which it would not be supposed could fly. As seen
from Fig. 9 its appearance is that of two light boxes without tops or
bottoms, fastened some distance above each other. The wind exerts its
lifting force chiefly upon the front and rear sides of the top box,
the lower box, which inclines to the rear, and so receives less
pressure, preserving the balance. The ends of the boxes, being in line
with the wind, keep the kite steady and serve the purpose of the
dihedral angle in the Malay kite. The Japanese are said to fly a
single box, which is the prototype of the Hargrave double cell.

At the present time some form of the Hargrave kite is generally
employed for scientific purposes. On account of the weight of the
large cord necessary to control these kites, and the surface which it
presented to the wind, a height of 2000 feet could not be reached, so,
during the winter of 1895-6, following Archibald's example and the
methods of deep-sea sounding employed by Captain Sigsbee, U. S. N.,
steel pianoforte wire was substituted for the cord. This wire is less
than half as heavy, and less than one-fourth the size of cord having
the same strength, and, moreover, its surface is polished, which
reduces the friction of the wind blowing past it. With the wire the
height of a mile was reached in July, and a mile and two-thirds above
Blue Hill in October 1896.

Up to this time a reel turned by two men sufficed to draw down the
kites, but the increasing pull and length of wire made recourse to
steam-power necessary. In January 1897 a grant of money was allotted
from the Hodgkins Fund of the Smithsonian Institution for the purpose
of obtaining meteorological records at heights exceeding ten thousand
feet, and no doubt the first application of steam to kite-flying was
the winch built by Mr. Fergusson with ingenious devices for
distributing, oiling, and measuring the length of wire. The cumulative
pressure of the successive coils of wire finally crushed the drum, and
the next apparatus applied the principle of Sir William Thomson's
deep-sea sounding apparatus, in which there is no accumulation of
pressure. In October 1897 records were brought down from eleven
thousand feet, or a thousand feet above the prescribed height.

The kites and apparatus at present employed at Blue Hill will now be
described.

The kites are all of the multiplane type, and mostly of Hargrave's
construction with two rectangular cells. These cells are covered with
cloth or silk, except at their tops and bottoms, and one is secured
above the other by four or more sticks. The wooden frames are as light
as possible, but are made rigid by guys of steel wire that bind them
in all directions. The average weight is about two ounces a square
foot of lifting surface, which is about the same weight a square foot
as the Eddy kites when all the surface is included in the estimate.
The largest of the Hargrave kites stands nine feet high, weighs eleven
pounds, and contains ninety square feet of lifting surface, which in
the recent kites is arched, resembling the curvature of a bird's
wings, a construction that was proposed many years ago by Phillips
(Fig. 10). These curved surfaces increase the lift, or upward pull,
more than the drift, or motion to leeward, and so the angular
elevation is augmented without materially adding to the total pull on
the wire, which should not exceed one-half its breaking strength.

  [Illustration: FIG. 10.--Modified Hargrave Kite at
   Blue Hill.]

Perhaps the most important factor in the success of the Blue Hill work
was the invention by Mr. Clayton of the regulating bridle which is
applied to every kite. An elastic cord is inserted in the lower part
of the bridle, to which the flying-line is attached, and when the
wind-pressure increases this cord stretches, and causes the kite to
diminish its angle of incidence to the wind until the gust subsides. A
kite can be set to pull only a fixed amount in the strongest wind,
when the kite will fly nearly horizontal. We are therefore able to
calculate the greatest pull which can be exerted on the wire by all
the kites. With this device the kites have flown through gales of
fifty or sixty miles an hour without breaking loose or injuring
themselves. Another efficient kite which has been used at Blue Hill is
the so-called "aero-curve kite" made by Mr. C. H. Lamson of Portland,
Maine. As is seen from Fig. 11, this kite resembles a soaring bird,
and it can be taken apart and folded up for storage or transportation.

In general, the angle of the flying lines of the Blue Hill kites is
50∞ or 60∞ above the horizon, and in winds of twenty miles an hour the
pull on the line is about one pound for each square foot of lifting
surface in the kite. Kites can be raised in a wind that blows more
than twelve miles an hour at the ground, and as the average velocity
of the wind for the year on Blue Hill is eighteen miles an hour, the
days are few when kites will not fly there.

   [Illustration: FIG. 11.--Lamson's Aero-curve Kite.]

The wire to which the kites are attached is steel music-wire, 32/1000
of an inch in diameter, weighing fifteen pounds a mile, and capable of
withstanding a pull of three hundred pounds. The wire is spliced in
lengths of more than a mile with the greatest care, special pains
being taken that no sharp bends or rust-spots occur which would cause
it to break. To lift the increasing weight of wire, kites are
attached at intervals of a few thousand feet, so that the angle may be
maintained as high as is consistent with a safe pull, and this is done
by screwing on the wire aluminium clamps, to which the kite-lines are
fastened. On account of the greater stability and strength of the new
kites, the meteorograph is suspended directly from the top kite. The
Richard meteorograph, contained in an aluminium cage of about a foot
cube, weighs less than three pounds, and it is only necessary to
screen the thermometer from the sun's rays to obtain the true
temperature of the air, since the wind insures a circulation of air
around the thermometer. Another meteorograph, constructed by Mr.
Fergusson, records the velocity of the wind in addition to the three
other elements, and it weighs no more than the French instrument.

The reeling apparatus is an example of how the same apparatus may
serve diametrically opposite purposes. In sounding the deep sea the
wire must be pulled upwards, whereas in sounding the heights of the
atmosphere the wire must be pulled in the reverse direction. Therefore
the deep-sea sounding apparatus has been altered by Mr. Fergusson to
pull obliquely downwards, the wire passing over a swivelling pulley
which follows its direction and registers on a dial the exact length
unreeled. Next the wire bears against a pulley carried by a strong
spiral spring, by which the pull upon it at all times is recorded on a
paper-covered drum turned by clockwork. The wire passes now several
times around a strain-pulley, and finally is coiled under slight
tension upon a large storage-drum. When the kites are to be pulled
down, the strain-pulley is connected with a two-horse-power
steam-engine, and the wire is drawn in at a speed of from three to six
miles an hour; but when the kites are rising the belt is removed, and
the pull of the kites unreels the wire.

  [Illustration: FIG. 12.--Meteorograph lifted by Kites
   at Blue Hill.]

The method of making a kite-flight for meteorological purposes at Blue
Hill is as follows: a kite, fastened by a long wire to the ring in the
main wire, being in the air, and the meteorograph suspended, another
kite is attached to the ring by a shorter cord (Fig. 12). They are
then allowed to rise, and to unreel the wire, until its angle with the
horizon becomes low, when, by means of the clamps described, other
kites are added, the number depending on the size of the kites and the
strength of the wind. After a pause at the highest attainable
altitude, the winch is connected with the steam-engine and the kites
are drawn down. The pauses at the highest point, and when kites are
attached or detached, are necessary to allow the recording instruments
to acquire the conditions of the surrounding air; and because at
these times the meteorograph is nearly stationary, measurements of its
angular elevation are made with a surveyor's transit, while
observations of azimuth give the direction of the wind at the
different heights. The time of making each angular measurement is
noted, so that the corresponding point on the trace of the
meteorograph may be found. From the length of the wire and its
vertical angle, the height of the meteorograph can be calculated, it
having been found that the sag of the wire, or its deviation either in
a vertical or a horizontal plane from the straight line joining the
kite and the reel, does not cause an error exceeding three per cent.
in the height so computed. When the meteorograph is hidden by clouds,
the height above the last point trigonometrically determined is
computed from the barometer record by Laplace's formula. At night
there is only the barometer from which to determine the height; for
although an attempt was made to use a lantern to sight upon, yet it
soon became invisible, or, when seen, was confounded with the stars.
Before and after the flight the meteorograph is hung upon a tripod in
the free air, in order that its thermometer and hygrometer may be
compared with the standards.

HEIGHTS ABOVE SEA-LEVEL OF KITE-FLIGHTS.

(_Blue Hill is 630 feet above the sea_)

 ----+----+----------------++-------------------------------------------
     | No.|Heights in Feet ||        Percentages of Records above
 Year| of +-------+--------++-------+--------+--------+--------+--------
     |Rec-|Mean of|Absolute|| 500 m.| 1000 m.| 1500 m.| 2000 m.| 3000 m.
     |ords|Maximum|Maximum || (1640 |  (3280 |  (4920 |  (6560 |  (9840
     |    |       |        ||  ft.) |   ft.) |   ft.) |   ft.) |   ft.)
 ----+----+-------+--------++-------+--------+--------+--------+--------
 1894|  2 | 1,860 |  2,070 ||   50  |    0   |     0  |     0  |    0
 1895| 28 | 1,673 |  2,490 ||   59  |    0   |     0  |     0  |    0
 1896| 86 | 2,772 |  9,327 ||   78  |   28   |     9  |     4  |    0
 1897| 38 | 4,557 | 11,716 ||   95  |   68   |    45  |    21  |    5
 1898| 35 | 7,350 | 12,070 ||  100  |   92   |    80  |    66  |   20
 ----+----+-------+--------++-------+--------+--------+--------+--------

Since the use of wire and more efficient kites, the heights have been
greatly increased. Thus the average height above the hill attained by
the meteorograph in thirty-five flights made during 1898 was more than
a mile and a quarter, whereas the average height of all the ascents
prior to 1897 was about a quarter of a mile (see Table). The average
height of the meteorograph above the hill, in all the flights during
August 1898, was nearly a mile and a half, and on August 26 the
meteorograph was raised 360 feet higher than ever before, its
altitude, determined trigonometrically, being 11,440 feet above Blue
Hill, or 12,070 feet above the neighbouring ocean. The meteorograph
was suspended from the topmost kite, one of the Lamson pattern, having
71 square feet of lifting surface, and this was increased to a total
of 149 square feet by four kites of the modified Hargrave type, that
were attached at intervals to the wire. The five miles of wire in the
air weighed 75 lbs., and the total weight including kites and
apparatus was 112 lbs. The meteorograph left the ground at 10:40 a.m.,
attained its greatest height at 4:15 p.m., and returned to the ground
at 8:40 p.m., a feat which it would be difficult for a man to equal on
a mountain. The cumulus clouds were traversed three-quarters of a mile
from the earth, and above them the air was found to be very dry. On
the hill the air temperature was 72∞, when it was 38∞ in the free air
11,440 feet above, and the wind velocity increased from twenty-two to
forty miles an hour. These figures give an idea of the change of
atmospheric conditions which occurs, but the conclusions deduced from
the Blue Hill kite-flights will be discussed in the next chapter.
However, the phenomena of atmospheric electricity, which have become
noticeable since the use of wire, may be described here. Generally,
whenever the kites rise above seventeen hundred feet, the wire becomes
strongly charged with electricity, and when the great heights are
reached the electricity is discharged in long and brilliant sparks at
the reel, often to the inconvenience of the attendants. Usually, the
electrical potential increases with altitude, and it is greatest
during snow-storms or when the conditions favour thunder-storms.
Notwithstanding its intensity, the quantity of electricity in the
atmosphere is probably insufficient to make its collection and
storage for practical purposes worth while.

It must not be imagined that kite-flying for meteorological purposes
is a sinecure. At Blue Hill about two hundred flights have been made
in all seasons and in all weathers, with temperatures varying from
-5∞ to +90∞, in gales, in rain, and in snow-storms, though not in
thunder-storms. Sometimes the kites are invisible from almost the time
they leave the earth until their return, but when the upper kites are
visible it is necessary to observe them with theodolites every few
minutes. Remembering that a high flight occupies ten or twelve hours,
and frequently terminates late at night, or even continues until
morning, it will be obvious that the work requires skill, energy, and
perseverance, which have been shown by my assistants at the Blue Hill
Observatory who have conducted the flights.

Occasionally, for lack of wind or from breakage of the line, the kites
fall to the ground, usually intact. If they were visible,
trigonometrical measurements on the hill enable the place of descent
to be located, and then the kites and meteorograph are sent for and
the wire is reeled up. But at night, or when clouds hide the kites,
the direction in which they fall is not known, because the azimuth of
the wire at the reel often differs from that of the kites; so last
autumn several hundred miles of road, path, wood, and swamp were
traversed before the aÎrial apparatus, which had been lost during a
flight at night, was found comparatively close at hand.

From what has been said it will be evident that a former toy has been
proved to be of the greatest importance for meteorological
investigation at the Blue Hill Observatory. On account of the success
there attained it is coming into use elsewhere for meteorological
observations. In 1898 the United States Weather Bureau created
seventeen kite stations, chiefly in the Mississippi Valley, with the
intention of obtaining data every day, at the height of a mile or
more, with which to plot a synoptic weather map similar to the map
that is now drawn from the data at the ground. From a knowledge of the
weather conditions prevailing simultaneously in the upper and lower
air, it was expected that the weather forecasts could be improved, but
unfortunately, on account of the light winds during the summer, it was
impossible to make enough simultaneous kite-flights to construct the
upper-air map, and therefore the scheme was abandoned. However, the
data obtained will no doubt furnish valuable information about the
vertical temperature gradient, etc., in various conditions of weather.
The chief meteorological bureaus of Germany and Russia are equipping
stations with kites and balloons, and M. Teisserenc de Bort, who has
provided his private observatory near Paris with kite apparatus of the
Blue Hill type, has already reached high altitudes. In Scotland too,
which was the birthplace of scientific kite-flying, experiments have
been resumed by a Scotchman and an American--a happy union of forces.

From these preparations it appears that the resolution of the
International Aeronautical Conference, recommending that all central
observatories should employ this method of investigation as being of
prime importance for the advancement of meteorological knowledge, is
being carried out, and seems likely to produce important results.

                               CHAPTER VI

   RESULTS OF THE KITE-FLIGHTS AT BLUE HILL--FUTURE WORK

Kites possess several advantages over other methods of exploring the
air up to heights of at least 12,000 feet whenever there is wind, but
their chief merit is, that with them the true conditions of the air
may be ascertained. The disadvantages of other methods of exploring
the air, as compared with kites, are these:

1. =Mountains= not only affect by contact the adjacent air, but by
deflecting the air-currents cause mixture and ascent, which give
conditions differing widely from those of the free air.

2. =Free Balloons= are more or less surrounded by heated or stagnant
air, because they drift with the wind, and on account of the
sluggishness of the thermometers, the temperatures observed at a given
height in a balloon are generally higher during the ascent, _i.e._
when passing from warm to cold air, than during the descent, when the
conditions are reversed. Again, it is not possible to study the
progressive changes in the atmospheric conditions at one place,
because observations in a drifting balloon are not comparable with
simultaneous ones made at a station on the ground below. With kites,
however, the possibility of making frequent and nearly vertical
ascents and descents permits observations to be obtained almost
simultaneously in superincumbent strata of air. The height of the kite
can usually be determined with an accuracy not attainable by the
barometer in a balloon.

3. =Captive Balloons=, although constructed so as not to be driven
down by wind, cannot rise nearly so high as kites on account of the
weight and resistance of the cable necessary to control them, and even
the German kite-balloon, on account of its large surface, would hardly
withstand the strong winds in which kites can fly.

4. =The Cost= of installing and operating either mountain stations or
balloons is much greater than for kites.

The exploration of the lower two miles of air with kites flown from
Blue Hill is no doubt the most complete ever made at one place. Nearly
two hundred records have been obtained in all kinds of weather
conditions, and the progressive attainment of greater and greater
heights is shown in the table in the preceding chapter. The records
from the flights have been discussed by Mr. Clayton; those until
February 1897, with the Blue Hill Observations, in Vol. xlii., Part I.,
of the _Annals of the Astronomical Observatory of Harvard College_,
and later records in two _Bulletins_ of the Blue Hill Observatory, in
which the changes of temperature and humidity with height, and their
relation to the positions of cyclones and anti-cyclones, are
investigated. The use of kites for weather predicting, as was said,
has been tried by the United States Weather Bureau, but it is certain
that further studies, such as have been made on Blue Hill, are
necessary before the sequence of the conditions at the earth's surface
to the phenomena observed in the upper air is definitely known, so that
the latter can be utilized in forecasting.

Some of the deductions from the observations with kites at Blue Hill
follow. Plate VIII. is a facsimile of the record of the
baro-thermo-hygrograph during two flights on October 8, 1896, when for
the first time the height of a mile and a half was attained. The
record-sheet, it may be said, is wrapped around a cylinder that turns
on its axis in twelve hours, and the curved lines in each of the three
horizontal sections divide them into quarter hours. The lower section
contains the trace of the barometer, the horizontal lines being the
heights in metres and feet that correspond to the barometric
pressure with a temperature of 32∞ Fahrenheit; in the middle section
is the trace of the hygrometer on a scale of relative humidity in
percentages, and in the upper section is the trace of the thermometer
on a scale of temperatures in Fahrenheit and Centigrade degrees. It
will be observed that the record of the barometer is reversed, _i.e._
the trace rises for falling pressure, and in the second flight when
the unexpected height of 8697 feet above Blue Hill was reached, the
limit of the altitude scale was exceeded.

   [Illustration: PLATE VIII.--METEOROGRAM FROM THE KITE-FLIGHT OF
   OCT. 8, 1896, AT BLUE HILL.]

In order to study the changes of these elements with height during the
higher flight, in Plate IX., Figs. 4 and 5, the temperature and
humidity of the automatic record are plotted as abscissÊ, with the
heights above sea-level in metres as ordinates. For those not familiar
with this unit of length, it may be said that 100 metres are about 330
feet, and that 1600 metres equal one mile approximately. When the
meteorograph was ascending, dots indicate the recorded temperatures
and humidities, which are each connected respectively by continuous
lines; when the meteorograph was descending, crosses indicate the
observations, which are connected by broken lines. Lines inclining
upwards to the left indicate decreasing temperature and humidity with
increase of height, and lines inclining to the right increasing
temperature and humidity with height. The straight dotted lines show
the adiabatic decrease of temperature for ascending dry air. The
ascent was made during the warmest part of the day, and the descent
for the most part after sunset. The two branches of the
temperature-lines typify the temperature change with height which
usually occurs in fair weather during the day and the night
respectively. The continuous line, representing the day observations,
shows a uniform fall of temperature at the adiabatic rate to the cloud
level. During the night, the lower part of the broken line bends
decidedly to the left, showing a body of relatively cold air near the
ground, caused by radiation. There is a rise of temperature with
increasing altitude above the ground up to a certain height, and
afterwards a comparatively uniform fall as high as the clouds, if they
exist; but the rate of fall with increasing altitude, shown by the
upper part of the diagram, is slower at night than during the day. It
appears that the diurnal change of temperature is very small at great
altitudes, compared with the change near the earth's surface. The
relative humidity (Fig. 5) up to 2000 metres varies inversely with the
temperature, and in the present case there was only a slight change in
the direction of the wind (Fig. 6).

   [Illustration: PLATE IX.--MEAN CHANGES WITH HEIGHT, AND CHANGES
   DURING THE KITE-FLIGHT OF OCT. 8, 1896.]

=Diurnal Changes of Temperature at Different Altitudes.=--The curve
representing the diurnal change in the air at some distance above the
ground is probably similar to one representing the change near the
ground, except that its amplitude is less. If this be true, then the
diurnal rate of fall for a given time at any two levels will be
proportional to the daily ranges of temperature at the two levels. It
is impossible in practice to keep a kite at exactly the same level for
twenty-four hours; hence the daily ranges for the different levels
must be found by comparing the rates of rise or fall of temperature
for given times with the rates found from records near the ground,
made simultaneously with those above. In Plate IX., Fig. 1, the
results for six stations, _i.e._ the kite at 1000 and 500 metres, the
Eiffel Tower in Paris (300 metres), the summit of Blue Hill, its base,
and the valley (200, 50, and 15 metres respectively), are connected,
and a smooth curve is drawn through them. The curve passes
approximately through every one of the observed and the computed
ranges, except the one at the summit of Blue Hill, which is too great.
This evidently is because insolation and radiation, acting through the
soil of the hill, heat and cool the air to a greater extent than the
free air is heated and cooled at the same altitude, and this must be
true at every mountain station. The smoothed curve passes also very
slightly to the left of the data for the Eiffel Tower, indicating
that the range there is about 1∞ greater than the true range on
account of the heating and cooling of the Tower. From this it appears
that the diurnal range of temperature diminishes rapidly with
increasing altitude in the free air, and almost disappears in the
average at a height of 1000 metres.

The records of the anemometer show that, as a rule, the wind increases
steadily as the kites rise, but the increase is greatest between
Boston and the top of Blue Hill, due probably to the retarding of the
lower winds by contact with the ground. The results are plotted in
Plate IX., Fig. 3, together with the mean wind velocity on Blue Hill
(209 metres), and the velocity on a tower in Boston (60 metres).
Single records of the kite-anemometer differ much, for sometimes the
wind velocity diminished with altitude, and at other times it
increased so rapidly that the kites were unable to rise higher. On
several occasions when the kites passed from one current into another,
having a different direction and a different temperature, the wind
suddenly increased, and was stronger between the two currents than
above or below that plane.

=Diurnal Changes of Humidity at Different Altitudes.=--It is found
that as night approaches the humidity at the altitude of 1000 metres
diminishes, while at the earth it increases. This agrees with the
evidence furnished by the cumulus clouds that form during the day
between 1000 and 2000 metres, and disappear at night, thus visibly
indicating an increase of humidity by day and a decrease by night. If
the trend of the humidity-curve at a height of 1000 metres is assumed
to be the reverse of its trend at the ground, then the results from
the kite-meteorograph show the minimum humidity to be at the coldest
and the maximum humidity at the warmest part of the day. The mean
daily ranges for different altitudes are plotted in Plate IX., Fig. 2.
The part of the curve at the left of the zero line shows the range at
different altitudes, with the minimum humidity near the warmest time
of day, while the part at the right of the zero shows the ranges at
different altitudes, with the minimum humidity at the coldest time of
day.

=Types of Change of Temperature with Altitude.=--When the records of
temperature and humidity made aloft by the kite-meteorograph and at
the stations near the ground are plotted in relation to altitude, they
are found to be easily divisible into a few types. In Plate X., Type 1
represents the decrease of temperature on most fair days from the
ground to altitudes of a mile or more, when no clouds are met. The
continuous line, plotted from the records of the ascent, represents
the day conditions, and the broken line, plotted from the records of
the descent, represents the night conditions. This curve shows that
with increasing altitude the temperature falls uniformly during the
day and approximately at the adiabatic rate represented by the dotted
lines. The fall of temperature with increasing altitude during the
night is slower than during the day, and in fact, from the earth's
surface to an altitude of a few hundred metres, there is often a rise
of temperature with height, so that the air at altitudes of from 300
to 500 metres may be considerably warmer than it is at the ground.
This was shown in the descent on October 8, 1896, and is found in Type
3.

   [Illustration: PLATE X.--CHANGES WITH HEIGHT RECORDED BY KITES
   AT BLUE HILL.]

When clouds are traversed during the flight, the temperature curve
assumes the form of Type 2. The continuous curve is plotted from the
records of an ascent; the broken curve from the records of the
descent, both occurring in the day-time. The temperature falls at the
adiabatic rate in unsaturated air till the base of the cumulus cloud
is reached. It falls at a slower rate in the cloud, the rate probably
being that computed by physicists as the adiabatic rate for air in
which condensation is taking place. Above the clouds, the fall of
temperature appears to be very slow.

Type 3 is a condition which persists throughout the day and night, and
it resembles the night form of Type 1. The temperature rises very
rapidly for a short distance above the ground and then falls, with
increase of height, somewhat slower than the adiabatic rate. The rise
of temperature near the ground with increasing height is more marked
after sunset than during the day-time.

Type 4 was illustrated by the ascent of October 8. This distribution
of temperature is caused by a warmer current overflowing colder air,
which is very commonly found at low altitudes in the atmosphere and
probably exists usually at some altitude, great or small. Recent
observations indicate that this type represents the normal condition
of the atmosphere in all sorts of weather. Frequently there are two or
more sudden rises of temperature at different heights, so that the
plotted data resemble inverted stair-steps. During the day there is a
decrease of temperature at the adiabatic rate (1∞∑8 in 100 metres)
from the ground to the height of several hundred metres, then a sudden
rise of temperature in the next one or two hundred metres, and above
this a slow fall of temperature with increasing altitude, usually much
less than the adiabatic rate. Generally, clouds are found near the
plane of meeting of the warm and cold current.

The reverse of Type 4, that is, a sudden fall of temperature, due to a
colder current overlying a warmer one, is probably impossible, because
the colder air, on account of its greater weight, would immediately
begin to sink and the warmer air would rise. This should cause a fall
of temperature at the adiabatic rate from the ground to the top of the
colder current, and is probably the origin of the "cold wave" shown in
Type 5. Both the continuous and broken curves (representing an ascent
and a descent) show a fall of temperature at the adiabatic rate of
unsaturated air, from about 500 metres to the highest point reached.
Up to 500 metres the decrease of temperature is more rapid than the
adiabatic rate, due to the rapid moving in of colder air above,
whereby air rising from the ground is cooled by contact as well as by
its expansion, and also because the air is heated more than usual by
contact with the ground, which under these conditions is abnormally
warmer. This is the special characteristic of the "cold wave" type of
curve during the day hours. The night form of Type 5, notwithstanding
the excessive radiation from the ground through the dry air, shows a
rapid decrease of temperature with increase of altitude from the
ground upward.

Type 6 shows a less common, but an interesting form, of vertical
distribution of temperature, in which the temperature is about the
same from 400 metres to 1400 metres or more. Up to 400 metres there is
a fall of temperature with increasing altitude during the day, and a
rise with increasing altitude at night. These last conditions can be
readily traced to the effects of insolation and radiation near the
ground. In the morning, if the temperature of the air be the same from
the ground up to 1000 metres or more, the heating of the ground by the
sun will cause ascending currents, until the warmest part of the day.
This air, cooling by expansion at the adiabatic rate, will rise to
about 440 metres before it assumes the mean temperature of the upper
air column. At night cooling takes place next the ground by radiation
and is gradually transferred upward a few hundred metres by
conduction, thus producing an increasing temperature with increasing
altitude, until sunrise. As a result of the conditions described, it
is evident that on certain days the diurnal range of temperature is
but little felt above 500 metres.

=Types of Change of Relative Humidity with Altitude.=--As in the
temperature types, the continuous lines represent the records of the
ascent, and the broken lines the records of the descent, generally
under changing conditions. Lines inclining upward to the left show a
decreasing humidity, and to the right an increasing humidity.

Type 1 may be called a normal type of curve when there are clouds. A
variation of this type was met with in the ascent on October 8, 1896,
and it differed from that now illustrated in indicating in its upper
part a fall of humidity rather than a rise. These two types can be
taken as the normal change of humidity with change of altitude in
cloudy or partly cloudy weather. The humidity increases steadily to
the base of the cloud, then there is complete saturation in the cloud,
and above it is a sudden fall of humidity, on entering the dry air
above the cloud, into which the ascending currents from the ground
have not penetrated.

Type 3 is a clear-weather form of curve in which the humidity
increases until a certain altitude is reached, probably at the upper
limits of the currents rising from the ground. Above this altitude the
humidity decreases rapidly.

Type 5 is also a clear-weather form and accompanies the "cold wave"
type of temperature, also numbered 5. The very dry descending air
mingles with air rendered damp by ascent, and the result is a nearly
uniform relative humidity at different altitudes, although the
absolute humidity diminishes on account of decreasing pressure and
temperature. In Type 6 both the relative and the absolute humidity
decrease rapidly, this type coinciding with the temperature, Type 6.

During the week of September 5 to 11, 1897, kite-flights were made
daily on Blue Hill. Twice the kites were maintained in the air, and
continuous records were obtained during most of twenty-four hours.
These records furnish an example of the small diurnal changes of
temperature in the free air at short distances above the ground, which
were deduced from the average changes at different hours and at
different heights. From 2 p.m. of the fifth to 2 p.m. of the sixth,
the altitude of the self-recording instruments varied between 500 and
1000 metres above sea-level, averaging about 700 metres and varying
little from this height during much of the night. The times when the
kite-meteorograph crossed the 700-metre level in ascending and
descending were determined from its barograph trace, and the
synchronous temperatures and humidities were read from the records of
its thermograph and hygrograph. The results have been plotted in Plate
XI., Figs. 1 and 2, together with the temperatures recorded
simultaneously at the summit and valley stations of the Observatory
and the humidities at the summit. Fig. 1 shows that the diurnal
variation of temperature, well marked at the lower levels, is very
slight or has entirely disappeared at 700 metres. Fig. 2 shows that
the course of the relative humidity at 700 metres is exactly opposite
in phase to that recorded at lower levels, for at 700 metres the
minimum humidity was recorded at night and the maximum during the day,
while the opposite conditions prevailed on the hill. Repeated
kite-flights indicate that these are the normal conditions at the two
levels.

In Plate XI., Fig. 3, is plotted a curve from the hourly readings of
the thermograph at the Blue Hill valley station (fifteen metres)
during the week, and also a curve connecting temperatures recorded by
the kite-meteorograph once or twice each day during the same week at a
level of 500 metres, obtained in the way described or computed from
the adiabatic change. All the night records show that it was decidedly
warmer at the height of 500 metres during the night than it was at the
ground, except during the cool wave on the seventh and eighth.
Furthermore, the curves in Fig. 3 indicate a control of the surface
temperatures during the day by those above. For instance, on the
seventh there was a distinct flattening of the day curve, evidently
because, as the temperature on the ground rose 10∞ above that at 500
metres, the air was in unstable equilibrium, and colder air descended
to take the place of the surface air so that its temperature could
rise no higher. On the tenth, the temperature at 500 metres was
considerably greater than the mean of the day at the ground, and the
air at the ground did not acquire the unstable condition in any volume
until the warmest part of the day, so that the diurnal curve at the
lower station forms a sharp peak.

   [Illustration: PLATE XI.--KITE OBSERVATIONS AT BLUE HILL,
   SEPT. 5-11, 1897.]

Since there appears to be no appreciable diurnal period in the
temperature at and above 500 metres, a better comparison of the
relative changes aloft and below during the passage of warm and cold
waves is obtained by smoothing out the diurnal period below. This has
been done in Plate XI., Fig. 4, with the data given by the kites at
500 and 1000 metres plotted in curves, which it was necessary to
complete by extrapolation. It is seen that there is a much greater
range in the temperature from the crest of a warm to the crest of a
cold wave at a height of 500 metres than at the ground. At 1000 metres
the range appears to be slightly greater than at 500 metres, and the
crests of the warm and cold waves occur successively earlier than they
do at the ground. On the approach, and until the passage of the crest
of the cold wave the air is colder aloft than at the ground, the
difference being apparently that of the adiabatic cooling of ascending
air. After the passage of the crest of the cold wave, the temperature
aloft rises much more rapidly than at the ground, and at the crest of
the warm wave the air at 500 metres is some 10∞ warmer than the mean
daily temperature at the ground. In many kite-flights the difference
was found to be even greater than this. Taking the mean temperature of
twenty-four-hours, it is seen that the average temperature at the
ground during a week or more is about the same as it is at 500
metres. Fig. 5 shows the change in the vertical distribution of
temperature during the oncoming of the warm wave on the eighth and
early morning of the ninth, as determined by four ascents, culminating
at 11 a.m., 9 p.m., 11 p.m., and 4 a.m. The lines of 59∞, 62∞, 65∞,
and 68∞ show that there was a gradual rise of temperature aloft, which
extended downwards to 200 metres, or to the top of Blue Hill. Clouds
formed at the level of lowest temperature, and these sank also until
they covered the top of the hill.

Plate XII. is a facsimile of the meteorogram during the kite-flight of
October 15, 1897, the lower part showing the trace of the barometer on
a scale of heights in metres, the middle section the trace of the
hygrometer, and the upper one the trace of the thermometer on a scale
of Centigrade degrees. The temperature followed the normal change,
which is as follows: during the day, up to a certain height, which
varies under different conditions, there is a decrease nearly at the
adiabatic rate of 1∞∑8 F. per hundred metres. Above that height the
air suddenly becomes warmer, and then cools with ascent at a rate
somewhat less than the adiabatic rate. During the night there is a
marked inversion of temperature between the ground and 200 or 300
metres.

   [Illustration: PLATE XII.--AUTOMATIC RECORDS DURING A HIGH
   KITE-FLIGHT AT BLUE HILL.]

Higher than this, the temperature decreases at a fairly uniform rate,
but more slowly than the adiabatic rate. Although no clouds were
visible, yet the relative humidity increased greatly, both during the
ascent and descent, near 1500 and 2700 metres, these being about the
heights at which cumulus and alto-cumulus clouds usually form.

During September 1898 four kite-flights were made on four successive
days when an anti-cyclone and a cyclone passed nearly over Blue Hill.
This is a rare occurrence, and the mechanism of these phenomena was
accordingly studied by Mr. Clayton, some of whose deductions will now
be given, illustrated by Plate XIII. Figs. 1 and 2 give the
temperature plotted according to height on September 21 in the
anti-cyclone, and on September 22, when the barometric pressure was
falling, the full lines, as in previous diagrams, indicating
observations during the ascents, and the broken lines observations
during the descents. It is seen that from the ground the lines all
incline upward to the left, indicating a fall of temperature, to a
certain height when the lines bend to the right sharply, showing a
sudden rise of temperature. Above this, the temperature again falls,
but more slowly than at lower levels. The general prevalence of this
phenomenon was noted by Welsh in his balloon ascents in England in
1854, and the high kite-flights at Blue Hill show it to be very
frequent below 2000 metres. The plane of increased temperature usually
determines the height of the tops of cumulus and strato-cumulus
clouds. Above 2000 metres other sudden rises of temperature are found
during the highest kite-flights.

   [Illustration: PLATE XIII.--RESULTS OF KITE-FLIGHTS AT BLUE
   HILL DURING AN ANTI-CYCLONE AND A CYCLONE.]

Figs. 3 to 6 show the changes in the various elements during the four
days at some of the following levels, viz. near sea-level, 200, 1000,
2000, and 3000 metres. Fig. 3 shows the changes in the barometer at
the four levels, from which it is evident that the fall of pressure
was greatest near sea-level.

Fig. 4 shows temperature changes at the different levels, and
indicates that the changes were of the same nature up to 3000 metres.
The greatest non-diurnal range of temperature is seen to be at 1000
metres, and it diminishes both at higher and at lower levels.

Fig. 5 shows changes in relative humidity at 200, 1000, and 2000
metres. The curves show that the greatest range of humidity was at
2000 metres. There the relative humidity rose from almost zero, in the
anti-cyclone on the twenty-first, to saturation at the same level in
the cyclone. At 200 metres the change is similar to that at 2000, but
is less in amount. At 1000 metres the relative humidity fell until the
twenty-second, but then rose rapidly, showing the very dry air at
2000 metres on the twenty-first had descended as low as 1000 metres on
the twenty-second.

Fig. 6 gives the change in wind velocity at the different levels.
There was an increase of wind at all the levels from the time of the
passage of the anti-cyclone to the passage of the cyclone. The minimum
of wind at 200 metres was in the anti-cyclone, with a secondary
minimum during the passage of the centre of the cyclone.

Figs. 7 to 10 show the changes in height from day to day of the equal
conditions at the different levels. Fig. 7 shows the change in level
of the isobars, which, although very small, is largest at the lower
levels. The light broken lines in Fig. 7 and subsequent figures
indicate the axes of the anti-cyclone and cyclone. That the axis of
the cyclone was inclined backward, and that the high pressure occurred
later at high than at low levels, was confirmed by the wind
observations on the twenty-first.

Fig. 8 shows the heights at which the same temperatures were found on
successive days. Since the isotherms rose until the twenty-third, the
temperature of the air up to 3000 metres was higher on the day of the
cyclone than on the day of the anti-cyclone. Previous high flights
indicate that this is the normal condition in the moving cyclones and
anti-cyclones of the eastern United States. As the light broken lines
represent the axes of the anti-cyclone and cyclone up to 3000 metres,
it is seen that at this level the temperature at the place of maximum
pressure is probably higher than at the place of minimum pressure,
although this is not true for a vertical column of air above the
earth.

Fig. 9 gives the positions of equal humidities on successive days,
saturated and cloudy areas being indicated by crossed shading, and
less humidity by single ruling. From the laws of thermo-dynamics the
unshaded curves should represent descending currents, and the shaded
portions ascending ones. In the first case, increased warmth and a
lower relative humidity are produced in the descent to a lower
altitude; in the last case, cooling, increasing relative humidity, and
condensation are produced by expansion in the ascent to a higher
altitude. Consequently, two regions of descending air are indicated,
one in the centre of the anti-cyclone, the other in the centre of the
cyclone.

Fig. 10 shows the change in height of the lines of equal wind
velocity. With ascending currents and precipitation, high wind
velocities were found at low levels, because of increased barometric
gradient, while with the descending currents in the anti-cyclone and
centre of the cyclone, the high velocities were found only at great
altitudes. The study of these data indicate that the cyclonic and
anti-cyclonic circulations observed in this latitude do not embrace
any air-movements at greater altitudes than 2000 metres, except in
front of the cyclone, when the air appears to be carried upward to a
great height. Above 2000 metres there are probably other weak cyclones
and anti-cyclones, or secondary ones, with their centres at different
places from those at the earth's surface and producing a different
circulation of wind. The observations of the cirrus clouds at Blue
Hill indicate that at their level exists a cyclonic circulation above
the anti-cyclone apparent at the earth's surface. The shallowness of
our anti-cyclones would be inferred from the great differences in
speed of the general atmospheric drift, for since the velocity of the
general drift from the west is more than thirty times greater at
10,000 metres than it is at 200 metres, a circulation of great depth
could not endure long. Cyclones and anti-cyclones appear to be but
secondary phenomena in the great waves of warm and cold air which
sweep across the United States from periodic causes.

The origin of cyclones and anti-cyclones is perhaps the most important
problem remaining for meteorological study. The theory that they are
produced by differences of temperature in adjacent masses of air, or,
as it is called, the convectional theory of the American
meteorologists, Espy and Ferrel, is opposed by the observations on
mountains in Europe which were collected by Dr. Hann of Vienna. If the
question can be solved by the use of kites, as seems to be
foreshadowed by the results just stated, another foundation-stone will
be laid in the science of meteorology and the status of the kite
established as an instrument of research. The kite fails when there is
little or no wind at the ground, but it seems possible in such cases
to lift the kite into the upper air, where there usually is wind, by
attaching it to a small balloon that, after the kite can support
itself, shall be detached automatically. While the height to which
kites can rise is limited, and the limit is probably being approached,
judging from the less gain of altitude in recent flights, yet it seems
reasonable to expect that, with favourable conditions, a height of at
least three miles will be reached.

Besides lifting the meteorological instruments described, kites can
carry apparatus for other investigations in the free air, such as the
measurement of atmospheric electricity, and the collection of samples
of air, to be examined for cosmic dust and bacteria. Cameras have been
lifted by kites, as already said, and for the purpose of photographing
the upper surfaces of clouds there is being constructed for the Blue
Hill Observatory a very light automatic camera, similar in principle
to M. Cailletet's apparatus for photographing the ground from a
balloon.

The use of the kite as an aeroplane can only be alluded to in this
book, and it may be sufficient to say that if a motor attached to a
kite can, by wings or screws, propel it against the wind, the
sustaining string is unnecessary, and we shall have the flying machine
which Professor Langley tells us will soon be realized. The surface of
our globe has been tolerably well explored; the exploration of the
atmosphere by balloons and kites will continue to make great progress
during the last year of the century, and at the end of the twentieth
century we may confidently expect that as the seas now are a medium
for transportation, so the ocean of air will have been brought
likewise into man's domain.

INDEX

 A

 Abercromby (R.), classification of clouds, 42
 Academy of Sciences, French, 18, 72-3
 Academy of Sciences, Russian, balloon ascent, 72
 Accademia del Cimento, 14
 Actinometer, ViollÈ's, 115
 Adiabatic rate of change of temperature, 29
 Aeronautical Conference at Chicago, 125
 ---- Conference at Strassburg, 97, 110
 ---- Committee; International, 108
 _AÈrophile_ balloons, 102, 104
 Aerostatic Commission, French, 111
 Air, collection and analysis of, 70, 73, 75, 82, 112
 ---- weight of, 16
 Aitken (J.), dust particles, 39
 Alhazen (B. A.), height of atmosphere, 12
 Altitudes, comparative, 20
 AndrÈe (S. A.), balloon voyage to North Pole, 90
 Anti-cyclones, 59, 170
 Aratus, _Diosemeia_, 11
 Archibald (D.), kites for meteorological observations, 122
 Archytas, supposed inventor of kite, 117
 Aristotle, 10, 11, 15
 Assmann (R.), 86, 94, 108
 Atmosphere, composition of, 24
 ---- energy of upper portion, 58
 ---- extent of, 28
 ---- methods of exploring same, 35 _et seq._, 145
 ---- moisture of, 34
 ---- origin of, 23
 ---- phenomena showing height of, 26
 ---- Pliny on, 9
 ---- temperature of, 28
 Atmospheric circulation in cyclones and anti-cyclones, 60
 ---- electricity, 70, 76, 121, 141

 B

 Balloon ascents, international, 108 _et seq._
 ---- crossing the Atlantic by, 92
 ---- invention of hot-air, 19
 ---- kite, 94
 Balloons, 19, 20, 21, 37, 68, _et seq._
 ---- captive, 76, 93, 146
 ---- changes of temperature observed in, 71, 73, 75, 77, 84, 88, 90
 ---- changing the direction of, 93
 _Ballons-sondes_, 98 _et seq._
 Barometer, 15, 16, 85, 113
 Baro-thermograph of Richard, 102
 Barral, balloon ascent, 73, 84
 Batavia, Java, international cloud measurements, 65
 Batut (A.), photography from kites, 123
 Berson (A.), balloon ascents, 81, 87 _et seq._
 Bert (P.), respiration of oxygen, 82
 BesanÁon (G.), 98, 99, 101, 104
 Bezold (W. von), wave-cloud, 40
 Biot (J. B.), balloon ascent, 73
 Birt (W. R.), kite at Kew Observatory, 122
 Bixio, balloon ascent, 73, 84
 Blanc, Mont, 20, 21
 Blanchard, balloon ascent with Jeffries, 70, 71
 Blue Hill Observatory, 47, 51, 53, 64, 108, 126 _et seq._
 Bonaparte (Prince Roland), patron of aeronautics, 112
 Bonpland (A.), ascent in Andes, 20
 Bonvallet (L.), exploring balloons, 99
 Bouguer (P.), height of freezing-point, 18
 Boyle (R.), 16

 C

 Cailletet (L.), 112, 113
 Cambridge, Mass., clouds measured at, 53
 Castelli (B.), invented rain-gauge, 13
 Cavallo (T.), showed lightness of hydrogen, 69
 Celsius (A.), thermometer, 14
 Charles (J. A. C.), ascent in hydrogen balloon, 19, 68
 Cimento, Accademia del, 14
 _Cirrus_ balloon, 106
 Clayton (H. H.), 45, 47, 53, 62, 133, 147, 167
 Cloud, amount of, 47 _et seq._
 ---- atlases, 42, 43, 44
 ---- Committee, International, 44, 65
 ---- -year, international, 65
 Clouds, classification of, 41 _et seq._
 ---- definitions of, 45
 ---- formation of, 39
 ---- observations of direction and relative velocity, 51, 65
 ---- measurements of height and velocity, 52, 53 _et seq._, 65, 121
 ---- on Jupiter, 51
 ---- relation to forecasting, 63-4
 Cotte (L.), on clouds, 38
 Coxwell (H.), aeronaut for Glaisher, 75 _et seq._
 CrocÈ-Spinelli (J.), ascent in _Zenith_, 82
 Cyclones, 59, 170

 D

 Dalton (J.), water-vapour in the air, 38
 Daniell (J. F.), mountains a registering thermometer, 13
 Davis (W. M.), cloud measurements, 53
 Deluc (J. A.), theory of clouds, 42
 De Saussure (H. B.), 20, 42, 73
 ---- (H. B.), ascent of Mont Blanc, 20
 Deutsche-Seewarte, Hamburg, 43
 Donaldson (W. H.), proposed crossing Atlantic in a balloon, 92

 E

 Eddy (W. A.), 123, 124 _et seq._
 Eiffel Tower, Paris, 23, 152
 Ekholm (N.), 53, 92
 Electricity, atmospheric, 70, 76, 121, 141
 Espy (J. P.), kites to verify calculated height of clouds, 121
 Etna, ascended by ancients, 12
 Euler (L.), theory of kites, 118
 Exploring the atmosphere, methods of, 35 _et seq._, 145-6

 F

 Fahrenheit (D. G.), thermometer, 14
 Ferdinand II. (Grand Duke), distributed meteorological instruments, 17
 Fergusson (S. P.), 35, 53, 126, 128, 131, 136
 Ferrel (W.), theory of cyclones, 173
 Flammarion (C.), balloon ascents, 81
 Flying machines, future, 59, 174
 Fonvielle (W. de), 81, 108
 Fˆrster (W.), hypothesis of _Himmelsluft_, 28
 Forecasting by kites, 143, 147
 Franklin (B.), experiment with kites, 121, 123
 Franklin Kite Club, 121

 G

 Galileo (G.), 14, 15
 Gay-Lussac (J. L.), balloon ascent, 20, 73
 German Emperor (William II.), patron of aeronautics, 86
 ---- Society for Promotion of AÎrial Navigation, 86, 93, 106
 Glaisher (J.), balloon ascents, 75 _et seq._, 93
 Green (C.), aeronaut for Welsh, 74
 Grimaldi (F. M.), first measured clouds trigonometrically, 52
 Guericke (O. von), experiment of Magdeburg hemispheres, 16

 H

 Hagstrˆm (K.), measured clouds, 53
 Halley (E.), measured heights by barometer, 17
 Hann (J.), 36, 173
 Han Sin, employed kites in warfare, 117
 Hargrave (L.), invented cellular kite, 129
 Harrington (M. W.), advocated exploring air with kites, 125
 Harvard College Observatory, 35, 48
 Hazen (H. A.), highest balloon ascent in America, 85
 Height of balloon, Cailletet's apparatus for obtaining, 113
 Heights of kite-flights at Blue Hill, 21, 140
 ---- how measured by barometer, 17, 101
 Heim (A.), voyage across the Alps, 92
 Hellmann (G.), historical researches, 12
 Helmholtz (H. von), wave-cloud, 40
 Hergesell (H.), President of Aeronautical Committee, 108
 Hermite (G.), 98, 99, 101
 Hildebrandsson (H. H.), 42, 65
 Hodgkins' Fund of Smithsonian Institution, grant from, 131
 Howard (L.), cloud nomenclature, 41
 Humboldt (A. von), 18, 20, 35
 Humidity, changes with altitudes, 34, 71, 73, 77, 151, 169, 171
 ---- diurnal changes at different altitudes, 153
 ---- types of change with altitudes, 159 _et seq._
 Hutton (J.), cause of precipitation, 38
 Hygrometer, invention of, 13

 J

 Jeffries (J.), first scientific balloon ascent, 69
 ---- first to cross the English Channel, 71
 Jourdanet (D.), hypothesis of descent of man, 24
 Jovis, balloon ascent, 84
 Jupiter, analogy between cloudiness on earth and on, 51

 K

 Kepler (J.), height of atmosphere, 12
 Kew Observatory, 53, 74, 122
 Kˆppen (W.), cloud atlas, 43
 Kirwan (R.), temperature at different latitudes, 18
 Kite, antiquity of the, 117
 ---- Eddy or Malay, 124, 129
 ---- flights at Blue Hill, 21, 137, 140, 142
 ---- Hargrave, 129, 132
 ---- Lamson's "aero-curve," 134
 ---- photography, 123, 126, 173
 ---- theory of, 118, 124, 130
 Kites, first scientific use of, 120
 ---- first self-recording instruments raised by, 126
 ---- oriental tailless, 118
 ---- scientific uses of, 173
 Kite-winch at Blue Hill, steam, 131, 136
 Krakatoa, eruption of volcano, 27

 L

 Lamarck (J. B.), first to classify cloud forms, 41
 Lamson (C. H.), aero-curve kite, 134
 Langley (S. P.), 28, 174
 Laplace (P. S. de), 17, 23, 101
 Lavoisier (A. L.), 19, 68
 Ley (W. C.), classification of clouds, 42
 Lunardi (V.), balloon ascent, 69

 M

 M'Adie (A.), 53, 123
 Magnetism, variation with height, 73, 77
 Mallet (M.), balloon ascent, 84
 Manila, Philippine Islands, cloud measurements at, 65-6
 Mariotte (E.), law of gases, 16
 Melvill (T.), aided in first scientific use of kites, 120
 Merle (W.), oldest weather chronicles, 12
 Meteorograph for kites, 128, 136
 Meteorological conferences, international, 42, 44, 108
 Meteorology, first treated by Aristotle, 10
 ---- origin of, 10
 Misti, El, highest station, 35
 Montgolfier brothers, invented hot-air balloon, 19
 M¸ntz (A.), analysis of air, 112-13

 N

 Nares (Sir G.), storm-kite, 122
 Nebular hypothesis of Laplace, 23
 Neumayer (G.), cloud-atlas, 42
 Newton (I.), improved kites, 118

 O

 Olympus, mountain ascended by ancients, 11
 Oxford, oldest weather chronicles, 12

 P

 Parseval (A. von), kite-balloon, 94
 Pascal (B.), experiment with barometer, 15, 16
 Perier (F.), _idem_, 15
 Photography from balloons, 81, 113
 ---- from kites, 123, 126, 173
 Pickering (E. C.), pole-star recorder, 48
 Pike's Peak, meteorological station, 35
 Pil‚tre de Rozier (J. F.), first to ascend in balloon, 19
 Pliny, the atmosphere, 9
 Pocock (G.), great kite, 122
 PoÎy (A.), classification of clouds, 42
 Priestley (J.), oxygen in the air, 18

 R

 Rain-gauge, invention of, 13
 RÈaumur (R. A. F. de), thermometer, 14
 Rey (J.), first thermometer filled with liquid, 14
 Riccioli (G. B.), first to measure clouds trigonometrically, 52
 Richard (AbbÈ), clouds, 38
 ---- (J.), self-recording instruments, 101, 125, 128, 136
 Robertson (E. G.), 72, 73
 ---- balloon ascent, 72
 Rotch (A. L.), balloon ascents, 85, 86

 S

 Sacharoff, balloon ascent, 72
 Siegsfeld (H. B. von), kite-balloon, 94
 Sigsbee (C. D.), 44, 131
 Sivel (T.), ascent in _Zenith_, 82
 Spelterini (E.), balloon voyage with Heim, 92
 Spencer (S.), aeronaut for Berson, 89
 Sweetland (A. E.), prognostics from clouds, 63
 Symons (G. J.), meteorologist and bibliophile, 12

 T

 Teisserenc de Bort (L.), 51, 116, 144
 Temperature, change with height, 18, 29, 71, 73, 75, 77, 89, 90, 104,
   107, 120, 121, 126, 141, 151, 167
 ---- diurnal changes at different altitudes, 152, 161-2
 ---- types of change with altitude, 154 _et seq._
 Theodolite, registering, 106
 Theophrastus, weather prognostics, 11
 Thermometer, aspiration, 74, 86
 ---- metallic, 116
 ---- sling, 85
 Thermometers, early, 14
 Tissandier (A.), sketches of optical phenomena, 81
 ---- (G.), ascent in _Zenith_, 81, 82
 Toronto, Canada, international cloud measurements, 65
 Torricelli (E.), invented barometer, 15
 Tycho Brahe, height of atmosphere, 12

 U

 United States hydrographic office, 44
 ---- weather bureau, 44, 65, 143
 Upsala, Sweden, clouds measured at, 53, 58, 66

 V

 Violle (J.), 112, 115

 W

 Washington (Mount), meteorological station, 35
 Weather chronicles, first, 12
 ---- forecasting by means of clouds, 63
 ---- prognostics of Aristotle and Theophrastus, 11
 ---- vane, oldest meteorological instrument, 13
 Weber (L.), measured electric potential with kites, 124
 Welsh (J.), balloon ascents, 74
 Wenz (E.), photography from kites, 123
 Wilson (A.), first scientific use of kites, 120
 Wind at different heights, 57 _et seq._, 98, 153, 170, 171
 Wire for kite-lines, 123, 131, 134
 Wise (J.), 84, 92
 Woglom (J. T.), photography from kites, 123

 Y

 Young (C. A.), limit of atmosphere, 28

 Z

 _Zenith_, catastrophe of balloon, 82
 Zero, absolute, 33

_Richard Clay & Sons, Limited, London & Bungay._

Transcriber's Notes

The text presented here is essentially that in the original printed
volume. The list of corrections (CORRIGENDA) which accompanied the
original has been applied. One additional  typo (see below) has also
been made. There may have been some minor corrections (missing period,
commas, etc. added) which are not detailed here. In the original
publication, several figures and plates were placed in the middle of
paragrahs. Here most were moved between paragraphs. The List of
Illustrations and any other page references still indicate the page
number of the original location. On page 88, the "oe" ligature was
replaced with the individual letters.

Typographical Correction

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