antiquity. As a consequence, the loose soil, which formerly was
planted, has been washed away and the dams and retaining walls, built
to prevent a too rapid drain of the water, have disappeared. Thus
the country has grown increasingly arid. Large cities, as Palmyra,
existed in desert regions where lack of water now prevents habitation.
But water was brought to the metropolitan cities of old through long
magnificent aqueducts, the ruins of which partly remain today. We
have all reasons to believe that the marked decrease in cultivation
and population, laid to changes in conditions of humidity, depended
altogether on man’s interference with nature. On certain rocks in
Morocco, carvings have been found portraying in a simple way large
mammals such as elephants, rhinoceroses, and giraffes, which do not
exist in these regions now because of lack of nourishment. But these
rough works of art, resembling those of the bushmen of today, date
from prehistoric time, the so-called paleolithic era, when the climate
admittedly was more humid in these regions than it is now.

Similar conditions obtain, according to Hedin, in Central Asia and in
Persia. The climate there has without doubt been more humid but not in
historic times. The march of Alexander toward India took place under
as adverse conditions as now are found in these regions (Baluchistan).
Their cities, now in ruins, received their water supply through
conduits from rivers some of which were then adjacent to the cities,
although they later have changed their course as pointed out by Leo
Berg.

In Western and Central Europe numerous marshes and morasses have
indeed been drained and rendered available for cultivation, but this
does not prove that the climate is become more dry. On the contrary
all observations, for instance those made by Tycho Brahe on the
island Hven, indicate that the difference between summer and winter
temperature has decreased during historic time; that is the climate is
become less continental, or more humid, than formerly. Furthermore,
many circumstances, such as the occurrence of hazel and of water
chestnut in far more northerly latitudes and the higher altitude of
the timber line in earlier time, prove that the summer in prehistoric
ages was warmer than now. Simultaneously it was dryer. A study of the
lacustrine pile dwellings in Switzerland shows that the levels of the
lakes then were not higher than now but very nearly the same, which
demonstrates that the precipitation has not changed perceptibly in
Switzerland since these buildings were made; the period in question
occurred, we believe, about 7000 years ago.

While great climatic changes have taken place since man’s first
appearance on earth, presumably before the end of the ice period,
historic time is too short to record any distinct modifications.
Local ones may be in evidence such as West Europe’s transition to a
less continental climate. A variation of this nature has been found
not longer back than since thermometrical observations commenced.
Thus the winters in Berlin during the period 1746–1847 were colder
and the summers warmer than during 1848–1907. The difference for
January amounted to -1.5° C. (-2.7° F.) and for May to +0.6° C.
(+1.08° F.). The tabulation below, quoted from Ekholm, shows the mean
temperature in Stockholm, Lund, London, and Paris, during winter
(December-February), spring (March-May), summer (June-August), and
autumn (September-November) and for the following periods:

  -------+-------------------+------------------
         |    _Stockholm_    |      _Lund_
         |-------------------+------------------
         |  _1799– |  _1849– |  _1753– | _1799–
         |  1848_  |  1898_  |  1798_  | 1898_
  -------+---------+---------+---------+--------
  Winter |+25.5° F.|+26.8° F.|+30.2° F.|30.9° F.
         | -3.6° C.| -2.9° C.| -1.0° C.|-0.6° C.
  -------+---------+---------+---------+--------
  Spring | 37.9° F.| 37.9° F.| 41.2° F.|41.5° F.
         |  3.3° C.|  3.3° C.|  5.1° C.| 5.3° C.
  -------+---------+---------+---------+--------
  Summer |   60° F.|   60° F.|   61° F.|60.2° F.
         | 15.6° C.| 15.6° C.| 16.1° C.|15.7° C.
  -------+---------+---------+---------+--------
  Autumn | 43.9° F.| 43.5° F.| 45.9° F.|45.9° F.
         |  6.6° C.|  6.4° C.|  7.7° C.| 7.7° C.
  -------+---------+---------+---------+--------
  Year   | 41.9° F.| 42.1° F.| 44.6° F.|44.6° F.
         |  5.5° C.|  5.6° C.|  7.0° C.| 7.0° C.
  -------+---------+---------+---------+--------

  -------+-----------------+-----------------
         |    _London_     |     _Paris_
         |-----------------+-----------------
         | _1799– | _1849– | _1806– | _1849-
         | 1848_  | 1898_  | 1848_  | 1898_
  -------+--------+--------+--------+--------
  Winter |38.5° F.|39.2° F.|37.9° F.|37.9° F.
         | 3.6° C.| 4.0° C.| 3.3° C.| 3.3° C.
  -------+--------+--------+--------+--------
  Spring |48.2° F.|  48° F.|50.5° F.|50.3° F.
         | 9.0° C.| 8.9° C.|10.3° C.|10.2° C.
  -------+--------+--------+--------+--------
  Summer |61.9° F.|62.2° F.|64.6° F.|64.8° F.
         |16.6° C.|16.8° C.|18.1° C.|18.2° C.
  -------+--------+--------+--------+--------
  Autumn |50.7° F.|50.5° F.|54.2° F.|51.8° F.
         |10.4° C.|10.3° C.|11.3° C.|11.0° C.
  -------+--------+--------+--------+--------
  Year   | 9.8° F.|  50° F.|51.3° F.|51.3° F.
         | 9.9° C.|10.0° C.|10.7° C.|10.7° C.
  -------+--------+--------+--------+--------

The difference is not great. For Stockholm the winter has grown warmer,
the autumn colder, for London the winter warmer and so slightly also
the summer, but spring and autumn a trifle colder, and for Paris the
summer a little warmer while the autumn is considerably colder. Lund
shows the least variation. The winter has grown 0.4° C. (0.72° F.)
warmer and the summer colder by the same amount. The annual mean
remains nearly constant, only slightly increased, but the climate is
become more marine. (This is hardly apparent from the figures cited as
far as Paris is concerned.)

From Tycho Brahe’s observations of the number of days when snow or rain
fell in the place where his observatory was situated on the island
Hven in Öresund not far from Copenhagen, Ekholm has calculated that
the temperature there during the time 1582–1597 was 1.4° C. (2.5° F.)
lower in February and 1° C. (1.8° F.) lower in March than in later
years (1881–1896). On the other hand the first autumn frost occurred at
the same time as now and the same was the case with the last frost in
spring, so that the temperatures on these dates in autumn and spring
were nearly identical at the end of the sixteenth century and now.
Ekholm drew the conclusion that the climate is become more marine.

Hildebrandson makes the objection that Tycho Brahe’s observations
were confined to an abnormally cold period judging by the tables
Speerschneider has prepared showing the ice formation in Danish
navigable waters. Nine out of the sixteen years in which Tycho Brahe
gathered his data were notable for abnormally cold winters, while only
nineteen out of the hundred years composing the sixteenth century were
characterized by equally severe winters.

Thus, the conclusion is not warranted that the winters of the sixteenth
century as a whole averaged colder than those of the nineteenth. Later
investigations (in 1917) in regard to the dates when the ice would
break up in Lake Mälar at Vesterås, in Neva River at Petrograd, and
in Dwina River at Riga, have led Ekholm to believe that he has found
a periodicity in winter temperatures of not less than 212 years, a
conclusion which would agree with Speerschneider’s statistics. If
so, we are at present living in a period remarkable for its mild
winters while a series of extremely severe winter seasons occurred at
the time of Tycho Brahe. This law would also have a bearing on the
preceding table of temperatures in Stockholm, Lund, London, and Paris
as a succession of severe winters came around in the beginning of the
nineteenth century while the reverse is true towards its end. On the
whole climatic variations during historic time have been insignificant,
if present at all, provided we extend our comparisons over two or more
centuries. Such is also the opinion of Hildebrandson.

The idea of a slow deterioration of the climate due to increasing
desiccation is of old lineage and is most likely related to the
venerable conception of a bygone golden age. Aristotle even at that
early date believed that a gradual arefaction of the Earth took
place. In recent times this faith has been particularly fostered by
Huntington in a number of treatises where he endeavours to prove that
Asia, represented for instance by Palestine, Syria, and Persia, and
further Africa and North America are subject to a rapid exsiccation
clearly traceable through historic time. The contrary, however, is true
about Western Europe. It was often said that Southern Russia in recent
times suffered a slow arefaction manifesting itself in the formation of
steppes. This led to careful investigations showing the assertion to
be erroneous and culminating in the work of Leo Berg. Rather, a slight
shifting in the opposite direction is detectable as the forest region
has expanded at the expense of the steppes in conformity with the
development toward the end of prehistoric time. The renowned American
astronomer, the late Dr. Lowell, embraced the idea of increasing
aridity which he observed himself in Arizona where his observatory
is located. The drying out of Arizona undoubtedly took place in
long bygone prehistoric time. The disappearance of a high culture in
Syria and Mesopotamia was the result of hostile devastation of their
waterworks; a compensation is now offered in the reclamation of the
deserts along the Nile, in California and Arizona, and in numerous
other places.

CHAPTER IV

ATMOSPHERE AND PHYSICS OF THE STELLAR BODIES

In a certain sense, we are justified in speaking of an atmosphere of
suns and stars. These bodies consist mainly of a comparatively dense
mass surrounded by a layer of very attenuated gas. The density of our
Sun is about 1.4 times that of water. In other stars it is considerably
lower, in some cases only a few hundredths of the density of water.
This applies particularly to those stars of variable magnitude, the
Cepheid type, named after their longest and best known representative,
the mysterious star Delta in the constellation Cepheus, and in general
to young stars. In any case the stars are gaseous throughout on account
of their high temperature. An exception must be made for the clouds
of matter precipitated by easily condensed vapours, such as gaseous
carbon, which clouds float in the outer strata, and are responsible for
the bright astral light.

The stars just mentioned belong to the comparatively young stellar
bodies, while the Sun in common with other yellow stars is considerably
older. Correlated with their age is undoubtedly the greater mean
density of the yellow stars. Around many of the young stars, for
instance around the brilliant Altair, the principal member of the
constellation Aquila, a gas shell of great expansion has been observed,
usually consisting of hydrogen but frequently also of helium. These
extensive gas appendages may be considered as a kind of atmosphere
surrounding the stars in question. Their density is no doubt
exceedingly small. Our own central orb, the Sun, is also endowed with
rarefied gases outside of the luminous clouds. By their absorption of
light they cause the dark lines in the Sun’s spectrum, after their
discoverer named Fraunhofer lines. The greatest height from the
surface of the Sun is reached by hydrogen, mixed with a small quantity
of helium and with a gas, unknown on Earth, which we call coronium
because it has been observed in the Sun’s corona. These gases may be
looked upon as the atmosphere of the Sun.

Similar conditions obtain no doubt on the major planets, which possess
a density not essentially different from that of the Sun. They have,
moreover, practically the same period of revolution around their own
axis, Jupiter 9.9 hours, Saturn 10.3, and Uranus (probably) 10.8 hours.
Judging by their density, they are, in all probability, like the Sun,
gaseous throughout except for the heavy cloud formations which appear
to constitute the outer limit of these stellar bodies. Their interior,
like that of the Sun, may contain comparatively sluggish gas-masses,
as certain peculiar patches appear on their exterior, similar to the
sunspots and persisting for long intervals, sometimes over a year. The
best known example of the kind is the so-called red spot on Jupiter,
which has remained since 1878, although it is not so pronounced now as
during its early days (see Fig. 11). Characteristic for these planets
are certain bands of a marked delineation and running parallel with the
equator (see Figs. 11 and 12). They are caused by the rapid peripheral
motion of these planets, with Jupiter 28 and with Saturn 24 times that
of the Earth.

  Fig. 11. The planet Jupiter in 1909 in Mercator’s projection by
           F. le Coultre of Geneva. The “red spot” over which the
           clouds are dispersed, is found at 355° Long. and 20° S.
           Lat. in the outcurving of the dark band. South direction
           is upward, as on all pictures obtained with astronomical
           tubes.
]

Which gases should we expect to find in the atmosphere of these
planets? According to the Kant-Laplace hypothesis, a theory generally
credited with a sound kernel, the planets were segregated from the
Sun’s substance at a time when the latter was expanded so as to include
the orbits of these planets and beyond. Naturally, therefore, their
atmospheres would originally be composed of the very gases that formed
the outmost part of the Sun’s atmosphere, notably hydrogen. Slipher,
who has photographed the spectra here reproduced of the outer planets,
believes that certain strong absorption bands in the spectra of
Neptune and Uranus correspond to the distinctive F and C lines of
hydrogen, using Fraunhofer’s denotation (see Fig. 13). But because
the bands in question, as shown on the figure, are very broad it is
difficult to identify them with certainty. Also other gases of unknown
origin enter into the vapour envelopes outside of the clouds and cause,
as apparent from their spectra, a strong absorption of the sunlight
reflected from the clouds below. The absorption increases with the
planet’s distance from the Sun; thus, it is most pronounced on Neptune
and least on Jupiter.

[Illustration: Fig. 12. The appearance of Saturn September 30, 1909,
according to F. le Coultre of Geneva.]

At any rate, the gas appendages of the heavenly bodies just considered
differ in one essential respect from the atmospheres of the inner
planets: Mars, Earth, Venus, and Mercury. On the Sun and on the outer
planets the atmosphere gradually merges into the interior gas-masses so
that no distinct boundary can be found between the rarer and the denser
layers. Widely different conditions obtain on the Earth. Here the range
of air is sharply defined below by the Earth’s solid crust or by the
oceans. In such case alone may we speak of an atmosphere proper, of
the kind that enters into our commonplace conceptions. Similar are the
conditions on all stellar bodies with a solid or a liquid surface.

  Fig. 13. Spectra of the major planets, compared with that of
           the moon. The latter corresponds to the spectrum
           of sunlight reflected from a planet which lacks a
           light-absorbing atmosphere. Photos by V. M. Slipher of the
           Lowell Observatory.
]

But it is not certain in every case that all such planets possess an
atmosphere. Observations of the Moon when passing some star show that
the air envelope, if present, is unable to deflect the light beam from
the star, or in other words it has no perceptible power of refraction.
From this we also infer that its density is very small, corresponding
at most to one or two mm. (.04 to .08 inches) barometric pressure. But
we have good reasons to believe that the Moon has been detached from
the Earth, carrying away parts of its lightest substance, which theory
is supported by the fact that the Moon’s mean density (3.3) is only
six-tenths of the Earth’s (which again is 5.53 times that of water),
and we might therefore have expected that the Moon in parting
should have shared in the very lightest constituents of the Earth,
namely its air-covering. Such was unquestionably the actual procedure,
but in the course of time the Moon has lost its originally no doubt
considerable atmosphere. The reason is that the molecules in a gas are
in a continuous rapid motion, which is the swifter the lighter the gas
and the higher its temperature. In hydrogen, the lightest gas known,
the velocity amounts to 1.84 km. (1.15 miles) per second at 0° C.
(32° F.). The parts of the Moon exposed to the strongest sunlight are
heated to about 150° C. (300° F.). At that temperature, the average
velocity of hydrogen molecules is 2.29 km. (1.43 miles) per second.
But a body departing from the surface of the Moon at a rate of 2 km.
(1.24 miles) per second, or more, cannot be retained by the attraction
of that globe and therefore never retraces its path but speeds away
for ever. In the same manner a bullet ejected from a cannon with an
initial velocity of 11.2 km. (7 miles)--a velocity not even approached
by present artillery--would fly away from Earth barring the resistance
of the air. Thus we see that we as yet are far from the realization of
the dreams of Jules Verne in his _A Voyage to the Moon_. At any rate,
gravity on the Moon is too weak to retain hydrogen over the hottest
point of the surface. This part of the gas flies away, new supplies
rush in from the sides, and in a short time all traces of hydrogen have
disappeared from the Moon. Probably it was mainly gathered in by the
Sun, where a velocity of 613 km. (380 miles) per second is necessary if
the molecules are to overcome the Sun’s attraction, while their actual
velocity there amounts to only about 8 km. (5 miles) per second.

In a similar manner we find that the second lightest gas, helium, at
a temperature of 150° C. (300° F.), possesses a molecular velocity of
1.62 km. (1.1 miles) per second. This is less than the 2 km. (1.24
miles) per second necessary to leave the Moon’s sphere of attraction.
But all helium molecules do not move at the same speed; some are
faster and some slower than the average. Those moving at a higher rate
than 2 km. (1.24 miles) per second constitute a considerable fraction
of the total. This fraction disappears. Equilibrium is soon restored
so that in less than a second the same fraction of helium molecules is
ready to depart. In this manner the Moon lost its helium atmosphere
speedily, although not quite as rapidly as its hydrogen.

More slowly yet vanished the gases which are most abundant in our
atmosphere, nitrogen and oxygen, but these too were not fettered
for ever by the limited gravity on the Moon. The same fate befell
aqueous vapour, which is nearly twice as light as oxygen. The loss of
water, however, was long delayed, as we later shall learn, because
new vapour masses were discharged from the lunar volcanoes. In these
considerations, we should also bear in mind that the Moon no doubt was
a fluid molten mass when separating from the Earth and its substance
resembled the lava from our volcanoes. In this condition it remained
until its exterior temperature had fallen to about 1200° C. (2200° F.).
At that point, the average velocity of oxygen molecules is about 1 km.
(.62 mile) per second, with variations in both directions, so that a
few per cent. of them reach a sufficient velocity to leave the Moon for
ever. Such gas molecules of medium weight return probably to the Earth
which, as experience tells us, is ponderous enough to hold them in
bonds.

All gases, that constitute any considerable fraction of the Earth’s
atmosphere, and which, therefore, most likely were divided with the
Moon in its parting from us, have again left that globe. The same
unquestionably holds true for other stellar bodies of equal or smaller
size, such as all the minor planets and for the great majority of the
satellites to the major ones. Only the very largest of Jupiter’s moons,
and possibly Neptune’s lonely companion, whose size is not known with
certainty, might possibly surpass our Moon in ability to retain gases.
Our reasoning with respect to the Moon applies also to Mercury. It is
true that the molecules there must possess a velocity one and a half
times as high as on the Moon if they are to leave the planet. But at
the same time the temperature on Mercury’s hottest point, always turned
toward the Sun, is far higher, about 400° C. (750° F.), so that the
molecules there move 1.26 times as fast as similar molecules over the
Moon’s hottest point. Mercury is consequently better able than the Moon
to retain gases, but the difference is slight. Direct observations (see
below) also lead us to believe that Mercury is very similar to the
Moon in these respects. We might possibly imagine that certain gases,
which on the Moon would condense into fluids or solids, on Mercury
might remain volatilized on account of the high temperature and thus
form an atmosphere. Such assumption, however, would be erroneous. The
investigations by Schiaparelli and by all his successors show that
Mercury in turning around its axis always presents the selfsame side
to the Sun. The opposite side, never reached by a ray of sunlight,
must assume an extremely low temperature, very close to the absolute
zero (-273.7° C. or -460.6° F.) and far below any cold existing on the
Moon. To this side, all bodies with an appreciable vapour pressure
must distil and freeze to solid lumps or frost-coverings without
perceptible vapour pressure. For these reasons, Mercury cannot possess
any atmosphere to speak of. There remain in the whole series of planets
and satellites in our solar system only two bodies besides the Earth
which are endowed with an atmosphere in the original sense of the
word--namely, Mars and Venus.

We reach the same conclusion when investigating the ability of the
planets to reflect the sunlight falling upon them. The bodies which
possess an atmosphere hold also suspended therein clouds of water or
ice, and also of dust, whirled up from below. These floating particles
reflect light far more efficiently than the solid or fluid surface of
a planet. The Moon can now reflect 7.3 per cent. of the sunlight and
Mercury 6.9 per cent. (H. N. Russell, _Proceedings Nat. Acad._, 1916).
These numbers lie so close that they may be considered practically the
same within errors of observation.

It is therefore probable that Mercury is as devoid of an atmosphere as
the Moon. The opposite extreme is represented by Venus, which reflects
not less than 59 per cent. of the sunlight received, according to
H. N. Russell. Terrestrial clouds were found by Abbot to return 65
per cent. We believe from astronomical observations that the entire
surface of Venus is hidden behind a thick opaque cloud-covering.
The slight difference between O.65 and O.59 may be due to errors of
observation, but also to a small absorption of light in those parts of
Venus’s atmosphere which are outside of the clouds. Saturn and Jupiter
are very similar to Venus in this regard with 63 and 56 per cent.
respectively. The gases above the clouds on these planets extinguish
to a considerable extent the sunlight reflected from the clouds, as
apparent from their spectra. (Compare Fig. 13.) Hence the value 0.63
given by Russell for Saturn is probably too high. Concerning Jupiter
it has been observed that its red light becomes deeper when the
sunspots are few, but whiter when the spots are numerous. The sunspots
have been found to favour the formation of high clouds, such as cirrus,
and this would seem to apply also to Jupiter; when spots are plenty,
the clouds are high, and consequently the absorbing layers above, which
cause the red colour, are thinner, so that Jupiter will then shine with
a whiter--less red--lustre than when the sunspots are rare.

The two outmost planets, Uranus and Neptune, return, according to
Russell, 63 and 73 per cent. respectively of the sunlight received.
These figures are probably too high. They do not agree well with
Slipher’s records of their spectra (Fig. 12).

There now remains Mars. This planet approaches the Moon inasmuch as
it reflects only 15.4 per cent. of the sunlight arriving to the orb.
Everything points to the conclusion that the atmosphere of Mars is very
thin. Lowell estimates, on somewhat meagre grounds, however, that on
each square metre of the planet rests only 22 per cent. of the mass of
air supported by each square metre of the Earth’s surface.

It would naturally be very interesting to ascertain the amount of
sunlight our Earth throws back into space. This we cannot measure, as
we cannot place our instruments outside of the Earth’s cloud-mists
nor can we read them there. Not less than 52 per cent. of the Earth
is covered with clouds, whose whiteness (Latin: _Albedo_) is 65. Thus
the clouds alone return 0.52 × 0.65 = 0.338 parts of the sunlight. Of
this portion a fraction amounting to about 4 per cent. is extinguished
in the air above. The remainder is 0.325. Atmosphere and suspended
dust reduce the sunlight over the cloud-free part, _i. e._ 48 per
cent. of the Earth, by 60 per cent, half of which returns to space,
while the other half reaches the ground in the form of light from the
sky, and of this fraction again about 4 per cent. is reflected into
space; these two items added give 0.15. Finally, the 40 per cent.
sunlight directly received by the Earth’s surface is reflected to the
extent of 6 per cent. by the oceans and by the generally moist ground;
deserts and bare rocks reflect about twice as much, but their total
area is comparatively small; of this 6 per cent. reflected light, 70
per cent. reaches outside space; thus we obtain 0.48 × 0.40 × 0.06 ×
0.70 = 0.008. In all, therefore, the amount of reflected sunlight is
0.338 + 0.15 + 0.008 = 49.6 per cent. If the air were free from clouds,
the reflexion-number or Albedo would be 33 per cent., or considerably
higher than that of Mars. When now half or a little more (52%) of the
Earth’s surface is overcast with clouds and this portion therefore has
the whiteness of Venus, the figure 49.6 (Russell calculates the figure
45) for the entire Earth naturally falls closer--almost 3.6 times--to
59, the figure of Venus, than to 15.4, the figure for Mars. We may
also compare the value 33 per cent., which applies to the cloud-free
portion of the Earth, with the value 15.4 per cent. for Mars, which is
almost without clouds, and with the value 7.3 per cent. for the Moon,
which has neither clouds nor dust, because it lacks an atmosphere. We
can then conclude that the atmosphere of our Earth holds almost three
times as much dust suspended over each square metre as does Mars, and
this in spite of the smaller gravitational force on Mars, which is
about 37.5 per cent. of that on Earth. Taking proper account of the
low temperature on Mars we may easily compute, by means of a formula
given by Stokes, that a particle of dust should sink 2.3 times slower
on Mars than on Earth. When, nevertheless and in spite of frequent
but thin mists, so few particles of dust float in the atmosphere of
Mars, the conception inevitably comes to our mind that the air on
that planet must be extremely rarefied so that the wind-puffs have
little power to raise the dust from the ground. Lowell estimated the
barometric pressure at the surface of Mars to be about 64 mm. (2.52
inches), and Proctor gives about twice this figure. There appears to
be ground for considering already the former value too high; both are
very uncertain. If we accept that of Lowell, we find that each square
metre of the surface of Mars supports a column of air, whose mass is
only about one-fifth of the mass resting on each square metre of ocean
surface on Earth.

  Fig. 14. The planet Venus, with sunlit atmosphere (to the
           left), as observed by Langley at the Venus passage
           December 6, 1882.
]

The dense clouds which float above Venus have long ago led to the
assumption that the atmosphere of that planet must be far deeper than
that of the Earth. Its strong refractory power has also contributed
to this belief. When Venus is close to the sun-disc the dark body
stands forth surrounded by a ring of light (see Fig. 14). It is,
however, recognized that this phenomenon requires no greater air
density than that on Earth for its appearance. In this connection we
should remember that the inside limit of the vapour shell which we
in this manner observe, is the cloud-wall, not the ground. And these
clouds, we have every reason to believe, float on account of the heat
prevailing at a great height in the atmosphere, so high in fact that
they form an impenetrable wall already where the cirrus clouds
appear in our sky. If these suppositions are correct, the light-ring
mentioned is caused by a quarter only of the air-masses on Venus, and
its total air-covering must be far deeper than that of the Earth.
The latter occupies probably in this respect as well as in reference
to position in space a middle ground between Mars with its extremely
thin and Venus with its comparatively dense atmosphere. If so, we
might expect the atmosphere on Mercury to be denser yet, while we
already have seen that it is almost wholly lacking on that planet.
The explanation is that Mercury has lost its spin around its own axis
and therefore always presents the same side to the Sun--just as the
Moon and probably all other satellites turn one side only toward their
respective central bodies--hence the opposite side becomes so cold
that all gases are there condensed to fluids or solids except the two
most volatile ones, hydrogen and helium, which on the other hand leave
the planet on its hot side. If Venus, therefore, as held by several
astronomers from Schiaparelli to Lowell, always turned one side only
toward the Sun, this planet also would be without any perceptible
air-covering. According to investigations by Bjelopolsky of Venus’s
spectrum, which investigations, however, are in complete disagreement
with corresponding measurements by Slipher, that planet has a period
of rotation on its own axis of about 29 hours. This figure is very
uncertain and a new determination is therefore highly desirable.

In order to understand the atmospheres of the planets, it is of great
interest to ascertain the composition of the air that surrounds the
Earth. Our knowledge in these matters has grown considerably of late.
We shall in the main follow the presentation by Dr. Wegener of Marburg.

We know at present with considerable accuracy which gases enter into
the air. Besides the previously well-known nitrogen and oxygen which
contribute the bulk, 78.1 and 20.9 per cent. respectively, of the total
volume at the earth’s surface, we find water vapour in proportions
changing with localities and times, and it is for this reason left
out when fixing the various percentages; further, carbon dioxide 0.03
volume per cent. and the rare gases discovered by Rayleigh and Ramsay,
argon, 0.932 per cent., neon 0.0012 per cent., helium 0.0004 per cent.,
krypton 0.000005 per cent., and xenon 0.0000006 per cent. Each one of
these constituents diminishes in quantity with height in accordance
with the so-called barometer-formula and the rate is the more rapid
the heavier the gas. Krypton and xenon, therefore, which are two and
one half and four times heavier than oxygen, occur mainly in the lower
strata. The percentage of helium on the other hand, a gas eight times
lighter than oxygen, should increase rapidly with height. If the air
consisted of a mixture of oxygen and helium at 0° C. (32° F.) the
former would decrease to one half at an elevation of 5 km. (3.1 miles)
but the latter not before we had ascended 40 km. (25 miles) (eight
times higher than for oxygen, as the weights are in the ratio of 1 to
8). At that altitude, oxygen would have decreased in the proportion
1:2^8 = 1:256. When, as actually is the case, there is 50,000 times
more oxygen than helium at the surface this ratio should decrease
in the proportion 128:1 at a height of 40 km. (25 miles). Ninety
kilometers (56 miles) above the surface helium should overbalance
oxygen and thereafter rapidly gain in preponderance. This holds true
provided no agitation takes place in the form of vertical currents of
air.

Similar laws apply to all light gases which do not turn into fluids or
solids at low temperatures. Aqueous vapour, on the other hand, which
when cooled condenses to clouds, diminishes much faster than the nearly
twice as heavy oxygen, because the temperature rapidly decreases as we
move upward or at a rate of about 5° C. per km. (14.5° F. per mile)
up to 2.5 km. (1.5 miles) and of 8° C. per km. (23° F. per mile) at a
height of 8.5 km. (5.3 miles). The quantity of water vapour shrinks to
one half at 1.9 km. (slightly more than a mile) above ground. Carbon
dioxide again follows the barometer-formula applicable to other gases
because it occurs in such minute quantity that it never condenses to
clouds. In fact, it is water vapour alone which must be treated as an
exception. Carbon dioxide is nearly one and one half times heavier than
the other gases of the atmosphere on an average. It should therefore
diminish in the proportion 1:2^{1.5} = 1:2.8 in a vertical distance of
5 km. (3.1 miles) while the density of the air decreases only in the
ratio 1:2. Several determinations of the presence of carbon dioxide
in the atmosphere as high up as 3.8 km. (2.33 miles) have been made,
by S. A. Andree among others, but the percentage of this gas remains
constant within the errors of observation. The same holds true to a
height of 7 km. (4.35 miles) for the proportion between oxygen and
nitrogen, although we might have expected a perceptible change as
oxygen is 14 per cent. heavier than nitrogen. How shall we explain this
fact which seemingly contradicts the theory just advanced?

The explanation is quite simple. The preceding statements hold true for
a mass of air at perfect rest. But, if the air is violently agitated,
the composition becomes homogeneous all through. We know that in the
barometric cyclones and anticyclones strong rising and descending air
currents flow. The composition of the atmosphere, therefore, becomes
the same as far up as this mixing action prevails. These currents
produce another effect, namely, a fall of temperature with rising
height. Because when a gas is transported upward the surrounding
pressure decreases, resulting in expansion and consequent cooling. It
is well known that a gas is heated when (rapidly) compressed, a quality
formerly made use of in the pneumatic fire-tool to ignite tinder. It is
evident that conversely a gas must cool off when expanding. If now the
mixing of the air were extremely rapid the thermometer would fall very
close to 10° C. (18° F.) with each km. (.62 miles) rise in elevation.
If, on the other hand, the air stood perfectly still in a vertical
direction, the temperature would remain constant at all heights
over the same point. Between these two extremes, we find the actual
condition, inasmuch as the temperature of the atmosphere decreases
upward 5° to 8° C. per km. (14.5° to 23° F. per mile) as observed
during balloon ascensions.

This applies to the so-called “troposphere”--mixing-zone. One of the
most remarkable discoveries in recent times, made by Teisserenc de Bort
and Assman, is the fact that the decrease of temperature with height
does not continue indefinitely but only up to a certain elevation,--in
middle Europe about 11 km. (7 miles), in Lapland about 7 km. (4.5
miles), and at the equator about 15 km. (10.5 miles)--and above this
point the temperature remains constant. We now meet the peculiar
condition that the temperature of this upper layer, which is called
stratosphere--“film-zone”--is lowest over the equator, because it
commences at a great height there, and lowest over the polar regions,
where it extends farther down. The stratosphere has received its
name from the fact that it consists so to speak of lamellæ almost
parallel to the earth’s surface and moving in a horizontal direction
while vertical motions are absent. The winds in these strata have a
marked westerly direction (_i. e._ they are east winds) and they become
stronger the higher the stratum--at an altitude of 83 km. (52.5 miles)
their velocity is about 100 m. (330 ft.) per second. In the troposphere
on the other hand west winds predominate. The wind direction in the
stratosphere was observed on the so-called luminous night-clouds which
were found as high as 80 km. (50 miles) above the Earth. These strata
consequently revolve slower around the earth’s axis than the solid
body of the planet itself. At an elevation of 80 km. (50 miles) the
rotational speed has decreased to 65 per cent. of the angular velocity
of the earth’s surface. We have reason to believe that the very highest
strata stand still, that is do not take part in the earth’s rotation on
its axis. This would follow if outside space were not entirely devoid
of vapour so that our atmosphere would merge imperceptibly into the
exceedingly attenuated gas masses of interplanetary space.

As high as the mixing-zone extends, so high also is the composition of
the air constant and like that at the surface of the earth. But above
this limit--in Scandinavia, we might say above an elevation of 10 km.
(6.2 miles)--commences a rapid tapering of the heavy gases, while the
percentage of the light ones correspondingly rises. Foremost among the
latter is hydrogen, with only half the weight of helium. The presence
of hydrogen in the atmosphere has been shown by Boussingault, and
the proportion in which it occurs has later been measured by Armand
Gautier. It is about one three hundredth part of one per cent. It
increases extremely rapidly with height in the stratosphere so that 80
km. (50 miles) above the earth and upward hydrogen is more abundant
than all other known gases of the atmosphere at the same altitudes.

We reproduce below a somewhat revised table by Dr. Wegener of Marburg,
who has made the most recent computation of the percentages of the
various constituents of air at different heights. Consideration has
been given to the fact that the composition of the air does not change
except as regards the percentage of moisture within the troposphere,
which is assumed to reach a height of 10 km. (6.2 miles). As usual in
similar cases the percentages refer to volume.

  -------------+-----------------+----------+--------+----------
    _Height_   |   _Pressure_    |          |        |
  -----+-------+--------+--------+          |        |
  _in  | _in   |  _in   |  _in   |_Hydrogen |_Helium |_Nitrogen
   km._| miles_|   mm._ | inches_|    2_    |   4_   |    28_
  -----+-------+--------+--------+----------+--------+----------
     0 |   0   |760     |29.9    |   0.0033 | 0.0005 |   78.1
  -----+-------+--------+--------+----------+--------+----------
    10 |   6.2 |197     | 7.75   |   0.0033 | 0.0005 |   78.1
  -----+-------+--------+--------+----------+--------+----------
    30 |  18.6 |  8.95  |  .352  |   ----   |  ----  |   85
  -----+-------+--------+--------+----------+--------+----------
    50 |  31.0 |  0.45  |  .0177 |   1      |  ----  |   88
  -----+-------+--------+--------+----------+--------+----------
    70 |  43.5 |  0.045 |  .00177|  13      | 1      |   80
  -----+-------+--------+--------+----------+--------+----------
    90 |  55.8 |  0.0157|  .00062|  68      | 5      |   26
  -----+-------+--------+--------+----------+--------+----------
   110 |  68.2 |  0.0116|  .00046|  94      | 5      |    1
  -----+-------+--------+--------+----------+--------+----------
   130 |  80.6 |  0.0097|  .00038|  96      | 4      |    0
  -----+-------+--------+--------+----------+--------+----------
   210 | 130.2 |  0.0055|  .00022|  99      | 1      |   ----
  -----+-------+--------+--------+----------+--------+----------
   310 | 192.6 |  0.0032|  .00013| 100      |  ----  |   ----
  -----+-------+--------+--------+----------+--------+----------
   410 | 254.2 |  0.0021|  .00008| 100      |  ----  |   ----
  -----+-------+--------+--------+----------+--------+----------
   510 | 316.2 |  0.0016|  .00006| 100      |  ----  |   ----
  -----+-------+--------+--------+----------+--------+----------

  -------------+-----------------+--------+-------+---------+-------
    _Height_   |   _Pressure_    |        |       |         |
  -----+-------+--------+--------+        |       |_Carbon  |
  _in  | _in   |  _in   |  _in   |_Oxygen |_Argon | Dioxide |_Water
   km._| miles_|   mm._ | inches_|   32_  | 39.9_ |   44_   |  18_
  -----+-------+--------+--------+--------+-------+---------+-------
     0 |   0   |760     |29.9    |  20.9  | 0.937 | 0.03    | 1.41
  -----+-------+--------+--------+--------+-------+---------+-------
    10 |   6.2 |197     | 7.75   |  20.9  | 0.937 | 0.03    | 0.14
  -----+-------+--------+--------+--------+-------+---------+-------
    30 |  18.6 |  8.95  |  .352  |  15    | 0.29  | 0.0064  | 0.5
  -----+-------+--------+--------+--------+-------+---------+-------
    50 |  31.0 |  0.45  |  .0177 |  10    | 0.10  | 0.0014  | 1.7
  -----+-------+--------+--------+--------+-------+---------+-------
    70 |  43.5 |  0.045 |  .00177|   6    | 0.05  | 0.0005  | ----
  -----+-------+--------+--------+--------+-------+---------+-------
    90 |  55.8 |  0.0157|  .00062|   1    | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   110 |  68.2 |  0.0116|  .00046|   0    | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   130 |  80.6 |  0.0097|  .00038|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   210 | 130.2 |  0.0055|  .00022|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   310 | 192.6 |  0.0032|  .00013|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   410 | 254.2 |  0.0021|  .00008|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   510 | 316.2 |  0.0016|  .00006|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------

Under the name of each gas its molecular weight is given as a measure
of corresponding specific weight. The quantity of water vapour was
not included when the percentages of the other gases were calculated,
because it changes considerably with locality and time. The number
given in the table for water is the mean for the entire globe--it
corresponds to 11.4 grammes per cubic metre (.31 oz. per cu. yd.)--or
the amount present in air saturated with moisture at 16.5° C.
(61.7° F.). The bulk of the water vapour forms a layer strongly
concentrated toward the surface of the earth. Carbon dioxide also
tapers rapidly with increasing height because its density is 1.5
times greater than that of air. This is apparent from the molecular
weight 44 stated under carbon dioxide, while the average molecular
weight of air is 29. Faster yet do krypton, with a molecular weight of
83, and xenon, with a molecular weight of 131, decrease as we ascend
in the atmosphere. These gases, like neon, whose percentage first
increases slightly with height, and argon, which decreases upward as
shown in the table, do not perceptibly influence the processes of
nature. The reverse is true about water vapour and carbon dioxide,
which nourish the plants and also protect the Earth against a too
rapid heat radiation into space. We well remember how abruptly the
temperature changes in the course of the day in the dry desert
climate, while corresponding variation is comparatively slight in
humid climates (compare page 86). This is the result of the ability of
water vapour to arrest the radiation from the Earth. Carbon dioxide is
about evenly distributed over the globe--although somewhat sparser
over highlands--and its heat-conserving and equalizing influence is,
therefore, not so manifest as that of moisture. Only by the most
accurate investigations has this influence been demonstrated.

In Wegener’s table a gas is included, called Geocoronium, whose
existence in the air has not been directly proved. Conspicuous is,
however, the green light displayed at great altitude by the Northern
Light arches, a green color which does not, as far as we are aware,
belong to any known constituent of the air. It is true that the
corresponding spectral line (557 µµ) lies very close to a line
belonging to krypton, but the latter is a heavy gas which cannot occur
to any traceable extent in the high strata, more than 300 km. (186
miles) above the earth, where occasionally the Northern Light arches
appear--their favoured height according to measurements by Störmer
is about 120 km. (75 miles). Wegener assumes, therefore, that this
green line belongs to an hitherto unknown substance, Geocoronium,
which should be five times lighter than hydrogen. Recent researches
present great difficulties to the acceptance of this assumption, and
for this reason further discussion of the problem will be omitted.
Above a height of 210 km. (130 miles), this gas, according to Wegener,
would preponderate. If such postulated gas does not exist, hydrogen
completely dominates in these regions and down to 85 km. (53 miles)
above the Earth. Because hydrogen is so light, the density of the
air in the range of a barometric pressure below 0.02 mm. (.0008
inches) increases but slowly as we descend toward the Earth. This
uppermost part of the atmosphere may appropriately be designated as
the hydrogen-zone. Even within this range, the “shooting stars” meet
sufficient resistance to flame into light at a height of about 120
km. (75 miles) and dissolve into dust which turns dark about 85 km.
(53 miles) above the Earth. E. C. Pickering recognized the spectrum
of hydrogen in the light of meteors passing at great height, but
decomposed water vapour might possibly be its source. Meteors crossing
lower strata show the spectrum of nitrogen. Nitrogen becomes important
from a height of about 85 km. (53 miles) downward and from 75 km. (46.5
miles) to the surface of the Earth it predominates. As a consequence
the pressure increases rapidly as we approach the ground. In these
regions or up to 80 km. (50 miles) floated the highest luminous night
clouds, observed by Jesse, indicating that here commenced a new range,
the nitrogen-zone. Only the heavy meteorites are able to penetrate
into the nitrogen sphere, which checks their speed and causes them to
explode, and thereafter the remnants fall with a velocity compatible
with the air resistance they meet. To these parts descend also the
lowest rays of the Northern Lights, the so-called draperies--Störmer
observed them once at a height of 37 km. (23 miles). Finally, water
vapour presents itself in appreciable quantities at an elevation of
about 10 km. (6.2 miles), where the troposphere commences. We now meet
the highest clouds, cirrus (with the exception of the “luminous night
clouds” observed only in the years 1883–1892 after the eruption on
Krakatoa). To these heights, reach the vertical air currents which are
essential to cloud formations. Only light clouds, however, float at
these altitudes; the heavy clouds (alto-cumulus) do not rise above 4
to 5 km. (2.5 to 3.1 miles) and the rain clouds proper (cumuli) occur
only below a height of 2 km. (1.25 miles). This is the result of the
downward concentration of water vapour within the troposphere.

If gravity decreased in intensity, the effect would be the same as if
the gases were lighter. On Venus, the intensity of gravity is eight
tenths of that on Earth. The difference is slight. If everything else
were similar the various air-zones would reach 25 per cent. higher on
Venus than they do on our globe. But one essential condition is varied
by the far higher temperature on our neighbour. The proportion of
moisture in the air is thereby vastly increased. The dense clouds rise
to much greater heights than on Earth. If there be ten times as much
water in the air on Venus as there is in the air on Earth--which might
fairly represent the actual condition--the heavy rainclouds would there
rise to a height of more than 10 km. (6.2 miles), and their smaller
weight on Venus would also contribute to their buoyancy. The light
cirrus clouds should appear as high as about 30 km. (18.5 miles) above
the ground. Under such circumstances, we cannot expect but that the
planet must be entirely hidden from our sight as well as from the rays
of the Sun.

On Mars, the intensity of gravity is 2.68 times smaller than on Earth.
In consequence barometric pressure falls 2.68 times slower with
increasing height there than here. The same ratio holds for decrease in
temperature and for shrinkage of proportion of moisture when comparing
conditions on the two planets. The strong cold precludes anything but
insignificant quantities of water vapour. The air on Mars is similar
to the atmosphere on Earth in and above the cirrus-region. The clouds
existing there are not only extremely thin and transparent--it is well
known that cirrus clouds throw no shadows--but they are confined to
small fractions of the planet’s sky. They are replaced by light mists.

We shall later return to the consequences of these peculiarities.

With help of the spectroscope we have ascertained that the gases on
the Sun are, in the main, also arranged according to specific weights,
so that the lightest reach the greatest heights. Somewhat similar
conditions obtain in the gas-appendages of the stars (compare page
119).

CHAPTER V

THE CHEMISTRY OF THE ATMOSPHERE

Of a very particular interest is the question of the atmosphere of
the planets. The great problem of habitability of the latter is most
intimately connected therewith. Primitive fancy, very early, populated
the stellar bodies, especially the stars and the Sun, with beings
similar to those on Earth. Gradually, however, the insight awoke that
these bodies are incandescent and therefore unfit to shelter life of
the kind with which we are familiar. Attention then turned to the
planets, as they and the Earth belonged to the same order of heavenly
bodies. Perhaps they furnished abodes for our kin. And the stars, suns
like our Sun, should not they be surrounded each with its throng of
planets, gravitating around their central source of light and heat?
This beautiful thought vied with the conception of Earth as the centre
of the universe and as wholly set apart from the other stellar bodies,
whose prime object it were merely to furnish light and to indicate time
for the inhabitants of the Earth. Much to be regretted, it was the
latter far less attractive theory which gained firm hold on the Church,
although a few of its unbiased men, like the renowned Cardinal Nicolaus
Cusanus (1401–1464), declared in favour of the contrary opinion, and
did so unmolested. But times grew harder, ironclad orthodoxy triumphed,
and Giordano Bruno, whose defence was that he simply accepted the
theory of the great Cusanus, was burned at the stake to expiate his
fearless assertion that other worlds, no less than ours, might be
blessed with the presence of living beings.

Undoubtedly, other planets are upbuilt of the same material which
enters into the composition of the Earth--as held already by Leonardo
da Vinci. Spectral analysis teaches us that the same constituents
form all the suns, including our own. If we agree that the suns have
furnished the original substance of the planets revolving around them
it is a natural conclusion that this matter should combine into similar
chemical compounds on planets equally advanced in their evolutionary,
that is cooling, process. And we know, indeed, that the same elements
compose the Sun and the Earth, and that the samples brought to us from
other worlds, _i. e._ the meteors, are of a composition which strongly
reminds us of the rocks in the interior of the Earth. We seek in vain
only for indications of the action of water, which substance has left
such obvious traces on the surface of our globe and in the immediate
strata below. But it should be remembered that the water, in the form
of vapour, as previously set forth, has left all the minor stellar
bodies, and the meteorites manifestly belong to the small or smallest
among the wanderers of the heavens.

There is then no reason to doubt that the material of which the
planets are built essentially is the same throughout the universe.
Their interior should, like that of our Earth, consist of the heavy
metals, principally iron,--strongly prevalent also in the Sun and in
the meteors,--and this metallic nucleus should be clothed with the
silicates, oxides, carbonates, sulphides, and hydrates of all metals,
particularly aluminum, and among the metals we may also count hydrogen.
The melting points of these exterior and lighter substances lies above
1000° C (1800° F.). No life could exist in such a molten mass, so
that not until a solid crust had been formed through cooling was the
possibility of life at hand.

Life is, at least on Earth, tied to certain so-called compounds, in
which carbon is the essential common element, while hydrogen, nitrogen,
and oxygen together with sulphur, phosphorus, iron, magnesium, and a
few other less important elements also enter therein. No substance but
carbon possesses this quality of being a prerequisite of life. Silica
is a close kin to carbon and a substitute in certain organic compounds,
but protoplasm, the main constituent of the living cell, cannot be
built without carbon. In the inorganic world, however, silica by
virtue of its affinity, which is kindred to that of carbon, plays a
rôle somewhat similar to the latter in the almost infinitely variably
silicates. Protoplasm cannot endure in a temperature above 60° C.
(140° F.) or thereabout--certain algæ, it is sometimes stated, thrive
in hot springs up to 80° to 90° C. (176° to 194° F.) but certainly not
over 100° C. (212° F.). At these temperatures--strictly speaking at
all temperatures between O° C. (32° F.) and 365° C. (689° F.)--water
can exist in fluid state and this too is a prerequisite of life. We
may say therefore that life is confined to a small temperature range
between the freezing and boiling points of water. But wherever water
occurs, except in a vessel which it completely fills, there exists also
in the adjacent space, if unoccupied by fluids or solids, water vapour
of at least 4.6 mm. (.18 inches) pressure. There is, therefore, always
an atmosphere of aqueous vapour on any planet whose surface is partly
covered by water.

The palæontologists have agreed that all life commenced in the water.
The manifold living beings which now inhabit the solid crust of the
Earth all descend from ancestors which floated in the waves of the
ocean, the cradle of all organisms. It is not absolutely certain
that oxygen is necessary to all living beings but many biologists
hold that opinion. Certain bacteria are able to draw the oxygen they
require for their development from compounds in which oxygen is bound
sometimes in a very intimate manner, as in sulphates. But these
bacteria are considered degenerate plants, and free oxygen is certainly
indispensable to the existence of the animals and probably also to the
plants with the exception just mentioned. As we shall see later, free
oxygen cannot be present on the planets until a solid crust has been
formed. We may therefore state that the conditions for the existence of
life on a planet are fulfilled when a true atmosphere containing oxygen
and water surround its body.

If we are to understand these conditions, we must study the processes
whereby oxygen is supplied to the atmosphere. As the planets are
segregations from the Sun, they should originally have the composition
of the Sun, particularly that of its outer layers. Here metals occur in
greatest abundance, but there are also a few oxides, especially those
of titanium and magnesium (according to Fowler), hydrogen in great
quantities, oxygen, carbon, cyanogen, and carbon monoxide. It may seem
strange that free oxygen exists side by side with a surplus of hydrogen
and sodium, strong so-called reducing substances which bind oxygen.
But at the high temperatures prevailing on the Sun compounds of oxygen
and the reducing substances, for instance hydrogen, _i. e._ water, are
largely dissociated into their constituents. But, if the temperature
should drop to about 1200° C. (2200° F.), at which point crust building
does not yet take place, the oxygen would be entirely absorbed in the
formation of the compounds mentioned. The compound substances of the
Earth, like those of the Sun, are also strongly reducing, so that we
must infer that free oxygen did not enter into the gas covering of
the Earth at the time when its solid crust was formed. We may gain
a conception of the gases which then existed in the outmost layers
of the Earth by studying the gases on the Sun and on other stellar
bodies, particularly the comets, and also by investigating the gases
absorbed by the molten interior of the Earth. Previous to the crust
formation, the entire mass of the Earth except the gases in its outmost
layers were of the same character as possessed by its molten interior
now. This molten mass, or magma, when in contact with the surrounding
gases partly imbibed them through the process called absorption. An
investigation of the gases present in the magma will therefore give
us an idea of those existing in the original vapours surrounding the
Earth. The magma occasionally comes into view in volcanic eruptions,
and the confined gases are then partly given up into the air, but they
are also partly retained in the solidifying lava, or volcanic rocks,
whence they may be driven off by high temperature and subsequently
analyzed. The direct gas emanations from the craters may also be
gathered and analyzed. Such investigations have been carried out on
a large scale by Albert Brun, Frenchman, Arthur Day, American, and
his co-labourers Shepherd and Perret. Brun reached the surprising
conclusion that water vapour, hitherto considered the most important of
the volcanic gases, in reality was not one of them but originated in
the crust of the earth. This theory, however, was completely refuted
through the investigations by Day and his associates. As an example,
we give the analysis (the mean of several determinations) of volcanic
gases from the crater Halemaumau on the volcano Kilauea in Hawaii:

           _May, 1912_
      _Per cent. of Volume_

      Carbon dioxide    55.4
      Carbon monoxide    4.3
      Hydrogen           7.7
      Nitrogen          29.6
      Sulphurous acid    2.9

        _December, 1912_
      _Per cent. of Weight_

      Carbon dioxide    42.9
      Nitrogen          25.8
      Water             27.5
      Sulphurous acid    3.7

In the latter case, it has been shown that air had gained entrance to
the volcanic gases, which air might have carried a quantity of water.
But this quantity could not have been large judging by the amount of
nitrogen present which corresponds to not quite 3 per cent. water by
weight. In the former case water was not included in the analysis. At
any event, a high percentage of water has frequently been observed in
volcanic gases.

When the gases were left in contact with water a considerable part was
absorbed thereby, particularly compounds of chlorine and fluorine and
also ammonia and sulphurous acid. An analysis of such water showed 10
per cent. more fluorine than sulphurous acid and two fifths as much
chlorine as fluorine. The ammonia amounted only to half of one per
cent. of the chlorine. None of the rare air gases were present, which
indicates that the nitrogen originated entirely from the magma and not
from the air.

Brun has analyzed lavas from different volcanoes. The gases extracted
therefrom naturally do not give as reliable information about the
original atmosphere of the Earth as do the gases directly emanating
from the volcanoes. As examples we quote the composition of the vapours
in lavas ejected March 4, 1901, from Stromboli, and from Vesuvius in
the well-known eruption of 1906. They show in percentages of volume:

                       _Stromboli_    _Vesuvius_
  Free chlorine           12.8            0
  Hydrate of chlorine      2.0            6.6
  Sulphurous acid          4.5           12.0
  Carbon dioxide          60.2           73.8
  Carbon monoxide         11.5          traces
  Hydrogen                 0.5            7.6
  Nitrogen                 6.9          traces
  Marsh-gas                1.6            0
                         -----          ------
                         100.0           100.0

It will be noticed that the composition varies considerably. Free
chlorine cannot very well be primigenial as it, like oxygen, combines
with reducing substances. Chlorine may be produced by heating chloride
of calcium with silica and ferro-silicates which are present in the
magma. At all events, carbon dioxide constitutes the bulk. Next
in importance are sulphurous acid and hydrate of chlorine. Carbon
monoxide, hydrogen, and nitrogen may occur in quite considerable
quantities but are sometimes almost wholly absent.

Day and Shepherd reached the conclusion that the gases emitted by the
hot lava in the Halemaumau crater are nitrogen, water, carbon dioxide,
carbon monoxide, sulphurous acid, hydrogen, sulphur vapour, also
small quantities of chlorides, fluorides, and possibly ammonia. Such,
at least approximately, should also the original composition of the
Earth’s atmosphere have been when the crust was newly formed. Nitrogen,
water, and carbon dioxide were the most important ingredients; in
the high strata hydrogen was present. Oxygen was totally lacking and
reducing gases such as hydrogen, sulphurous acid, and carbon monoxide
abounded instead. If we further note the composition of comets and
meteorites we find that cyanogen, carbohydrates, and carbon monoxide
also are present in the former, argon and helium in the latter. It
is therefore probable that these substances, although absent in the
emanations from Kilauea, yet belonged to the primary atmospheres of the
planets. The rare gases of the air especially should originally have
come from the exterior parts of the sun, as did the nitrogen.

An atmosphere of such a composition would be utterly unendurable to
living beings. It must, if organisms are going to thrive therein,
be purified from such poisons as carbon dioxide, gaseous sulphur,
cyanogen, and sulphurous acid. We know that such a process has taken
place and that the sunlight has been the great chemist who produced
oxygen and carbon from the carbon dioxide. The just mentioned poisonous
gases were subsequently oxidized through electrical discharges. We
all know that the plants up-build their framework under the influence
of sunlight, consuming in the meantime carbon dioxide, water, and a
little ammonia. In the process, oxygen is formed, as are also starch,
cellulose, sugars, and albuminous substances with the aid of the
green colouring matter in plants, chlorophyll, which considerably
accelerates the action. Subsequently these new substances, which all
(except the albuminous ones) belong to the carbohydrate group, are
converted into principally carbon and water. The final result is that
carbon dioxide through the agency of sunlight is split into its two
constituents, carbon and oxygen. This process, which is comparatively
rapid in the presence of chlorophyll, should also, although more
slowly, take place without that medium; and in recent trials the
chemists--particularly Daniel Berthelot--have, indeed succeeded in
imitating without chlorophyll this important function of the plants
through the application of light of a short wave length. In the course
of the many millions of years which geology has proved necessary
for the evolution of our planet, the carbon dioxide in the air was
gradually converted into oxygen and carbon. As long as reducing gases,
such as the poisonous ones mentioned, or any considerable quantities
of carbohydrates and hydrogen yet remained in the atmosphere, the
oxygen was consumed in their combustion. If a solid crust had not
existed to prevent the oxygen from entering into the interior molten
mass, it would also have found its way there and would have oxidized
the reducing substances in the magma. The separation of the interior
from the surrounding gas shell is therefore a necessary requisite for
the existence of free oxygen in the air. Another condition is that the
combustible gases escaping from the volcanoes must be added to the air
at a sufficiently slow rate not to consume all of the simultaneously
formed oxygen. A third requirement is that the liberated carbon should
not in a renewed process of oxidation bind the oxygen just recovered.
As long as the air still was reducing, this last condition was no doubt
fulfilled for that very reason. At all events, the crust once formed
and the original violent volcanic activity somewhat abated, the time
finally arrived when free oxygen existed in the air. The previously
present reducing gases were, except for small fractions, burned into
water, carbon dioxide, and sulphuric acid and the nitrogen compounds
had no doubt yielded free nitrogen to the stores of that gas already
a part of the atmosphere. The time was now ripe for the first plants,
probably low forms of algæ, which, in the oceans, commenced life on our
planet. The carbon dioxide and hydrochloric acid of the air as well as
the newly formed sulphuric acid were absorbed by the running water and
caused a rapid disintegration producing silica and acid silicates. As
plant life developed and extended the formation of oxygen increased.
Falling vegetable matter was imbedded in slime which prevented the
access of oxygen during decay and in this manner the fossil fuels were
deposited. Koene of Brussels first pointed out that the carbon and the
sulphuric compounds accumulated in the Earth would suffice to bind the
oxygen of the air. Later investigations lead to the conclusion that the
carbon alone is sufficient for the purpose. It would therefore appear
that all the oxygen of the air is derived from carbon dioxide belonging
to the original atmosphere or contributed thereto by the volcanoes.

The reason why carbon dioxide and water continuously are liberated from
the magma is undoubtedly that the acid silicates are lighter than the
basic and therefore accumulate in the exterior parts of the magma. A
great surplus of silica exists there. Compounds containing water and
carbon dioxide, _i. e._ hydrates and carbonates, are also light and
should therefore congregate to the same strata where silica abounds,
there partly to be dissolved by the free silica and thus setting
water and carbonic acid free. The latter are, in contrast to silica,
volatile and evaporate therefore into the air leaving the silica
behind. This process is yet in evidence wherever the magma emerges as
through the volcanoes. But also a few other acids in the magma are
highly volatile, as sulphurous, thiosulphuric, and hydrochloric acids.
These also belong to the volcanic gases, are dissolved by the water,
and partake in the processes of disintegration. The carbon dioxide
and hydrochloric acid form carbonates and chlorides. The former are
extracted from the sea water by crustacea, sometimes also by plants,
and form part of our sedimentary strata; the latter are soluble and
remain in the water, chiefly as sodium chloride or common salt. The
thiosulphuric acid, probably a product of ferrous sulphide and acids in
the magma, has entered into numerous insoluble metallic sulphides found
in the Earth. Partly, it has also been oxidized, like the sulphurous
acid, into sulphuric acid and has then assisted in the processes
of disintegration, forming gypsum which has been deposited in the
sedimentary rocks.

The geologists believed formerly that the Earth gradually and
continuously has grown cooler. This theory, however, struggled with
the difficulty that certain cold time intervals, ice periods, were
succeeded by warmer epochs. To begin with efforts were made to
surmount this obstacle by assuming that an ice period on the north
hemisphere was counterbalanced by a warm period on the south hemisphere
and vice versa. In this manner, the mean temperature for the entire
surface of the globe might possibly have been continuously decreasing
although fluctuations had occurred on the two hemispheres. But this
view has proved untenable, because the ice period has left traces also
within the tropics, near the equator, as on Kilimanjaro, in New Guinea,
and so on. It is now practically agreed that the last great ice period
was characterized by a temperature between 4° to 5° C. (7° to 9° F.)
below the present all over the surface of the Earth. This determination
has been accomplished by measuring the difference in height between the
terminals of the glaciers at present and the lowest points where their
grinding action has left obvious traces. The ice coverings of North
Europe, North-East America, South America, along the coast of Chile
and in Argentina, as well as on the southern island of New Zealand,
all appear to have existed simultaneously. Also during earlier eras,
for instance during the Algonkian and the Permian epochs, ice periods
have occurred. The latter, which was felt in Australia, India, and
South Africa, is called the Gondwana-time. This period was formerly
supposed not to have caused any temperature drop except in the tracts
mentioned. Later investigations lead us to believe, as asserted by
Holland in his presidential address to the geological section of the
British Association at its 1914 meeting, that also this ice period
simultaneously embraced the entire globe.

As the Algonkian time belongs to the oldest epochs of geological
history it appears that the temperature on the Earth as long as life
has existed on our planet on the whole has been nearly constant,
with important alternations, however, of warm and cold periods. For
an explanation of these fluctuations, our well-nigh only recourse is
the assumption that the heat-conserving quality of the atmosphere has
changed by virtue of a varying composition. Warm periods occurred when
carbon dioxide was abundant in the atmosphere due to volcanic activity,
cold periods again accompanied a paucity of carbon dioxide. With rising
temperature, the percentage of water vapour in the air also increased,
affording further protection against radiation loss of heat.

Thus, it would seem as if the mean temperature of the Earth’s surface
hardly had changed to any extent worth mentioning during immense spans
of time estimated to about 500 million years. Nevertheless, a slow
process of cooling proceeding toward the centre of the planet probably
takes place. Ever growing quantities of matter are transported from the
interior of the earth through volcanic action. Sedimentary deposits
increase continuously while the interior becomes hollow. As a result
the crust must gradually settle, causing large cracks in the process.
For these weakened places the volcanic products show a special liking
and the craters are strung out in lines along such fissures. In other
places, where volcanic action is less pronounced, hot springs appear
instead, generally emitting carbon dioxide in abundance, occasionally
also sulphurous acid and sulphuretted hydrogen. The dislocations in the
crust also take place along these cracks accompanied by earthquakes.
The study of these various phenomena has enabled us to map out the
fissures, which generally radiate in nearly straight lines from one
point, the so-called centre of collapse, as the cracks in a pane of
glass issue from the point of breakage caused by a swift blow. We shall
later see that such breakage lines and centres of collapse are common
on all stellar bodies which possess a solid crust and are observable
from the Earth.

We may now easily form an idea of the general trend in the development
of the atmosphere. The gases originally present were all, except the
hydrogen, the nitrogen, and the rare gases, strongly absorbent of light
and in particular of heat. It is, therefore, natural that the planets
which have not formed a solid crust possess a strongly absorbing
vapour-shell, as indeed is the case with the large planets (compare
Fig. 13). The crust once formed and the air gradually purged of these
gases, thanks to the sunlight, so that mainly nitrogen and oxygen,
small quantities of the rare gases, and carbon dioxide besides water
remained, the temperature fell rather rapidly. Carbon dioxide formed
the last effective heat-conserving ingredient. As the crust grew
thicker, the supply of this gas diminished and was further used up
in the processes of disintegration. As a consequence the temperature
slowly decreased, although decided fluctuations occurred with the
changing volcanic activity during different periods. Supply and
consumption of carbon dioxide fairly balanced as disintegration ran
parallel with the proportion of this gas in the air. But evolution on
the whole can only proceed in one direction toward a final cooling of
the Earth. This must occur if for no other reason because the store of
energy in the Sun and therefore its radiation must slowly decrease.
With deepening crust and disappearing carbon dioxide vegetation must
ebb, and with it the production of oxygen. This gas also partakes in
the general disintegration through oxidation of iron protoxides in the
mineral rocks. The oxygen portion of the air must therefore finally
reach its maximum and start on the decline. Calculations point to the
conclusion that the carbon dioxide of the air would be consumed in a
few tens of thousand years if new supplies were not furnished from the
interior. Water is also absorbed in the processes of decay as hydrated
compounds are formed, increasing in quantity with falling temperature.
As the amount of water in the ocean is immensely larger (about 50,000
times) than the stores of carbon dioxide in the air and in the seas
the lack of the latter will undoubtedly first become serious. But a
slow desiccation of the planet must subsequently take place, and will
proceed at an accelerated rate with the continued cooling of the Earth.
Then the vapour in the air and consequently precipitation will wane.
Then, as during the ice periods, mighty ice caps will cover the poles
and impound a large portion of the water in the ocean. Finally, the
entire planet, perhaps after having harboured life during trillions
of years, becomes an ice waste with a few cracks in its hard crust
through which warm and acid vapours rise and create small melted areas
characterized by a darker colour than the desert and ice-landscape in
general. Organic life lacks the conditions for existence and ceases
therefore to cheer the planet with its interesting variations. The
planet is dead but continues in obedience to gravitation to describe
its orbit in space.

CHAPTER VI

THE PLANET MARS

Through the works of Schiaparelli, Flammarion, and Lowell the vivid
interest of the general public has been directed toward our neighbour
planet Mars. Several investigators, Flammarion and Lowell among them,
assert with full confidence that Mars has intelligent inhabitants, who
have built and maintain the curious “canals,” which, it is stated,
could not have been created except by intelligent beings far superior
to man.

Air, water, and sunshine exist there, says Flammarion in his well-known
great work, _La Planète Mars_ (1902, page 515). “It appears incongruous
to us to condemn a world like Mars, where all the conditions for life
exist, to such a fate” (to be a dry desert). No doubt sentiment and the
desired result play a part in all such deductions, as indeed the words
chosen by Flammarion would indicate.

As contrasted to the Earth, Mars is, on the other hand, considerably
further removed from the Sun, whose radiation therefore on Mars
possesses only 43 per cent. of its intensity on Earth. Judging by this
fact, the mean temperature of Mars should fall far below that of the
Earth and considerably below the freezing point of water and under such
conditions it is hard to imagine a vegetation near the poles of Mars
as Lowell does in his volume, _Mars as the Abode of Life_ (1909), or
even in the neighbourhood of the canals anywhere on the surface of the
planet as assumed by Flammarion.

With such ideas in vogue we can well understand that the astronomers
would point their lately extremely sharpened instruments toward our
ruby-coloured neighbour in the sky when, in 1909, it passed very close
to the earth under conditions particularly favourable to accurate
observations, more so in fact than they had been during the seventeen
preceding years.

Numerous astro-physicists, among them the world’s foremost
representatives of their discipline, have repeatedly turned their
spectroscopes toward Mars in order to ascertain whether water vapour
was present there or not. In the spectrum of the Sun we find several
so-called “rain-bands” due to the fact that the light before it reaches
the apparatus has passed through the moisture of the air. The more
humid the air the more strongly developed these rain-bands are. If we
direct the instrument first on the Moon, which lacks an atmosphere
and therefore also moisture, and then on Mars, which for the sake of
simplicity we assume standing close to the Moon-disk, a difference
should appear in the spectra of these two bodies, provided moisture
is present in the atmosphere of Mars. The rain-bands ought to be more
pronounced in sunlight that has passed the atmosphere of Mars (passed
it twice as the light is reflected by the surface of the planet) than
in light reflected from the naked Moon. The bands appear of course
in both spectra as the light on its final stage to the spectroscope
passes the atmosphere of the Earth, which never is free from moisture.
In this manner Huggins and Janssen, scientists of world-wide fame,
believed that they had demonstrated the presence of water vapour on
Mars. Campbell, on the other hand, the prominent director of the
Lick Observatory, made similar investigations of the planet in 1894,
and so did a French astronomer, Marchand, in 1896 and 1898, both
under unusually favourable circumstances as the former installed
his instrument 1283 m. (5200 ft.) and the latter 2860 m. (9370 ft.)
above sea level, but neither found any indication of moisture in the
atmosphere of Mars.

It is evident that the observations would be far more accurate if we
could eliminate the moisture from the atmosphere of the Earth, in which
case no rain-bands would appear in the spectrum of the Moon. It would
then be unnecessary to compare the two spectra; we would only have to
determine whether rain-bands were present in the spectrum of Mars or
not. We can never entirely avoid the water vapour of the air, but its
influence may be greatly reduced by making our observations from high
mountains or in desert climates where the air is comparatively dry,
that is free from water vapour. Investigations undertaken where the
air is dry deserve therefore more confidence than those handicapped by
greater humidity. The observations by Campbell and Marchand fall in the
former category, and it would, therefore, appear that the presence of
water vapour on Mars to any extent worth mentioning is highly doubtful.

In later trials, Campbell and Keeler have employed an improved method,
using photographs of the spectra on sensitive plates, but neither has
succeeded in discovering any water vapour in the atmosphere of Mars.

Obviously photography offers a great advantage over direct ocular
observation. The two pictures may be placed side by side and very
accurate measurements may be made at leisure. We may also choose the
moments for exposure when the two stellar bodies stand equally high
over the horizon so that the sunlight reflected from them traverses
equal distances in the humid atmosphere of the earth.

It now devolved upon Lowell to test his theories by means of the
magnificent resources at his disposal in Flagstaff observatory in the
desert of Arizona 2200 m. (7200 ft.) above sea level. In the months of
January and February the dew-point there is about -7° C. (+19.4° F.)
_i. e._, each cubic meter (1.3 cu. yds.) of air contains 2.8 grammes
(43.25 grains) of water vapour while saturated air at zero temperature
(32° F.) holds nearly twice this amount or 4.8 grammes (74 grains)
per cubic meter (1.3 cu. yds.). Slipher, working in this observatory,
pushed the sensitiveness of his plates to the utmost then obtainable,
and photographed the spectrum of Mars in January and February, 1908.
He found that the most important rain-band always was more prominent
in the spectrum of Mars than in the spectrum of the Moon photographed
later during the same night. Peculiarly enough, it was only the
rain-band designated “A,” and located in the red spectral field, that
was of a marked difference in the two spectra. Other bands gave no
indication of the presence of water vapour on Mars. This result did not
directly contradict the conclusions reached by Campbell and Keeler,
also by means of photography; they had investigated other bands than
“A.” The “A”-line might therefore possibly be more sensitive to water
vapour than the others.

Slipher’s discovery was considered so valuable that it must be employed
to the limit. The well-known physicist Very was therefore called in
consultation; he made careful measurements of the intensity of the
“A”-lines on the various plates and calculated that the atmosphere of
Mars contained 1.75 times as much water vapour as that of the Earth at
the point of observation. If we desire to determine the proportion
of water vapour in the air at the surface of Mars from this statement
we may figure in the following manner. The amount of water vapour in
a vertical column of air one square meter (1.2 sq. yds.) in section
is, according to Hann, 2500 times the amount in a cubic meter (1.3 cu.
yds.) at the surface of the earth. At the time of the observation, the
latter amount was 2.29 grammes (35.4 grains); on each square meter (1.2
sq. yds.) of the ground rested therefore 5725 grammes (12 lbs. 11 oz.)
of water vapour. That the quantity of water is not larger, although
the depth of the atmosphere far exceeds 2500 m. (1.5 miles), is due
to the fact that the temperature rapidly decreases with distance from
the ground. On Mars, the temperature ought not to fall so quickly with
change in height because the intensity of gravity there is 2.68 times
smaller than on the Earth. The temperature drops there 2.68 times
slower with ascent in the atmosphere, and a column of air on Mars one
square meter (1.2 sq. yds.) in section should therefore contain 6680
times as much water vapour as a cubic meter (1.3 cu. yds.) at its
surface. As Mars did not stand in zenith, the distance traversed by the
light-ray in the atmosphere was greater--in fact 1.43 times greater
than if such had been the case. A column of air in the direction of
the light-ray, one square meter (1.2 sq. yds.) in section contained
therefore 8175 grammes (18 lbs. 3 oz.) water vapour. In the atmosphere
of Mars which the light passed in a vertical direction there was, if
we are to believe Very, 1.75 times as much, or 14,300 grammes (31
lbs. 8 oz.) and in a cubic meter (1.3 cu. yds.) at the surface of the
planet, consequently 6680 times less, or 2.14 grammes (33.1 grains).
The corresponding dew-point is then, according to this determination,
-10.3° C. (+13.5° F.). It is agreed upon that a desert climate
prevails on Mars. It might at the time of the observation conform to
the extremely dry climate at Salt Lake City in the height of summer
when the humidity there is only 31 per cent. of saturation. Under
such conditions saturated air at noon in the equatorial belt on Mars
should contain 7 grammes (108 grains) per cubic meter (1.3 cu. yds.)
corresponding to a temperature of 5.3° C. (41.5° F.).

It must be admitted that this was not very encouraging to Lowell. If
the temperature in the middle of the day, when the sunlight falls
perpendicularly on the surface of the planet, rises only to about 5° C.
(41° F.), the mean temperature for twenty-four hours, even in the midst
of the summer, must in this entirely clear, light air be far below
freezing and vegetation on Mars is therefore not very well conceivable.
In spite of this, Lowell saw in Slipher’s measurements a confirmation
of his theory that Mars is the abode of an intelligent race that
utilizes, in their wrestle with existence, a verdant vegetation pushed
even into the polar regions.

Campbell, however, went one step further than Slipher. In August and
September, 1909, Mars occupied a position in the sky particularly
favourable to observations. Campbell decided to benefit thereby.
With the support of a rich patron of science, a Mr. Crocker, who
on several occasions has made magnificent contributions toward
astronomical research, Campbell equipped an expedition to Mount Whitney
in California, 4425 m. (14,502 ft.) high and the loftiest peak in the
United States. He was accompanied by an able scientific staff, the most
prominent of which were Dr. Abbot, head of the observatory belonging
to the Smithsonian Institution, and a well-known German astronomer,
Albrecht. The members of the expedition were affected by mountain
sickness and suffered many severe hardships when the wind was high,
reaching about 25 m. per sec. (56 miles per hour), and at the same time
cold, falling below zero (freezing) during the night. The barometric
pressure was only 447 mm. (17.6 inches). During the nights, when the
observations were made, the water content of the air fell to between
0.5 and 0.9 grammes (7.7 grains to 13.9 grains) per cubic meter (1.3
cu. yd.) or 2.5 to 4 times less than Slipher had to contend with. The
spectra of the Moon and of Mars were photographed in close succession,
two exposures being made in each case. The band “A” was plainly visible
on several plates. No indication of greater prominence of this band
in the spectrum of Mars could be found. Other rain-bands were also
investigated with the same result. Neither were the characteristic
bands of oxygen stronger in the spectrum of Mars, than in that of the
Moon. Slipher believed that he had discerned a difference, although of
a hair’s breadth, which would indicate the presence of oxygen in the
atmosphere of Mars. The conclusion itself is not improbable, but the
amount of oxygen there is in any case considerably smaller than in the
Earth’s atmosphere.

Several statements by Campbell, as well as Slipher’s observations,
indicate that a difference ought to have appeared between the spectrum
of Mars and that of the Moon if the water content in Mars’ atmosphere
had been the same as in the Earth’s at the time of the observation.
This content, as stated before, was about 3 times smaller on Mount
Whitney than at Flagstaff. At the latter place, the measurements gave
1.75 as the ratio of water vapour on Mars to that on the Earth. The
amount of water vapour with the Sun in zenith on Mars should therefore,
according to Campbell’s observations, only reach 0.4 gramme (6.1
grains) per cubic meter (1.3 cu. yds.) corresponding to a dew-point of
-28° C. (-18.4° F.) or to an actual temperature of -17° C. (+1.4° F.)
allowing also for a desert climate with only 31 per cent. saturation.
This temperature is probably higher than the mean for a summer day as
the observations were made at noon on Mars.

It must now be evident that we should consider Mars as unfit to
harbour living beings. There is possibly a slight amount of oxygen in
the thin air but the extremely low temperature and the scant supply
of water vapour form insurmountable obstacles to the subsistence of
even the simplest forms of life in the equatorial regions on Mars.
The temperature difference between day and night must be enormous on
account of the desert climate. Even if life could develop during the
day, which has nearly the same duration as with us--Lowell fixed it at
24 hours, 37 minutes, 22.6 seconds--and during which the temperature
possibly might rise above the freezing point, it would nevertheless be
destroyed without mercy by the bitter frost at night.

Campbell has offered an explanation of the indications of water vapour
on Mars, apparent on Slipher’s photographs. An analysis of the latter’s
observations shows that the Moon was photographed about four hours
later in the night than was Mars. On all occasions except one, clouds
appeared in the sky. This indicates the presence of moisture in the
air, so that the humidity should change with the temperature which
latter rapidly falls in the course of the night. Campbell, himself,
found during the clear nights, when he made his observations, that
the humidity in the hours of the night up to midnight falls to a
fraction--a half or a third--of its original value an hour or so after
sunset. This rapid temperature drop is probably confined to the strata
immediately above the observation point but the moisture is strongly
concentrated downward so that this change in humidity undoubtedly
should have been taken into account. Or better, observations should
be avoided in the beginning of the night and the Moon and Mars
photographed as soon after each other as possible, precautions taken
by Campbell but not by Slipher. That the latter found less traces of
water on the lunar photos than on the martian ones is, therefore,
probably due to the fact that the former were taken about midnight but
the latter not long after sunset, when the atmosphere contained much
more water vapour. Thus, we learn how a small slip, more obvious to the
meteorologist than to the astronomer, may spoil a labour otherwise done
with extraordinary care.

To Campbell’s critique Very answered by the suggestion that the
meteorological conditions during the Mount Whitney observations should
have been exceptionally unfavourable. The entire south-west of the
United States and the north of Mexico were visited at that time by
cloudy weather and heavy downpours. Very contends that this humidity
should partly have extended to the high strata above Mount Whitney and
therefore rendered the calculation of the moisture content of the air
entirely unreliable.

Simultaneously (August, 1910) new measurements were published of
Slipher’s photo-plates from February, 1908, which Very had examined.
The result was now that the rain-band “A” was 2.5 times more pronounced
in the spectrum of Mars than in that of the Moon. Furthermore, the
oxygen absorption-band “B” was 1.5 times stronger for Mars than for the
Moon. Great quantities of water vapour and oxygen should, therefore,
undoubtedly exist in the atmosphere of Mars.

In the meantime Campbell had not been idle. The difficulty with the
older measurements consisted in the fact that the absorption line
of water vapour in the atmosphere of Mars occupies the identical
place of the line due to vapour in the Earth’s atmosphere. There
exists, however, a method, as already pointed out by Campbell in 1896,
of separating the two, which method is available when Mars either
approaches or departs from the Earth with sufficient velocity. The
latter could be determined both from the known motions of the two
planets and from the displacement of certain spectral lines of the Sun.
These two determinations were in almost perfect agreement; for instance
January 26–27, 1910, astronomical calculations gave a relative velocity
of 19.1 km. (11.86 miles) per second and spectroscopical measurements
19.2 km. (11.93 miles) per second, while on February 3–4 the relative
velocity was 18.1 km. (11.24 miles) a difference of 1 km. (.62 miles)
per second. This trial shows the accuracy of the method. Among the
absorption lines of water vapour and of oxygen there was, however, none
due to the atmosphere of Mars. Campbell assumes that such lines would
certainly have been visible if they had been only one-fifth as strong
as the so-called tellurian lines. The advantage of this method is
evidently that the “martian” and the “tellurian” lines lie close beside
each other on the same plate so that differences in sensitiveness,
exposure, and atmospheric conditions are entirely eliminated.

From these and the following data we may calculate water content and
temperature of the atmosphere on Mars anew: water vapour at the point
of observation was 1.9 grammes (29.3 grains) per cubic meter (1.3 cu.
yds.), zenith-distance of Mars 55° and incident as well as reflected
sun-rays formed an angle of 70° with the surface of Mars; hence, the
amount of moisture at the surface was only 0.12 gramme (1.85 grains)
per cubic meter (1.3 cu. yds.), corresponding to -38° C. (-36.4°
F.) for saturated air and to -27° C. (-16.6° F.) for air of 31 per
cent. saturation. Oxygen content per cubic meter (1.3 cu. yds.) at
the surface of Mars would be only a sixteenth part of corresponding
numerical value on the Earth. This determination is more accurate
than any of the previous ones and reduces the temperature another
10° C. (18° F.) below the lowest value derived earlier in this chapter.
We should remember, however, that, during the trial of September,
1909, the sun stood practically in zenith on Mars, while in January
and February, 1910, we are concerned with a point where sunrise had
occurred about four and a half hours previously. The latter observation
should give a value close to, but slightly above, the mean diurnal
temperature on Mars.

No determination comparable in precision with this one by Campbell
appears to have been made. We must therefore recognize it as conclusive.

We may easily calculate the surface temperature of a planet from the
intensity of the solar radiation received, or insolation, provided
the surrounding vapour shell contains no heat retarding gas. The most
important gases of this kind are water vapour, which, as we just have
seen, is very sparse in the atmosphere of Mars, and carbon dioxide,
of which there probably also, for reasons stated below, is only a
scant supply in the martian gas shell. Such calculations were first
performed by Christiansen of Copenhagen, who assumed 2.5 calories as
the solar constant on Earth, _i. e._, the amount of energy received
through insolation per minute by each square centimeter (.15 sq. in.)
of the Earth’s surface when at right angle to the radiation and on
mean distance from the Sun. On Mars, the radiating energy received
under similar conditions is only about 1.1 calories. The surface of
the planet is heated until it radiates as much energy into space as it
receives from the Sun. In this way we obtain an average temperature
of -37° C. (-34.6° F.) for the entire surface of Mars. The regions,
exposed to the Sun in zenith at noon, might, if heat were not conveyed
therefrom, possibly reach a daily mean temperature of +8° C. (46.4° F.)
and perhaps slightly more at noon. Probably not even the freezing
point is reached, as the heat is rapidly carried away by the freely
circulating air. The above-mentioned mean temperature of -37° C.
(-34.6° F.) seems on the whole to agree well with the observations by
Campbell on Mount Whitney.

Recent accurate determinations of the intensity of the solar radiation
by Abbot, K. Ångström and others, indicate that it has been estimated
about 20 per cent. too high. If we take the solar constant to an even
2.0 calories, which is a trifle high, we reach the conclusion that the
mean temperature on Mars would fall about 50 degrees below freezing.
Equatorial regions might then reach an average of -8° C. (+17.6° F.)
and at noon the temperature might possibly rise slightly above zero
(32° F.). A higher temperature yet might be attained at the pole where
the Sun during the summer remains for months above the horizon or a
high mark of +8° C. (46.4° F.) provided no heat were carried away by
air currents. Such losses naturally must occur, and the temperature
probably hovers around freezing. At the martian poles we might possibly
imagine the existence of some low forms of vegetation (snow-algæ,
etc.) during the height of the short summer.

When we hitherto on the authority of Lowell, Very, and others, have
assumed an average temperature of +10° C. (50° F.) on Mars, we have
done so on the supposition that the atmosphere of the planet contained
great quantities of heat-conserving gases. This assumption appears
to be no more tenable than the belief in the high temperature on
Mars. After all, the temperature is probably about 10° C. (18° F.)
higher than our last calculation would indicate--or about -40° C.
(-40° F.)--because the air on Mars is very clear and admits, therefore,
all sun-rays, retaining also a fraction by virtue of what little water
vapour, carbon dioxide, and other heat-conserving gases there may be
present in the atmosphere. The mean summer temperature at the martian
equator (-27° C. or -16.6° F. acc. to Campbell’s data) would then lie
about 13° C. (23.4° F.) above the mean for the planet. This agrees
closely with conditions on the Earth where the highest mean in July
at the equator is 27° C. (80.6° F.) and the mean for the earth 16° C.
(60.8° F.).

We are consequently obliged to revise in their entirety our ideas about
Mars. The belief that organic life (green vegetation) causes the colour
of the so-called seas on Mars, as assumed by Lowell, or that the red
tints belong to the gorgeous dress in which autumn arrays the plants
before their leaves are shed under the attacks of frost, as intimated
by Flammarion, must nowadays take its place in the shadowy realm of
dreams.

Those who do not believe that the so-called canals are real
waterways, devoted to freight carrying and irrigation, or illusions,
which conception the photographs contradict (for example Fig. 18),
generally consider that they signify cracks or fissures. As in the
crust of the Earth, they generally run in nearly straight lines or
in regularly bent curves (Fig. 17 and 17a). Flammarion mentions that
the renowned physicist Fizeau looked upon the “canals” as cracks in
the ice-coverings of the oceans on Mars. Penard, in 1888, expressed
the more likely opinion that they correspond to the fissures in the
crust of the Earth. Flammarion contends that such fissures do not have
the rectilinear configuration of the “canals.” This is completely in
error, as shown on the map here reproduced (Fig. 16). It is also stated
that they are so inexplicably long, for instance the canal Phison is
2250 English miles (Lowell) or 3620 km. in length. The longest known
earthquake crack along the entire length of which a dislocation has
taken place _at one time_ is 600 km. (373 miles) in extension; the
violent earthquake in California, 1906, originated from this crack.
There is no doubt, moreover, that a great fissure in the Earth follows
the coast of Chile from Arica to the Strait of Magellan in a nearly
north and south direction for a distance of about 32 parallels or 3560
km. (2210 miles). This fissure is almost as long as Phison on Mars.
Such cracks exist along the entire coast of the Pacific Ocean. As yet,
we do not know their position in much detail, because long stretches
run below the sea or through territories not yet occupied by civilized
people. As an example of a small fissure, a picture taken by Sederholm
from Segelskär, east of Hangö in the Baltic, may serve (see Fig. 15).
As the studies of earthquakes are prosecuted with increasing interest
in later years, fissures of all dimensions will undoubtedly soon be
discovered. The solid crust on Mars is, furthermore, somewhat thicker
than that of the Earth as the cooling of that planet has progressed
further. The sections broken off at the bursting of the Martian crust
ought therefore to be much larger both in breadth and in length.
No doubt, the facts that the intensity of gravity on Mars is only
three-eights of its intensity on the Earth and that the curvature of
the Martian surface is twice as sharp as the Earth’s contribute to
this result. Imagine two vaults, one built with higher and broader
wedge-shaped stones than the other and with half the radius and
furthermore loaded only one-third as heavily as the other, and it
will become evident that we can permit a much larger span in the former
than in the latter case without fear of collapse. In other words, it
requires a much more extensive caving or shrinkage of the molten mass
beneath the crust of Mars to cause a rupture than under the terrestrial
crust.

  Fig. 15. Narrow bay on the left side of Segelskar, east of
           Hango in Finland. The bay owes its existence to the fact
           that the ice has laid bare a territory coursed through by
           fissures. Photo by I. I. Sederholm.
]

  Fig. 16. Earthquake centres in Calabria and in Sicily. On the
           larger map the damaged localities are indicated, on the
           smaller the most prominent tremor lines. Drawn by I. I.
           Sederholm.
]

As a consequence, the fissures on Mars ought to be longer than the
corresponding formations on the Earth. A thorough study of the large
fissure in Calabria shows that it consists of a veritable network of
smaller straight cracks (as is apparent on Figure 16, which is taken
from a work by the well-known Finnish geologist Sederholm). On this
map the radial cracks (see _Worlds in the Making_, Fig. 16), charted
by Suess, are also shown, and their direction under the sea designated
by dotted lines. The sketch in the upper left corner of Fig. 16 is in
striking similarity to a picture drawn by Schiaparelli in Mercator’s
projection of the planet Mars (see map at the end of book). We notice
on both, the numerous equidistant lines corresponding to parallel
cracks and duplex canals. Not every fissure has its parallel and not
every canal its mate--generally only one of the latter is visible and
sometimes both disappear.

As the radial cracks in the drawing by Suess if extended meet in the
Lipari Islands, so also several canals on Mars run together in a
so-called lake (Lowell called them “groves” or “oases”) which evidently
is a centre of collapse (many appear on Fig. 17). It is plain that all
crossings of the “canals” are not necessarily such centres of collapse.
(See maps Figs. 17 and 17a at end of book).

We shall consequently assume that the canals on Mars correspond to the
geological dislocation fissures on the Earth. Along these fissures
emerge the gases liberated in the cooling process on both planets;
which are similar gases to those which escape through the volcanoes.
These vapours are primarily water, next carbon dioxide and, in
considerably smaller quantities, sulphuretted gases and hydrochloric
acid. They discharge through cracks in regions which, geologically
speaking, not so long ago were the scene of volcanic activity. In the
dislocation-grooves, lakes, and water courses are often formed, as we
may observe in several places in Sweden, for instance near Stockholm.

Assume now a gradual cooling of our earth. Most territories are covered
by stratified, comparatively light rocks. To the dislocation fissures
water gathers from the surrounding strata and occasionally from the
interior, partly washes away the loose material and transforms the
fissures into furrows, generally with flat bottoms. Dissolved salts
are carried to the sea. As cooling proceeds, the ocean commences to
freeze. Each summer the surface melts to a certain extent, as is the
case now in our polar regions. Finally, the entire ocean freezes to the
bottom, the ice is now to be considered as a kind of rock, flexures and
dislocations cease and the ice assumes a smooth surface. In the strong
sunlight during the summer this surface thaws, as do the water-courses
on the mainland, and these continue to carry their salts to the open
surface water. At the approach of winter the latter solidifies again
but not as the water in our inland lakes from the top but from the
bottom, as ordinary sea-water possesses its greatest density below
the freezing point while the opposite is true of fresh water. The
consequence is that the ice foundation grows upward and as the surface
water becomes increasingly shallow it is turned into a concentrated
salt solution. With a further drop in temperature the ice formation is
accompanied by crystallization of the salts.

Something similar takes place on the Martian mainland in its flat river
basins, which correspond to the salt lakes in our deserts. On account
of the bitter cold and the consumption of the water in the process of
disintegration (the carbon dioxide has been largely used up in the
same manner), precipitation has almost ceased on Mars and most of the
water in circulation emerges from the interior of the planet along
the fissures. As it contains hydrochloric acid and carbon dioxide it
extracts from the soil salts, such as the chlorides of sodium (common
salt), of calcium and of magnesium, all present in common sea-water
to which it was brought by the rivers. The compounds of calcium and
magnesium are not precipitated as carbonated salts through the medium
of crustacea as is the case on earth. The strong solar radiation during
the summer partly evaporates the water into the thin air, leaving the
salts behind. On account of the low temperature, this vaporization on
Mars is probably slower than on the Earth. Along the cracks in the
crust, a kind of dry salt-lakes are formed similar to the generally
shallow and occasionally dry lakes common in the deserts of Central
Asia as described by Hedin. We know that Mars possesses a pronounced
desert climate. There finally remains in the lowest sections of the
water courses a concentrated salt solution, which parts with its water
more and more reluctantly, so that the salts which most strongly hold
the water crystallize at the deepest points. If the winter’s cold is
sufficiently severe (below -55° C. or -67° F.) ice is extracted even
from the most concentrated solutions, which mainly contain chloride of
calcium. In spite of such extreme temperatures, evaporation into the
rare atmosphere is not negligible and the ice crystals partly vanish,
to reappear in the coldest regions of the planet, that is, around the
pole which at the time is turned away from the sun. On the ocean, now
frozen solid throughout, a polar-cap of snow and hoar-frost is formed
which finally reaches as far as the 38th parallel on the southern
hemisphere (see Figs. 18 and 19), where winter occurs when Mars is most
removed from the sun, and to the 58th parallel (see Fig. 19) on the
northern hemisphere where winter reigns while Mars is nearest to the
Sun and consequently not quite so cold. Similar conditions obtain on
the Earth although not to such a marked degree.

  Fig. 18. Photograph of Mars, enlarged and retouched; taken by
           Lampland
]

  Fig. 19. Appearance of Mars April 8, 1907, as observed by
           Quénisset. Below one pole a dark line is plainly visible.

]

  Fig. 20. The south pole spot on Mars observed by Jarry-Desloges
           July 10, 1909.
]

  Fig. 21. The appearance of Mars during an observation by
           Antoniadi in 1909. The whole disk is somewhat hazy; below,
           the details are hidden by clouds of sand.
]

In the vicinity of the snow-white polar cap, whether there be continent
or sea, bodies of water occur with solidly frozen surface covered by
crystals of very hygroscopic salts, such as the chlorides of calcium,
magnesium, and sodium. When the summer warmth returns and the polar
cap is heated, the hoar-frost evaporates, and the now comparatively
humid air spreads over the surrounding territory. We observe also
frequent mist-formations in these places. The ground near the edge
of the polar snow assumes then often a dark hue on account of the
moisture (Fig. 19). Occasionally canals and lakes appear in the polar
cap (see Fig. 20). This is evidently due to hot emanations along the
cracks. The moist air sweeps over the salts, which then absorb water
and dissolve into concentrated solutions. New quantities of water
vapour are supplied from the pole as they distill over toward the
other pole, where winter now exists, and push on toward the equator
which they finally pass. In their course they dissolve the salts
in the depressions along the fissures and particularly at the deep
crossings where the centres of collapse or the so-called “oases” are
located. Lowell has observed that the “canals” in this manner gradually
“liquify” from 78° N. Lat. to the equator in fifty-two days.

The canal theory presents great difficulties to the explanation of this
curious phenomenon. In order to make the water flow it must be assumed
that the surface of Mars is entirely smooth or at least very nearly so
and that the inhabitants convey the water melted at the poles through
pumping stations. The canals vary in width; according to Lowell, their
mean is 16 km. (10 miles), according to Flammarion between 300 and 60
km. (185 and 37 miles) which latter estimate probably is too high. The
same canal differs widely in breadth in successive years and sometime
disappears altogether. When the supply of water vapour is scant, only
the most hygroscopic salts are dissolved, _i. e._, those deposited in
the deepest furrow of the canal, but when the moisture sweeping over
the canal is more abundant the broader portions absorb water, darken
and thus become visible. The same holds true in regard to the inland
lakes (“oases”). As the water vapour diffuses in the air, the canal
becomes liquid along its entire length independent of the altitude of
its various parts.

All agree upon the desert climate of the mainland on Mars. Like most
deserts on the Earth, it is, therefore, probably a table-land, where
one plateau mounts above the other, each one nearly level. By the
action of the wind, the upper layers have been transformed into fine
sand. On the dead planet no further sediments are deposited by the
sea. The only accretions to the planet are meteorites and cosmic dust
which slowly rains down. It contains among other substances iron,
partly metallic and partly in the form of protoxides (which have a
light green colour).[5] The oxygen in the atmosphere of Mars transforms
these compounds to ferro-oxide which has different colours according
to its coarseness, but generally is ochre. The surface of Mars is also
described as possessing this colour. Dross has, therefore, assumed
that the Martian soil is mingled with ferro-oxide. The finest dust,
however, is yellow while larger crystals tend toward violet. We often
observe on Mars that the details are covered by a yellow veil. This is
of course finely powdered ferro-oxide probably mixed with less coloured
sand which the desert wind whirls up over large portions of Mars. Vast
sections of the planet bore such wrappings in the autumn of 1909, as
observed and described by Antoniadi in Paris (see Fig. 21). Similar
observations have previously been made by W. H. Pickering and others.

    [5] In the deepest portions of the oceans on the Earth where
        no sediments from the coasts are deposited, large
        accumulations containing iron compounds have been found
        and the presence of certain minerals therein points to
        their meteoric origin.

As a rule, only the central and the polar regions of the surface of
Mars can be seen. Territories near the equator more than 40 to 50
degrees removed from the point in line with the Sun and the centre of
the planet are generally hidden behind a thin, white veil of mist. As
soon as the Sun leaves the zenith and reaches half-way to the horizon
the moisture of the air is precipitated near the ground. This shows
that the planet does not possess any quantities of heat-conserving
vapours in its gas shell. The mist does not extend to the poles, whose
white caps always appear distinctly, because the Sun cannot greatly
affect the evaporation in regions where the Sun’s altitude is neither
very high nor very variable. The same holds true for other snow-covered
tracts, even if they are not located in the immediate vicinity of the
pole.

When the supply of water vapour is scant, only the most salient canals
come into view. As a rule they do not then appear double, as one of the
mates is always less prominent. Lowell showed, he believed, that it
always is the same canal out of a pair which first comes to sight and
that its position always remains unchanged in contrast to Schiaparelli
who has reached the opposite conclusion. This, of course, is quite
natural.

On account of the small amount of water vapour in the atmosphere of
Mars true clouds are rare. Figure 22 shows such a cloud at the edge
of the planet. The aforementioned mists are often called clouds, for
instance by Pickering.

  Fig. 22. Cloud at the upper right edge of Mars observed by
           Molesworth, March 7, 1901
]

  Fig. 23. Mars as observed by Lowell, July 11, 1907. Even the
           dark portions appear coursed through by “canals.”
]

  Fig. 24. Mars, as observed by E. M. Antoniadi, October 6, 1909.
]

That elevations really may be found on Mars is evident from the fact
that snow or hoar-frost often remains in patches near the pole and
occasionally quite far therefrom, for instance on the large island
Hellas (40° S. Lat.), while it disappears from the surroundings and
sometimes from the pole itself (the south pole). Such a highland
covered by ice exists near the south pole, and is shown near the
upper edge of Fig. 24. In places where snow always remains, a feeble
glacier formation may occur. Most investigators assume that mountains
and plateaus exist on Mars, although of modest altitude (Campbell
believes that he has observed peaks 3000 m. (9800 ft.) high). Lowell
who diligently has looked for mountains at the edge of the illuminated
part of Mars, has reached the conclusion that they, if present,
cannot rise more than 600 to 900 m. (2000 to 3000 ft.) above the
surrounding plains. It were indeed improbable that all inequalities
of the Martian surface should have been removed in the process of
disintegration, which although at work for enormous extensions of time,
has long been extremely feeble and is unassisted by torrents of rain
which might rapidly wash the products into the valleys. At present, it
is mainly the sand carried by the desert wind that slowly reduces the
roughnesses and in this process extensive highlands are hardly touched.
But, without the assumption, in itself very unlikely, of a nearly level
surface on Mars it becomes difficult to comprehend how the canals, if
filled with pure water, can proceed in straight lines without reference
to existing differences in altitude. Like the rivers on the Earth they
ought to bend according to the topography, even if constructed by
engineers.

When the canals freeze at the approach of winter, they invariably
have been observed to disappear in company with the lakes or oases
at their crossings. They are then all covered by the reddish-yellow
dust carried by wind from the surroundings. When a canal is about to
reappear it frequently first comes to sight as a dark streak evidently
the result of moistened ferro-oxide. Occasionally a mist formation
precedes the appearance of the canal. It is plain that the cold, misty
air settles in the valleys, there as here, and gives up its moisture to
the salts on their bottom and the canal is thus brought out as a dark
line. Sometimes the vicinity also assumes a darker shade indicating
the absorption of some moisture. On the sides of the canals the less
hygroscopic salts are deposited. Possibly the green colour of the
canals is partly a contrast-effect due to the red surrounding, possibly
also the result of finely divided matter in the liquid. It is also
conceivable that the cause is the reducing influence on the ferro-oxide
of the sulphuric gases emerging from the fissures; an exceedingly
small quantity accomplishes in this case large results. F. le Coultre
describes the colour as being sometimes a dead black.

Something similar applies to the seas. When these freeze, especially in
shallow places, yellowish-red dust from the continent settles on their
surface and lends it hues between the original dark green and the light
yellowish-red. When the ice subsequently melts this dust sinks in the
water which latter resumes its dark green colour.

Chloride solutions, if concentrated, freeze at the following
temperatures; that of calcium at -55° C. (-67° F.) that of magnesium
at -44° C. (-48.2° F.) and that of sodium at -22° C. (-7.6° F.). If
now, as we previously have seen, the mean temperature of Mars as
a whole is about -40° C. (-40° F.), of the equatorial belt about
-10° C. (+14° F.), and of the pole in the height of summer about 0° C.
(32° F.), it is evident that a liquefaction of the ocean surface and of
the canals, particularly where salts are deposited, very readily may
take place. We should in this connection remember that the ice on Mars
is stationary while on Earth it is in motion. The consequence is that
sand and dust in the course of thousands of years have accumulated on
the bottom of the shallow basins in the polar ice. These seas appear
therefore dark in spite of their exceedingly small depth and the white
salt and ice-crystals remaining undissolved are unable to display
their light colour. Even in the “ocean,” Lowell was persuaded that he
had observed canals (see Fig. 23), and it may be possible that cracks
are in evidence there, particularly in the most shallow sections, as
is the case in the Tyrrhenian sea north of Sicily. It is significant
that Flammarion has reached the conclusion, which at first appears
highly hazardous, that the freezing point of water is lower on Mars
than on Earth. This is entirely correct, if we let water stand for salt
solutions.

It is customary to point to the strictly uniform breadth and the
rectilinear appearance of the canals as clear evidence of their being
artificial, _i. e._, the work of engineers. The Italian astronomer
Cerulli strongly objected to this conception. “In the exceedingly
rare cases when both sides of the canal plainly may be seen,” states
Schiaparelli, “I have observed curves and notches in the borders.”
This occurred with the canals Euphrates and Triton in 1879, and with
the Ganges in 1888. And it would seem obvious that watercourses
produced in old furrows would not, as a rule, be of uniform breadth.
Antoniadi, by his observations in the autumn of 1909 (see Fig. 17a and
24), has confirmed this opinion, as has le Coultre, who found twice
as many irregular canals as rectilinear ones. Antoniadi remarks that
some canals appear to be collections of lakes strung out in a certain
direction while others are narrow lines which bend and twist. “The
complicated network of straight lines is probably illusory.” The spots
on Mars, he continues, are very irregular, and “present by no means
any geometrical form” (on which the belief largely is founded that
they are the product of intelligent beings). “The appearance of the
planet reminds one of that of the Moon (except that the latter is dead,
_i. e._, unchangeable) or of a terrestrial landscape viewed from a
balloon.”--“In a word, the ‘geometry’ of Mars is revealed as a pure
illusion.” Exceedingly instructive is a comparison between the two maps
of Mars drawn by Schiaparelli (1886) and by Antoniadi (1909) reproduced
here and found at the end of the volume. While Schiaparelli as a rule
represents the canals as narrow, straight, or slightly curved bands
of uniform width, these formations on the Antoniadi chart frequently
dissolve into a series of dark spots joined by less obscure sections
(see for example the canals Nectar and Oeroe at the Sunlake). The
same is true about several of the so-called “seas,” particularly the
Tyrrhenian (Mare Tyrrhenum), and the Sunlake (Lacus Solis); also about
the “Ocean-bays” such as the well-known Syrtis major which with the
Sunlake form the most conspicuous objects on the surface of Mars. These
maps are, moreover, of great interest because several canals and other
features present on one are absent on the other and vice versa. In this
way, we obtain a vivid conception of the remarkable changeableness of
the Martian surface as contrasted with the exterior of the Earth. The
latter, if viewed from Mars, would not have presented any noticeable
change in historical time except for the seasonal variation of the snow
fields. This peculiarity of Mars is only explained by the fact that the
geographical features of that planet as a rule are surface formations
of a slight depth and therefore subject to rapid transformations.

Frequently, large white spots suddenly appear, especially near the
lakes, such as the spot at Lake Phœnix near the centre of Fig. 24 which
represents Mars on October 6, 1909, according to Antoniadi. These
white spots disappear as suddenly as they show forth. The white colour
is probably due to a very thin snow or hoar-frost, which is easily
condensed in the vicinity of the lakes but which as readily vanishes at
the approach of a warm draft or of sunshine.

Occasionally, dark spots on Mars are described as dissolving under
strong enlargement into dark and light squares giving the appearance
of a chessboard. This reminds one of the bayirs in Turkestan (see Fig.
9).

The collections of lakes along the cracks on Mars which appear to
us as “canals” are repeatedly filled up by sand and dried out. They
are revived through new depressions along the dislocation fissures,
corresponding to our earthquakes, when vapours of water and other
gases pour forth and condense to lakes in the deepest pockets of the
fissures. Canals are therefore created rather rapidly, sometimes over
night, and vanish occasionally as suddenly. The most remarkable case of
“new” canals was made known through a communication by Lowell. Two new
canals, at the time the most conspicuous on the surface of Mars, were
observed east of “Syrtis magna” on September 30, 1909, from Flagstaff
observatory, when they also were photographed, which precludes an
illusion. (On the other hand there was no sign of the great canal
Amenthes, shown on the map Fig. 17, a short distance to the left,
_i. e._, east of Syrtis in the very section where the new canals were
observed.) Also two new oases through which the new slightly curved
canals passed were observed for the first time, as were also a few
minor canals in the neighbourhood.

In 1913, the double canal Æthiops (see map at Long. 240°; the canal is
there single) was rediscovered from the Lowell observatory after an
absence of fifteen years.

These data make it evident that one or possibly several rather strong
earthquakes took place east of Syrtis major just prior to September
30, 1909 with the two oases as centres of collapse. The fissures now
made visible have probably existed before but filled with sand and have
now reappeared as a result of the condensation of water vapour when it
emerged into the cold Martian air.

This fact, that the most prominent canals in such manner now suddenly
appear and now as rapidly vanish, ought to convince us beyond doubt
that they are not magnificent products of engineering skill, for the
construction of which we should require centuries on the Earth.

The theory that intelligent men exist on Mars is very popular. With
its help everything may be explained, particularly if we attribute an
intelligence vastly superior to our own to these beings, so that we
not always are able to fathom the wisdom with which their canals are
constructed. The crossings of the latter are said to be cities (Lowell)
fifty times greater than London. The trouble with these “explanations”
is that they explain anything, and therefore in fact nothing. If we
would endeavour to understand the phenomena on Mars, we must in the
first place avoid the formerly so popular principle of “purposiveness”
which led even the most prominent scientists into so many amusing
errors. Neither may we base our conceptions, as does Flammarion, on the
assumption of natural forces unknown to us, no matter how much such a
course may appeal to mystics. Only forces with which we are familiar
can be resorted to, if we really are to understand nature. It seems to
me that such method of research might with good results be applied also
to the planet Mars.

CHAPTER VII

MERCURY, THE MOON, AND VENUS

The planet Mercury probably resembles Mars in many respects, but
differs particularly in lacking an atmosphere. The fissures in the
crust of the Earth or of Mars are as a rule rapidly filled and their
contours largely hidden from sight by alluvium or sand carried by
windstorms, so that they reveal themselves only through tremors and
various emanations along their course. The fissures on Mercury on
the other hand must remain as yawning chasms. It is probable that
reducing gases stream out of these cracks as on Earth, and colour
the environment in a darker shade than the other visible part of
the planet’s surface, that is the hemisphere turned toward the Sun.
Not very volatile gases, such as sal-ammoniac, other chlorides and
sulphur, which on the Earth are deposited inside the fissures, may
here spread over large areas and discolour surrounding territory,
particularly where iron compounds are present, and, under the attack of
sulphur, turn black. Lowell has made drawings of the dark spots visible
on Mercury, one of which is reproduced as Fig. 25. These spots lie, as
on Antoniadi’s drawing of the surface of Mars (Fig. 17a), arranged in
lines which are almost straight or of a slight curvature only. This
seems to indicate that the spots belong to areas immediately adjoining
enormous fissures. According to Lowell’s drawing, these cracks are
far more regularly distributed on Mercury than on Mars. Very close to
the centre of the ever-sunny side we see a dark spot, a “lake.” It is
evident that this spot is located in by far the hottest point on the
surface of Mercury. This gives rise to the following conception. The
hottest part of Mercury was naturally the last to solidify. Mercury
evidently ceased to rotate around its own axis, leaving one side
continually exposed to the Sun, while its surface yet consisted of
lava that was fluid, at least where the sunshine was most intense. The
weakest point on the planet was therefore the one just opposite the
Sun. When later collapses occurred the cracks commenced at this weak
point. We see on the figure how not less than six fissures radiate
from this centre. Others were formed where the crust broke off from
adjacent solid portions. These latter fissures have a less rectilinear
appearance than those diverging from the centre of collapse. Along
these faults, reducing gases no doubt issue from the interior of the
planet and give a dark tone to the surface layers, which probably
consist of ferruginous dust falling from space. In the neighbourhood
of the Sun, such dust ought to be more plentiful, concentrated as it
were by the gravitation of the Sun. Mercury lies five times nearer
the Sun than the Earth does, and twelve times nearer than Mars. There
probably also exist on Mercury, as on the Moon, large mountains which
are not subject to the wear of running water and blowing sand. We
cannot, however, observe them from the Earth. Possibly they correspond