THE SUPERIOR PLANETS
Having, in a previous chapter, noted the various aspects which an
inferior planet presents to our view, in consequence of its orbit being
nearer to the sun than the orbit of the earth, it will be well here to
consider in the same way the case of a superior planet, and to mark
carefully the difference.
To begin with, it should be quite evident that we cannot ever have a
transit of a superior planet. The orbit of such a body being entirely
_outside_ that of the earth, the body itself can, of course, never pass
between us and the sun.
A superior planet will be at its greatest distance from us when on the
far side of the sun. It is said then to be in _conjunction_. As it comes
round in its orbit it eventually passes, so to speak, at the _back_ of
us. It is then at its nearest, or in _opposition_, as this is
technically termed, and therefore in the most favourable position for
telescopic observation of its surface. Being, besides, seen by us at
that time in the direction of the heavens exactly opposite to where the
sun is, it will thus at midnight be high up in the south side of the
sky, a further advantage to the observer.
Last of all, a superior planet cannot show crescent shapes like an
interior; for whether it be on the far side of the sun, or behind us,
or again to our right or left, the sunlight must needs appear to fall
more or less full upon its face.
THE PLANETOID EROS
The nearest to us of the superior planets is the tiny body, Eros, which,
as has been already stated, was discovered so late as the year 1898. In
point of view, however, of its small size, it can hardly be considered
as a true planet, and the name "planetoid" seems much more appropriate
to it.
Eros was not discovered, like Uranus, in the course of telescopic
examination of the heavens, nor yet, like Neptune, as the direct result
of difficult calculations, but was revealed by the impress of its light
upon a photographic plate, which had been exposed for some length of
time to the starry sky. Since many of the more recent additions to the
asteroids have been discovered in the same manner, we shall have
somewhat more to say about this special employment of photography when
we come to deal with those bodies later on.
The path of Eros around the sun is so very elliptical, or, to use the
exact technical term, so very "eccentric," that the planetoid does not
keep all the time entirely in the space between our orbit and that of
Mars, which latter happens to be the next body in the order of planetary
succession outwards. In portions of its journey Eros, indeed, actually
goes outside the Martian orbit. The paths of the planetoid and of Mars
are, however, _not upon the same plane_, so the bodies always pass clear
of each other, and there is thus as little chance of their dashing
together as there would be of trains which run across a bridge at an
upper level, colliding with those which pass beneath it at a lower
level.
When Eros is in opposition, it comes within about 13-1/2 million miles
of our earth, and, after the moon, is therefore by a long way our
nearest neighbour in space. It is, however, extremely small, not more,
perhaps, than twenty miles in diameter, and is subject to marked
variations in brightness, which do not appear up to the present to meet
with a satisfactory explanation. But, insignificant as is this little
body, it is of great importance to astronomy; for it happens to furnish
the best method known of calculating the sun's distance from our
earth--a method which Galle, in 1872, and Sir David Gill, in 1877,
suggested that asteroids might be employed for, and which has in
consequence supplanted the old one founded upon transits of Venus. The
sun's distance is now an ascertained fact to within 100,000 miles, or
less than half the distance of the moon.
THE PLANET MARS
We next come to the planet Mars. This body rotates in a period of
slightly more than twenty-four hours. The inclination, or slant, of its
axis is about the same as that of the earth, so that, putting aside its
greater distance from the sun, the variations of season which it
experiences ought to be very much like ours.
The first marking detected upon Mars was the notable one called the
Syrtis Major, also known, on account of its shape, as the Hour-Glass
Sea. This observation was made by the famous Huyghens in 1659; and, from
the movement of the marking in question across the disc, he inferred
that the planet rotated on its axis in a period of about twenty-four
hours.
There appears to be very little atmosphere upon Mars, the result being
that we almost always obtain a clear view of the detail on its surface.
Indeed, it is only to be expected from the kinetic theory that Mars
could not retain much of an atmosphere, as the force of gravity at its
surface is less than one-half of what we experience upon the earth. It
should here be mentioned that recent researches with the spectroscope
seem to show that, whatever atmosphere there may be upon Mars, its
density at the surface of the planet cannot be more than the one-fourth
part of the density of the air at the surface of the earth. Professor
Lowell, indeed, thinks it may be more rarefied than that upon our
highest mountain-tops.
Seen with the naked eye, Mars appears of a red colour. Viewed in the
telescope, its surface is found to be in general of a ruddy hue, varied
here and there with darker patches of a bluish-green colour. These
markings are permanent, and were supposed by the early telescopic
observers to imply a distribution of the planet's surface into land and
water, the ruddy portions being considered as continental areas (perhaps
sandy deserts), and the bluish-green as seas. The similarity to our
earth thus suggested was further heightened by the fact that broad white
caps, situated at the poles, were seen to vary with the planet's
seasons, diminishing greatly in extent during the Martian summer (the
southern cap in 1894 even disappearing altogether), and developing again
in the Martian winter.[18] Readers of Oliver Wendell Holmes will no
doubt recollect that poet's striking lines:--
"The snows that glittered on the disc of Mars
Have melted, and the planet's fiery orb
Rolls in the crimson summer of its year."
A state of things so strongly analogous to what we experience here,
naturally fired the imaginations of men, and caused them to look on Mars
as a world like ours, only upon a much smaller scale. Being smaller, it
was concluded to have cooled quicker, and to be now long past its prime;
and its "inhabitants" were, therefore, pictured as at a later stage of
development than the inhabitants of our earth.
Notwithstanding the strong temptation to assume that the whiteness of
the Martian polar caps is due to fallen snow, such a solution is,
however, by no means so simple as it looks. The deposition of water in
the form of snow, or even of hoar frost, would at least imply that the
atmosphere of Mars should now and then display traces of aqueous vapour,
which it does not appear to do.[19] It has, indeed, been suggested that
the whiteness may not after all be due to this cause, but to carbonic
acid gas (carbon dioxide), which is known to freeze at a _very low_
temperature. The suggestion is plainly based upon the assumption that,
as Mars is so much further from the sun than we are, it would receive
much less heat, and that the little thus received would be quickly
radiated away into space through lack of atmosphere to bottle it in.
We now come to those well-known markings, popularly known as the
"canals" of Mars, which have been the subject of so much discussion
since their discovery thirty years ago.
It was, in fact, in the year 1877, when Mars was in opposition, and thus
at its nearest to us, that the famous Italian astronomer, Schiaparelli,
announced to the world that he had found that the ruddy areas, thought
to be continents, were intersected by a network of straight dark lines.
These lines, he reported, appeared in many cases to be of great length,
so long, indeed, as several thousands of miles, and from about twenty to
sixty miles in width. He christened the lines _channels_, the Italian
word for which, "canali," was unfortunately translated into English as
"canals." The analogy, thus accidentally suggested, gave rise to the
idea that they might be actual waterways.[20]
In the winter of 1881-1882, when Mars was again in opposition,
Schiaparelli further announced that he had found some of these lines
doubled; that is to say, certain of them were accompanied by similar
lines running exactly parallel at no great distance away. There was at
first a good deal of scepticism on the subject of Schiaparelli's
discoveries, but gradually other observers found themselves seeing both
the lines and their doublings. We have in this a good example of a
curious circumstance in astronomical observation, namely, the fact that
when fine detail has once been noted by a competent observer, it is not
long before other observers see the same detail with ease.
An immense amount of close attention has been paid to the planet Mars
during recent years by the American observer, Professor Percival Lowell,
at his famous observatory, 7300 feet above the sea, near the town of
Flagstaff, Arizona, U.S.A. His observations have not, like those of most
astronomers, been confined merely to "oppositions," but he has
systematically kept the planet in view, so far as possible, since the
year 1894.
The instrumental equipment of his observatory is of the very best, and
the "seeing" at Flagstaff is described as excellent. In support of the
latter statement, Mr. Lampland, of the Lowell Observatory, maintains
that the faintest stars shown on charts made at the Lick Observatory
with the 36-inch telescope there, are _perfectly visible_ with the
24-inch telescope at Flagstaff.
Professor Lowell is, indeed, generally at issue with the other observers
of Mars. He finds the canals extremely narrow and sharply defined, and
he attributes the blurred and hazy appearance, which they have presented
to other astronomers, to the unsteady and imperfect atmospheric
conditions in which their observations have been made. He assigns to the
thinnest a width of two or three miles, and from fifteen to twenty to
the larger. Relatively to their width, however, he finds their length
enormous. Many of them are 2000 miles long, while one is even as much
as 3540. Such lengths as these are very great in comparison with the
smallness of the planet. He considers that the canals stand in some
peculiar relation to the polar cap, for they crowd together in its
neighbourhood. In place, too, of ill-defined condensations, he sees
sharp black spots where the canals meet and intersect, and to these he
gives the name of "Oases." He further lays particular stress upon a dark
band of a blue tint, which is always seen closely to surround the edges
of the polar caps all the time that they are disappearing; and this he
takes to be a proof that the white material is something which actually
_melts_. Of all substances which we know, water alone, he affirms, would
act in such a manner.
The question of melting at all may seem strange in a planet which is
situated so far from the sun, and possesses such a rarefied atmosphere.
But Professor Lowell considers that this very thinness of the atmosphere
allows the direct solar rays to fall with great intensity upon the
planet's surface, and that this heating effect is accentuated by the
great length of the Martian summer. In consequence he concludes that,
although the general climate of Mars is decidedly cold, it is above the
freezing point of water.
The observations at Flagstaff appear to do away with the old idea that
the darkish areas are seas, for numerous lines belonging to the
so-called "canal system" are seen to traverse them. Again, there is no
star-like image of the sun reflected from them, as there would be, of
course, from the surface of a great sheet of water. Lastly, they are
observed to vary in tone and colour with the changing Martian seasons,
the blue-green changing into ochre, and later on back again into
blue-green. Professor Lowell regards these areas as great tracts of
vegetation, which are brought into activity as the liquid reaches them
from the melting snows.
[Illustration: PLATE XII. A MAP OF THE PLANET MARS
We see here the Syrtis Major (or "Hour-Glass Sea"), the polar caps,
several "oases," and a large number of "canals," some of which are
double. The South is at the top of the picture, in accordance with the
_inverted_ view given by an astronomical telescope. From a drawing by
Professor Percival Lowell.
(Page 216)]
With respect to the canals, the Lowell observations further inform us
that these are invisible during the Martian winter, but begin to appear
in the spring when the polar cap is disappearing. Professor Lowell,
therefore, inclines to the view that in the middle of the so-called
canals there exist actual waterways which serve the purposes of
irrigation, and that what we see is not the waterways themselves, for
they are too narrow, but the fringe of vegetation which springs up along
the banks as the liquid is borne through them from the melting of the
polar snows. He supports this by his observation that the canals begin
to appear in the neighbourhood of the polar caps, and gradually grow, as
it were, in the direction of the planet's equator.
It is the idea of life on Mars which has given this planet such a
fascination in the eyes of men. A great deal of nonsense has, however,
been written in newspapers upon the subject, and many persons have thus
been led to think that we have obtained some actual evidence of the
existence of living beings upon Mars. It must be clearly understood,
however, that Professor Lowell's advocacy of the existence of life upon
that planet is by no means of this wild order. At the best he merely
indulges in such theories as his remarkable observations naturally call
forth. His views are as follows:--He considers that the planet has
reached a time when "water" has become so scarce that the "inhabitants"
are obliged to employ their utmost skill to make their scanty supply
suffice for purposes of irrigation. The changes of tone and colour upon
the Martian surface, as the irrigation produces its effects, are similar
to what a telescopic observer--say, upon Venus--would notice on our
earth when the harvest ripens over huge tracts of country; that is, of
course, if the earth's atmosphere allowed a clear view of the
terrestrial surface--a very doubtful point indeed. Professor Lowell
thinks that the perfect straightness of the lines, and the geometrical
manner in which they are arranged, are clear evidences of artificiality.
On a globe, too, there is plainly no reason why the liquid which results
from the melting of the polar caps should trend at all in the direction
of the equator. Upon our earth, for instance, the transference of water,
as in rivers, merely follows the slope of the ground, and nothing else.
The Lowell observations show, however, that the Martian liquid is
apparently carried from one pole towards the equator, and then past it
to the other pole, where it once more freezes, only to melt again in due
season, and to reverse the process towards and across the equator as
before. Professor Lowell therefore holds, and it seems a strong point in
favour of his theory, that the liquid must, in some artificial manner,
as by pumping, for instance, be _helped_ in its passage across the
surface of the planet.
A number of attempts have been made to explain the _doubling_ of the
canals merely as effects of refraction or reflection; and it has even
been suggested that it may arise from the telescope not being accurately
focussed.
The actual doubling of the canals once having been doubted, it was an
easy step to the casting of doubt on the reality of the canals
themselves. The idea, indeed, was put forward that the human eye, in
dealing with detail so very close to the limit of visibility, may
unconsciously treat as an actual line several point-like markings which
merely happen to lie in a line. In order to test this theory,
experiments were carried out in 1902 by Mr. E.W. Maunder of Greenwich
Observatory, and Mr. J.E. Evans of the Royal Hospital School at
Greenwich, in which certain schoolboys were set to make drawings of a
white disc with some faint markings upon it. The boys were placed at
various distances from the disc in question; and it was found that the
drawings made by those who were just too far off to see distinctly, bore
out the above theory in a remarkable manner. Recently, however, the
plausibility of the _illusion_ view has been shaken by photographs of
Mars taken during the opposition of 1905 by Mr. Lampland at the Lowell
Observatory, in which a number of the more prominent canals come out as
straight dark lines. Further still, in some photographs made there quite
lately, several canals are said to appear visibly double.
Following up the idea alluded to in Chapter XVI., that the moon may be
covered with a layer of ice, Mr. W.T. Lynn has recently suggested that
this may be the case on Mars; and that, at certain seasons, the water
may break through along definite lines, and even along lines parallel to
these. This, he maintains, would account for the canals becoming
gradually visible across the disc, without the necessity of Professor
Lowell's "pumping" theory.
And now for the views of Professor Lowell himself with regard to the
doubling of the canals. From his observations, he considers that no
pairs of railway lines could apparently be laid down with greater
parallelism. He draws attention to the fact that the doubling does not
take place by any means in every canal; indeed, out of 400 canals seen
at Flagstaff, only fifty-one--or, roughly, one-eighth--have at any time
been seen double. He lays great stress upon this, which he considers
points strongly against the duplication being an optical phenomenon. He
finds that the distance separating pairs of canals is much less in some
doubles than in others, and varies on the whole from 75 to 200 miles.
According to him, the double canals appear to be confined to within 40
degrees of the equator: or, to quote his own words, they are "an
equatorial feature of the planet, confined to the tropic and temperate
belts." Finally, he points out that they seem to _avoid_ the blue-green
areas. But, strangely enough, Professor Lowell does not so far attempt
to fit in the doubling with his body of theory. He makes the obvious
remark that they may be "channels and return channels," and with that he
leaves us.
The conclusions of Professor Lowell have recently been subjected to
strenuous criticism by Professor W.H. Pickering and Dr. Alfred Russel
Wallace. It was Professor Pickering who discovered the "oases," and who
originated the idea that we did not see the so-called "canals"
themselves, but only the growth of vegetation along their borders. He
holds that the oases are craterlets, and that the canals are cracks
which radiate from them, as do the rifts and streaks from craters upon
the moon. He goes on to suggest that vapours of water, or of carbonic
acid gas, escaping from the interior, find their way out through these
cracks, and promote the growth of a low form of vegetation on either
side of them. In support of this view he draws attention to the
existence of long "steam-cracks," bordered by vegetation, in the deserts
of the highly volcanic island of Hawaii. We have already seen, in an
earlier chapter, how he has applied this idea to the explanation of
certain changes which are suspected to be taking place upon the moon.
In dealing with the Lowell canal system, Professor Pickering points out
that under such a slight atmospheric pressure as exists on Mars, the
evaporation of the polar caps--supposing them to be formed of
snow--would take place with such extraordinary rapidity that the
resulting water could never be made to travel along open channels, but
that a system of gigantic tubes or water-mains would have to be
employed!
As will be gathered from his theories regarding vegetation, Professor
Pickering does not deny the existence of a form of life upon Mars. But
he will not hear of civilisation, or of anything even approaching it. He
thinks, however, that as Mars is intermediate physically between the
moon and earth, the form of life which it supports may be higher than
that on the moon and lower than that on the earth.
In a small book published in the latter part of 1907, and entitled _Is
Mars Habitable?_ Dr. Alfred Russel Wallace sets himself, among other
things, to combat the idea of a comparatively high temperature, such as
Professor Lowell has allotted to Mars. He shows the immense service
which the water-vapour in our atmosphere exercises, through keeping the
solar heat from escaping from the earth's surface. He then draws
attention to the fact that there is no spectroscopic evidence of
water-vapour on Mars[21]; and points out that its absence is only to be
expected, as Dr. George Johnstone Stoney has shown that it will escape
from a body whose mass is less than one-quarter the mass of the earth.
The mass of Mars is, in fact, much less than this, _i.e._ only
one-ninth. Dr. Wallace considers, therefore, that the temperature of
Mars ought to be extremely low, unless the constitution of its
atmosphere is very different from ours. With regard to the latter
statement, it should be mentioned that the Swedish physicist, Arrhenius,
has recently shown that the carbonic acid gas in our atmosphere has an
important influence upon climate. The amount of it in our air is, as we
have seen, extremely small; but Arrhenius shows that, if it were
doubled, the temperature would be more uniform and much higher. We thus
see how futile it is, with our present knowledge, to dogmatise on the
existence or non-existence of life in other celestial orbs.
As to the canals Dr. Wallace puts forward a theory of his own. He
contends that after Mars had cooled to a state of solidity, a great
swarm of meteorites and small asteroids fell in upon it, with the result
that a thin molten layer was formed all over the planet. As this layer
cooled, the imprisoned gases escaped, producing vents or craterlets; and
as it attempted to contract further upon the solid interior, it split in
fissures radiating from points of weakness, such, for instance, as the
craterlets. And he goes on to suggest that the two tiny Martian
satellites, with which we shall deal next, are the last survivors of his
hypothetical swarm. Finally, with regard to the habitability of Mars,
Dr. Wallace not only denies it, but asserts that the planet is
"absolutely uninhabitable."
For a long time it was supposed that Mars did not possess any
satellites. In 1877, however, during that famous opposition in which
Schiaparelli first saw the canals, two tiny satellites were discovered
at the Washington Observatory by an American astronomer, Professor Asaph
Hall. These satellites are so minute, and so near to the planet, that
they can only be seen with very large telescopes; and even then the
bright disc of the planet must be shielded off. They have been
christened Phobos and Deimos (Fear and Dread); these being the names of
the two subordinate deities who, according to Homer, attended upon Mars,
the god of war.
It is impossible to measure the exact sizes of these satellites, as they
are too small to show any discs, but an estimate has been formed from
their brightness. The diameter of Phobos was at first thought to be six
miles, and that of Deimos, seven. As later estimates, however,
considerably exceed this, it will, perhaps, be not far from the truth to
state that they are each roughly about the size of the planetoid Eros.
Phobos revolves around Mars in about 7-1/2 hours, at a distance of about
only 4000 miles from the planet's surface, and Deimos in about 30 hours,
at a distance of about 12,000 miles. As Mars rotates on its axis in
about 24 hours, it will be seen that Phobos makes more than three
revolutions while the planet is rotating once--a very interesting
condition of things.
A strange foreshadowing of the discovery of the satellites of Mars will
be familiar to readers of _Gulliver's Travels_. According to Dean
Swift's hero, the astronomers on the Flying Island of Laputa had found
two tiny satellites to Mars, one of which revolved around the planet in
ten hours. The correctness of this guess is extraordinarily close,
though at best it is, of course, nothing more than a pure coincidence.
It need not be at all surprising that much uncertainty should exist with
regard to the actual condition of the surface of Mars. The circumstances
in which we are able to see that planet at the best are, indeed, hardly
sufficient to warrant us in propounding any hard and fast theories. One
of the most experienced of living observers, the American astronomer,
Professor E.E. Barnard, considers that the view we get of Mars with the
best telescope may be fairly compared with our naked eye view of the
moon. Since we have seen that a view with quite a small telescope
entirely alters our original idea of the lunar surface, a slight
magnification revealing features of whose existence we had not
previously the slightest conception, it does not seem too much to say
that a further improvement in optical power might entirely subvert the
present notions with regard to the Martian canals. Therefore, until we
get a still nearer view of these strange markings, it seems somewhat
futile to theorise. The lines which we see are perhaps, indeed, a
foreshortened and all too dim view of some type of formation entirely
novel to us, and possibly peculiar to Mars. Differences of gravity and
other conditions, such as obtain upon different planets, may perhaps
produce very diverse results. The earth, the moon, and Mars differ
greatly from one another in size, gravitation, and other such
characteristics. Mountain-ranges so far appear typical of our globe, and
ring-mountains typical of the moon. May not the so-called "canals" be
merely some special formation peculiar to Mars, though quite a natural
result of its particular conditions and of its past history?
THE ASTEROIDS (OR MINOR PLANETS)
We now come to that belt of small planets which are known by the name of
asteroids. In the general survey of the solar system given in Chapter
II., we saw how it was long ago noticed that the distances of the
planetary orbits from the sun would have presented a marked appearance
of orderly sequence, were it not for a gap between the orbits of Mars
and Jupiter where no large planet was known to circulate. The suspicion
thus aroused that some planet might, after all, be moving in this
seemingly empty space, gave rise to the gradual discovery of a great
number of small bodies; the largest of which, Ceres, is less than 500
miles in diameter. Up to the present day some 600 of these bodies have
been discovered; the four leading ones, in order of size, being named
Ceres, Pallas, Juno, and Vesta. All the asteroids are invisible to the
naked eye, with the exception of Vesta, which, though by no means the
largest, happens to be the brightest. It is, however, only just visible
to the eye under favourable conditions. No trace of an atmosphere has
been noted upon any of the asteroids, but such a state of things is only
to be expected from the kinetic theory.
For a good many years the discoveries of asteroids were made by means of
the telescope. When, in the course of searching the heavens, an object
was noticed which did not appear upon any of the recognised star charts,
it was kept under observation for several nights to see whether it
changed its place in the sky. Since asteroids move around the sun in
orbits, just as planets do, they, of course, quickly reveal themselves
by their change of position against the starry background.
The year 1891 started a new era in the discovery of asteroids. It
occurred to the Heidelberg observer, Dr. Max Wolf, one of the most
famous of the hunters of these tiny planets, that photography might be
employed in the quest with success. This photographic method, to which
allusion has already been made in dealing with Eros, is an extremely
simple one. If a photograph of a portion of the heavens be taken through
an "equatorial"--that is, a telescope, moving by machinery, so as to
keep the stars, at which it is pointed, always exactly in the field of
view during their apparent movement across the sky--the images of these
stars will naturally come out in the photograph as sharply defined
points. If, however, there happens to be an asteroid, or other planetary
body, in the same field of view, its image will come out as a short
white streak; because the body has a comparatively rapid motion of its
own, and will, during the period of exposure, have moved sufficiently
against the background of the stars to leave a short trail, instead of a
dot, upon the photographic plate. By this method Wolf himself has
succeeded in discovering more than a hundred asteroids (see Plate XIII.,
p. 226). It was, indeed, a little streak of this kind, appearing upon a
photograph taken by the astronomer Witt, at Berlin, in 1898, which first
informed the world of the existence of Eros.
[Illustration: PLATE XIII. MINOR PLANET TRAILS
Two trails of minor planets (asteroids) imprinted _at the same time_
upon one photographic plate. In the white streak on the left-hand side
of the picture we witness the _discovery_ of a new minor planet. The
streak on the right was made by a body already known--the minor planet
"Fiducia." This photograph was taken by Dr. Max Wolf, at Heidelberg, on
the 4th of November, 1901, with the aid of a 16-inch telescope. The time
of exposure was two hours.
(Page 227)]
It has been calculated that the total mass of the asteroids must be
much less than one-quarter that of the earth. They circulate as a rule
within a space of some 30,000,000 miles in breadth, lying about midway
between the paths of Mars and Jupiter. Two or three, however, of the
most recently discovered of these small bodies have been found to pass
quite close to Jupiter. The orbits of the asteroids are by no means in
the one plane, that of Pallas being the most inclined to the plane of
the earth's orbit. It is actually three times as much inclined as that
of Eros.
Two notable theories have been put forward to account for the origin of
the asteroids. The first is that of the celebrated German astronomer,
Olbers, who was the discoverer of Pallas and Vesta. He suggested that
they were the fragments of an exploded planet. This theory was for a
time generally accepted, but has now been abandoned in consequence of
certain definite objections. The most important of these objections is
that, in accordance with the theory of gravitation, the orbits of such
fragments would all have to pass through the place where the explosion
originally occurred. But the wide area over which the asteroids are
spread points rather against the notion that they all set out originally
from one particular spot. Another objection is that it does not appear
possible that, within a planet already formed, forces could originate
sufficiently powerful to tear the body asunder.
The second theory is that for some reason a planet here failed in the
making. Possibly the powerful gravitational action of the huge body of
Jupiter hard by, disturbed this region so much that the matter
distributed through it was never able to collect itself into a single
mass.
[18] Sir William Herschel was the first to note these polar changes.
[19] Quite recently, however, Professor Lowell has announced that his
observer, Mr. E.C. Slipher, finds with the spectroscope faint traces of
water vapour in the Martian atmosphere.
[20] In a somewhat similar manner the term "crater," as applied to the
ring-mountain formation on the moon, has evidently given a bias in
favour of the volcanic theory as an explanation of that peculiar
structure.
[21] Mr. Slipher's results (see note 2, page 213) were not then known.