A LUNAR ATMOSPHERE

At the close of the preceding chapter we stated that any force acting in
opposition to that of gravity would be six times more effective on the
moon than on the earth. But, in fact, it would in many cases be still
more so; at all events, so far as projectile forces are concerned; for
the reason that “the powerful coercer of projectile range,” as the
earth’s atmosphere has been termed, has no counterpart, or at most a
very disproportionate one, upon the moon.

The existence of an atmosphere surrounding the moon has been the subject
of considerable controversy, and a great deal of evidence on both sides
of the question has been offered from time to time, and is to be found
scattered through the records of various classes of observations. Some
of the more important items of this evidence it is our purpose to set
forth in the course of the present chapter.

With the phenomena of the terrestrial atmosphere, with the effects that
are attributable to it, we are all well familiar, and our best course
therefore is to examine, as far as we are able, whether counterparts of
any of these effects are manifested upon the moon. For instance, the
clouds that are generated in and float through our air would, to an
observer on the moon, appear as ever changing bright or dusky spots,
obliterating certain of the permanent details of the earth’s surface,
and probably skirting the terrestrial disc, like the changing belts we
perceive on the planet Jupiter, or diversifying its features with less
regularity, after the manner exhibited by the planet Mars. If such
clouds existed on the moon it is evident that the details of its surface
must be, from time to time, similarly obscured; but no trace of such
obscuration has ever been detected. When the moon is observed with high
telescopic powers, all its details come out sharp and clear, without the
least appearance of change or the slightest symptoms of cloudiness other
than the occasional want of general definition, which may be proved to
be the result of unsteadiness or want of homogeneity in our own
atmosphere; for we must tell the uninitiated that nights of pure, good
definition, such as give the astronomer opportunity of examining with
high powers the minute details of planetary features, are very few and
far between. Out of the three hundred and sixty-five nights of a year
there are probably not a dozen that an astronomer can call really fine:
usually, even on nights that are to all common appearance superbly
brilliant, some strata of air of different densities or temperatures, or
in rapid motion, intervene between the observer and the object of his
observation, and through these, owing to the ever-changing refractions
which the rays of light coming from the object suffer in their course,
observation of the delicate markings of a planet is impossible: all is
blurred and confused, and nothing but bolder features can be recognized.
It has in consequence sometimes happened that a slight indistinctness of
some minute detail of the moon has been attributed to clouds or mists at
the lunar surface, whereas the real cause has been only a bad condition
of our own atmosphere. It may be confidently asserted that when all
indistinctness due to terrestrial causes is taken account of or
eliminated, there remain no traces whatever of any clouds or mists upon
the surface of the moon.

This is but one proof against the existence of a lunar atmosphere, and,
it may be argued, not a very conclusive one; because there may still be
an atmosphere, though it be not sufficiently aqueous to condense into
clouds and not sufficiently dense to obscure the lunar details. The
probable existence of an atmosphere of such a character used to be
inferred from a phenomenon seen during total eclipses of the sun. On
these occasions the black body of the moon is invariably surrounded by a
luminous halo, or glory, to which the name “corona” has been applied;
and, further, besides this corona, apparently floating in it and
sometimes seemingly attached to the black edge of the moon, are seen
masses of cloud-like matter of a bright red colour, which, from the form
in which they were first seen and from their flame-like tinge, have
become universally known as the “red-flames.” It used to be said that
this corona could only be the consequence of a lunar atmosphere lit up
as it were by the sun’s rays shining through it, after the manner of a
sunbeam lighting up the atmosphere of a dusty chamber; and the red
flames were held by those who first observed them to be clouds of denser
matter floating in the said atmosphere, and refracting the red rays of
solar light as our own clouds are seen to do at sunrise and sunset. But
the evidence obtained, both by simple telescopic observation and by the
spectroscope, from recent extensively observed eclipses of the sun has
set this question quite at rest; for it has been settled finally and
indisputably that both the above appearances pertain to the sun, and
have nothing whatever to do with the moon.

[Illustration: Fig. 11.]

The occurrence of a solar eclipse offers other means in addition to the
foregoing whereby a lunar atmosphere would be detected. We know that all
gases and vapours absorb some portion of any light which may shine
through them. If then our satellite had an atmosphere, its black nucleus
when seen projected against the bright sun in an eclipse would be
surrounded by a sort of penumbra, or zone of shadow, in contact with its
edge, somewhat like that we have shown in an exaggerated degree in the
annexed cut (Fig. 11), and the passage of this penumbra over solar spots
and other features of the solar photosphere would to some extent obscure
the more minute details of such features. No such dusky band has however
been at any time observed. On the contrary, a band somewhat brighter
than the general surface of the sun has frequently been seen in contact
with the black edge of the moon: this in its turn was held to indicate
an atmosphere about the moon; but Sir George Airy has shown that a lunar
atmosphere, if it really did exist, could not produce such an
appearance, and that the cause of it must be sought in other directions.
If this effect were really due to the passage of the solar rays through
a lunar atmosphere a similar effect ought to be produced by the passage
of the sun’s rays through the terrestrial atmosphere: and we might hence
expect to see the shadow of the earth projected on the moon during a
lunar eclipse surrounded by a sort of bright zone or halo: we need
hardly say such an appearance has never manifested itself. Similarly as
we stated that the delicate details of solar spots would be obscured by
a lunar atmosphere, small stars passing behind the moon would suffer
some diminution in brightness as they approached apparent contact with
the moon’s edge: this fading has been watched for on many occasions, and
in a few cases such an appearance has been suspected, but in by far the
majority of instances nothing like a diminution of brightness or change
of colour of the stars has been seen; stars of the smallest magnitude
visible under such circumstances retain their feeble lustre unimpaired
up to the moment of their disappearance behind the moon’s limb.

Again, in a solar eclipse, even if there were an atmosphere about the
moon not sufficiently dense to form a hazy outline or impair the
distinctness of the details of a solar spot, it would still manifest its
existence in another way. As the moon advances upon the sun’s disc the
latter assumes, of course, a crescent form. Now if air or vapour
enveloped the moon, the exceedingly delicate cusps of this crescent
would be distorted or turned out of shape. Instead of remaining
symmetrical, like the lower one in the annexed drawing (Fig. 12), they
would be bent or deformed after the manner we have shown in the upper
one. The slightest symptom of a distortion like this could not fail to
obtrude itself upon an observer’s eye; but in no instance has anything
of the kind been seen.

Reverting to the consequences of the terrestrial atmosphere: one of the
most striking of these is the phenomenon of diffused daylight, which we
need hardly remind the reader is produced by the scattering or diffusion
of the sun’s rays among the minute particles of vapour composing or
contained in that atmosphere. Were it not for this reflexion and
diffusion of the sun’s light, those parts of our earth not exposed to
direct sunshine would be hidden in darkness, receiving no illumination
beyond the feeble amount that might be reflected from proximate
terrestrial objects actually illuminated by direct sunlight. Twilight is
a consequence of this reflexion of light by the atmosphere when the sun
is below the horizon. If, then, an atmosphere enveloped the moon, we
should see by diffused light those parts of the lunar details that are
not receiving the direct solar beams; and before the sun rose and after
it had set upon any region of the moon, that region would still be
partially illuminated by a twilight. But, on the contrary, the shadowed
portions of a lunar landscape are pitchy black, without a trace of
diffused-light illumination, and the effects that a twilight would
produce are entirely absent from the moon. Once, indeed, one observer,
Schroeter, noticed something which he suspected was due to an effect of
this kind: when the moon exhibited itself as a very slender crescent, he
discovered a faint crepuscular light, extending from each of the cusps
along the circumference of the unenlightened part of the disc, and he
inferred from estimates of the length and breadth of the line of light
that there was an atmosphere about the moon of 5376 feet in height. This
is the only instance on record, we believe, of such an appearance being
seen.

[Illustration: Fig. 12.]

Spectrum analysis would also betray the existence of a lunar atmosphere.
The solar rays falling on the moon are reflected from its surface to the
earth. If, then, an atmosphere existed, it is plain that the solar rays
must first pass through such atmosphere to reach the reflecting surface,
and returning from thence, again pass through it on their way to the
earth; so that they must in reality pass through virtually twice the
thickness of any atmosphere that may cover the moon. And if there be any
such atmosphere, the spectrum formed by the moon’s light, that is, by
the sun’s light reflected from the moon, would be modified in such a
manner as to exhibit absorption-lines different from those found in the
spectrum of the direct solar rays, just as the absorption-lines vary
according as the sun’s rays have to pass through a thinner or a denser
stratum of the terrestrial atmosphere. Guided by this reasoning, Drs.
Huggins and Miller made numerous observations upon the spectrum of the
moon’s light, which are detailed in the “Philosophical Transactions” for
the year 1864; and their result, quoting the words of the report, was
“that the spectrum analysis of the light reflected from the moon is
wholly negative as to the existence of any considerable lunar
atmosphere.”

Upon another occasion, Dr. Huggins made an analogous observation of the
spectrum of a star at the moment of its occultation, which observation
he records in the following words:—“When an observation is made of the
spectrum of a star a little before, or at the moment of its occultation
by the dark limb of the moon, several phenomena characteristic of the
passage of the star’s light through an atmosphere might possibly present
themselves to the observer. If a lunar atmosphere exist, which either by
the substances of which it is composed, or by the vapours diffused
through it, can exert a selective absorption upon the star’s light, this
absorption would be indicated to us by the appearance in the spectrum of
new dark lines immediately before the star is occulted by the moon.”

“If finely divided matter, aqueous or otherwise, were present about the
moon, the red rays of the star’s light would be enfeebled in a smaller
degree than the rays of higher refrangibilities.”

“If there be about the moon an atmosphere free from vapour, and
possessing no absorptive power, but of some density, then the spectrum
would not be extinguished by the moon’s limb at the same instant
throughout its length. The violent and blue rays would lay behind the
red rays.”

“I carefully observed the disappearance of the spectrum of η Piscium at
its occultation of January 4, 1865, for these phenomena; but no signs of
a lunar atmosphere were detected.”

But perhaps the strongest evidence of the non-existence of any
appreciable lunar atmosphere is afforded by the non-refraction of the
light of a star passing behind the edge of the lunar disc. Refraction,
we know, is a bending of the rays of light coming from any object,
caused by their passage through strata of transparent matter of
different densities; we have a familiar example in the apparent bending
of a stick when half plunged into water. There is a simple schoolboy’s
experiment which illustrates refraction in a very cogent manner, but
which we should, from its very simplicity, hesitate to recall to the
reader’s mind did it not very aptly represent the actual case we wish to
exemplify. A coin is placed on the bottom of an empty basin, and the eye
is brought into such a position that the coin is just hidden behind the
basin’s rim. Water is then poured into the basin and, without the eye
being moved from its former place, as the depth of water increases, the
coin is brought by degrees fully into view; the water refracting or
turning out of their course the rays of light coming from the coin, and
lifting them, as it were, over the edge of the basin. Now a perfectly
similar phenomenon takes place at every sunrise and sunset on the earth.
When the sun is really below the horizon, it is nevertheless still
visible to us because it is _brought up_ by the refraction of its light
by the dense stratum of atmosphere through which the rays have to pass.
The sun is, therefore, exactly represented by the coin at the bottom of
the basin in the boy’s experiment, the atmosphere answers to the water,
and the horizon to the rim or edge of the basin. If there were no
atmosphere about the earth, the sun would not be so brought up above the
horizon, and, as a consequence, it would set earlier and rise later by
about a minute than it really does. This, of course, applies not merely
to the sun, but to all celestial bodies that rise and set. Every planet
and every star remains a shorter time below the horizon than it would if
there were no atmosphere surrounding the earth.

To apply this to the point we are discussing. The moon in her orbital
course across the heavens is continually passing before, or occulting,
some of the stars that so thickly stud her apparent path. And when we
see a star thus pass behind the lunar disc on one side and come out
again on the other side, we are virtually observing the setting and
rising of that star upon the moon. If, then, the moon had an atmosphere,
it is clear, from analogy to the case of the earth, that the star must
disappear later and reappear sooner than if it has no atmosphere: just
as a star remains too short a time below the earth’s horizon, or behind
the earth, in consequence of the terrestrial atmosphere, so would a star
remain too short a time behind the moon if an atmosphere surrounded that
body. The point is settled in this way:—The moon’s apparent diameter has
been measured over and over again and is known with great accuracy; the
rate of her motion across the sky is also known with perfect accuracy:
hence it is easy to calculate how long the moon will take to travel
across a part of the sky exactly equal in length to her own diameter.
Supposing, then, that we observe a star pass behind the moon and out
again, it is clear that, if there be no atmosphere, the interval of time
during which it remains occulted ought to be exactly equal to the
computed time which the moon would take to pass over the star. If,
however, from the existence of a lunar atmosphere, the star disappears
too late and reappears too soon, as we have seen it would, these two
intervals will not agree; the computed time will be greater than the
observed time, and the difference, if any there be, will represent the
amount of refraction the star’s light has sustained or suffered, and
hence the extent of atmosphere it has had to pass through.

Comparisons of these two intervals of time have been repeatedly made,
the most recent and most extensive was executed under the direction of
the Astronomer-Royal several years ago, and it was based upon no less
than 296 occultation observations. In this determination the measured or
telescopic semidiameter of the moon was compared with the semidiameter
deduced from the occultations, upon the above principle, and it was
found that the telescopic semidiameter was greater than the occultation
semidiameter by two seconds of angular measurement or by about a
thousandth part of the whole diameter of the moon. Sir George Airy,
commenting on this result, says that it appears to him that the origin
of this difference is to be sought in one of two causes. “Either it is
due to irradiation[3] of the telescopic semidiameter, and I do not doubt
that a part at least of the two seconds is to be ascribed to that cause;
or it may be due to refraction by the moon’s atmosphere. If the whole
two seconds were caused by atmospheric refraction this would imply a
horizontal refraction of one second, which is only 1/2000 part of the
earth’s horizontal refraction. It is possible that an atmosphere
competent to produce this refraction would not make itself visible in
any other way.” This result accords well, considering the relative
accuracy of the means employed, with that obtained a century ago by the
French astronomer Du Séjour, who made a rigorous examination of the
subject founded on observations of the solar eclipse of 1764. He
concluded that the horizontal refraction produced by a possible lunar
atmosphere amounted to 1″·5—a second and a half—or about 1/1400 of that
produced by the earth’s atmosphere. The greater weight is of course to
be allowed to the more recent determination in consideration of the
large number of accurate observations upon which it was based.

But an atmosphere 2,000 times rarer than our air can scarcely be
regarded as an atmosphere at all. The contents of an air-pump receiver
can seldom be rarefied to a greater extent than to about 1/1000 of the
density of air at the earth’s surface, with the best of pneumatic
machines; and the lunar atmosphere, if it exist at all, is thus proved
to be twice as attenuated as what we are accustomed to recognise as a
vacuum. In discussing the physical phenomena of the lunar surface, we
are, therefore, perfectly justified in omitting all considerations of an
atmosphere, and adapting our arguments to the non-existence of such an
appendage.

And if there be no air upon the moon, we are almost forced to conclude
that there can be no water; for if water covered any part of the lunar
globe it must be vapourised under the influence of the long period of
uninterrupted sunshine (upwards of 300 hours) that constitutes the lunar
day, and would manifest itself in the form of clouds or mists obscuring
certain parts of the surface. But, as we have already said, no such
obliteration of details ever takes place; and, as we have further seen,
no evidence of aqueous vapour is manifested upon the occasion of
spectrum observations. Since, then, the effects of watery vapour are
absent, we are forced to conclude that the cause is absent also.

Those parts of the moon which the ancient astronomers assumed, from
their comparatively smooth and dusky appearance, to be seas, have long
since been discovered to be merely extensive regions of less reflective
surface material; for the telescope reveals to us irregularities and
asperities covering well nigh the whole of them, which asperities could
not be seen if they were covered with water; unless, indeed, we admit
the possibility of seeing to the bottom of the water, not only
perpendicularly, but obliquely. Some observers have noticed features
that have led them to suppose that water was at one time present upon
the moon, and has left its traces in the form of appearances of erosive
action in some parts. But if water ever existed, where is it now? One
writer, it is true, has suggested as possible, that whatever air, and we
presume he would include whatever water also, the moon may possess, is
hidden away in sublunarean caves and hollows; but even if water existed
in these places it must sometimes assume the vapoury form, and thus make
its presence known.

[Illustration: Fig. 13.]

[Illustration: Fig. 14.]

Sir John Herschel pointed out that if any moisture exists upon the moon,
it must be in a continual state of migration from the illuminated or
hot, to the unilluminated or cold side of the lunar globe. The
alternations of temperature, from the heat produced by the unmitigated
sunshine of 14 days’ duration, to the intensity of cold resulting from
the absence of any sunshine whatever for an equal period, must, he
argued, produce an action similar to that of the _cryophorus_ in
transporting the lunar moisture from one hemisphere to the other. The
cryophorus is a little instrument invented by the late Dr. Wollaston; it
consists of two bulbs of glass connected by a bent tube, in the manner
shown in the annexed illustration, fig. 13. One of the bulbs, A, is
half-filled with water, and, all air being exhausted, the instrument is
hermetically sealed, leaving nothing within but the water and the
aqueous vapour which rises therefrom in the absence of atmospheric
pressure. When the empty bulb, B, is placed in a freezing mixture, a
rapid condensation of this vapour takes place within it, and as a
consequence the water in the bulb A gives off more vapour. The
abstraction of heat from the water, which is a natural consequence of
this evaporation, causes it to freeze into a solid mass of ice. Now upon
the moon the same phenomenon would occur did the material exist there to
supply it. In the accompanying diagram let A represent the illuminated
or heated hemisphere of the moon, and B the dark or cold hemisphere; the
former being probably at a temperature of 300° above, and the latter
200° below Fahrenheit’s zero. Upon the above principle, if moisture
existed upon A it would become vapourised, and the vapour would migrate
over to B, and deposit itself there as hoarfrost; it would, therefore,
manifest itself to us while in the act of migrating by clouding or
dimming the details about the boundary of the illuminated hemisphere.
The sun, rising upon any point upon the margin of the dark hemisphere,
would have to shine through a bed of moisture, and we may justly
suppose, if this were the case, that the tops of mountains catching the
first beams of sunlight would be tinged with colour, or be lit up at
first with but a faint illumination, just as we see in the case of
terrestrial mountains whose summits catch the first, or receive the last
beams of the rising or setting sun. Nothing of this kind is, however,
perceptible: when the solar rays tip the lofty peaks of lunar mountains,
these shine at once with brilliant light, quite as vivid as any of those
parts that receive less horizontal illumination, or upon which the sun
is almost perpendicularly shining.

All the evidence, then, that we have the means of obtaining, goes to
prove that neither air nor water exist upon the moon. Two complicating
elements affecting all questions relating to the geology of the
terraqueous globe we inhabit may thus be dismissed from our minds while
considering the physical features of the lunar surface. Fire on the one
hand and water or the other, are the agents to which the configurations
of the earth’s surface are referrable: the first of these produced the
igneous rocks that form the veritable foundations of the earth, the
second has given rise to the superstructure of deposits that constitute
the secondary and tertiary formations: were these last removed from the
surface of our planet, so as to lay bare its original igneous crust,
that crust, so far as reasoning can picture it to us, would probably not
differ essentially from the visible surface of the moon. In considering
the causes that have given birth to the diversified features of that
surface, we may, therefore, ignore the influence of air and water action
and confine our reasoning to igneous phenomena alone: our task in this
matter, it is hardly necessary to remark, is materially simplified
thereby.

We have now reached that stage of our subject at which it behoves us to
repair to the telescope for the purpose of examining and familiarising
ourselves with the various classes of detail that the lunar surface
presents to our view.

That the moon is not a smooth sphere of matter is a fact that manifested
itself to the earliest observers. The naked eye perceives on her face
spots exhibiting marked differences of illumination. These variations of
light and shade, long before the invention of the telescope, induced the
belief that she possessed surface irregularities like those that
diversify the face of the earth, and from analogy it was inferred that
seas and continents alternated upon the lunar globe. It was evident,
from the persistence and invariability of the dusky markings, that they
were not due to atmospheric peculiarities, but were veritable variations
in the character or disposition of the surface material. Fancy made
pictures of these unchangeable spots: untutored gazers detected in them
the indications of a human countenance, and perhaps the earliest map of
the moon was a rough reproduction of a man’s face, the eyes, nose and
mouth representing the more salient spots discernible upon the lunar
disc. Others recognised in these spots the configuration of a human
form, head, arms and legs complete, which a French superstition that
lingers to the present day held to be the image of Judas Iscariot
transported to the moon in punishment for his treason. Again, an Indian
notion connects the lunar spots with a representation of a roebuck or a
hare, and hence the Sanskrit names for the moon, _mrigadhara_, a
roebuck-bearer, and _’sa’sabhrit_, a hare-bearer. Of these similitudes
the one which has the best pretensions to a rude accuracy is that first
mentioned; for the resemblance of the full moon to a human countenance,
wearing a painful or lugubrious expression, is very striking. Our
illustration of the full moon (Plate III.) is derived from an actual
photograph;[4] the relative intensities of light and shade are hence
somewhat exaggerated; otherwise it represents the full moon very nearly
as the naked eye sees it, and by gazing at the plate from a short
distance,[5] the well-known features will manifest themselves, while
they who choose may amuse themselves by arranging the markings in their
imagination till they conform to the other appearances alluded to.

We may remark in passing that by one sect of ancient writers the moon
was supposed to be a kind of mirror, receiving the image of the earth
and reflecting it back to terrestrial spectators. Humboldt affirmed that
this opinion had been preserved to his day as a popular belief among the
people of Asia Minor. He says, “I was once very much astonished to hear
a very well educated Persian from Ispahan, who certainly had never read
a Greek book, mention when I showed him the moon’s spots in a large
telescope in Paris, this hypothesis as a widely diffused belief in his
country: ‘What we see in the moon,’ said the Persian, ‘is ourselves; it
is the map of our earth.’” Quite as extravagant an idea, though perhaps
a more excusable one, was that held by some ancient philosophers, to the
effect that the spots on the moon were the shadows of opaque bodies
floating in space between it and the sun.

[Illustration: PLATE III.
FULL MOON.]

An observer watching the forms and positions of the lunar face-marks,
from night to night and from lunation to lunation, cannot fail to notice
the circumstance that they undergo no easily perceptible change of
position with respect to the circular outline of the disc; that in fact
the face of the moon presented to our view is always the same, or very
nearly so. If the moon had no orbital motion we should be led from the
above phenomenon to conclude that she had no axial motion, no movement
of rotation; but when we consider the orbital motion in connection with
the permanence of aspect, we are driven to the conclusion—one, however,
which superficial observers have some difficulty in recognising—that the
moon has an axial rotation equal in period to her orbital revolution.
Since the moon makes the circuit of her orbit in twenty-seven days and
one-third (more exactly 27d. 7h. 43m. 11s.) it follows that this is the
time of her axial rotation, as referred to the stars, or as it would be
made out by an observer located at a fixed position in space outside the
lunar orbit. But if referred to the sun this period appears different;
because the moon while revolving round the earth is, with the earth,
circulating around the sun. Suppose the three bodies, moon, earth, and
sun, to be in a line at a certain period of a lunation, as they are at
full moon: by the time the moon has completed her twenty-seven days’
journey around the earth, the latter will have moved along twenty-seven
days’ march of its orbit, which is about twenty-seven degrees of
celestial longitude: the sun will apparently be that much distant from a
straight line passing through earth and moon, and the moon must
therefore move forward to overtake the sun before she can assume the
full phase again. She will take something over two days to do this;
hence the solar period of her revolution becomes more than twenty-nine
days (to be exact, 29d. 12h. 44m. 2s. ·87). This is the length of a
solar day upon the moon—the interval from one sunrise to another at any
spot upon the equator of our satellite, and the interval between
successive reappearances of the same phase to observers on the earth.
The physical cause of the coincidence of times of rotation and
revolution was touched upon in a previous chapter.

We have said that the moon continuously presents to us the same
hemisphere. This is generally true, but not entirely so. Galileo, by
long scrutiny, familiarised himself with every detail of the lunar-disc
that came within the limited grasp of his telescopes, and he recognised
the fact that according as the position of the moon varied in the sky,
so the aspect of her face altered to a slight degree; that certain
regions at the edge of her disc, alternately came in sight and receded
from his view. He perceived, in fact, an _apparent_ rocking to and fro
of the globe of the moon; a sort of balancing or _libratory_ motion.
When the moon was near the horizon he could see spots upon her uppermost
edge, which disappeared as she approached the zenith, or highest point
of her nightly path; and as she neared this point, other spots, before
invisible, came into view, near to what had been her lower edge. Galileo
was not long in referring this phenomenon to its true cause. The centre
of motion of the moon being the centre of the earth, it is clear that an
observer on the surface of the latter, looks down upon the rising moon
as from an eminence, and thus he is enabled to see more or less over or
around her. As the moon increases in altitude, the line of sight
gradually becomes parallel to the line joining the observer and the
centre of the earth, and at length he looks her full in the face: he
loses the full view and catches another side face view as she nears the
horizon in setting. This phenomenon, occurring as it does, with a daily
period, is known as the _diurnal libration_.

But a kindred phenomenon presents itself in another period, and from
another cause. The moon rotates upon her axis at a speed that is
rigorously uniform. But her orbital motion is not uniform, sometimes it
is faster, and at other times slower than its average rate. Hence, the
angle through which she moves along her orbit in a given time, now
exceeds, and now falls short of the angle through which she turns upon
her axis. Her visible hemisphere thus changes to an extent depending
upon the difference between these orbital and axial angles, and the
apparent balancing thus produced is called the _libration in longitude_.
Then there is a _libration in latitude_ due to the circumstance that the
axis of the moon is not exactly perpendicular to the plane of her orbit;
the effect of this inclination being, that we sometimes see a little
more of the north than of the south polar regions of our satellite, and
_vice versâ_.[6]

The extent of the moon’s librations, taking them all and in combination
into account, amounts to about seven degrees of arc of latitude or
longitude upon the moon, both in the north-south and east-west
directions. And taking into account the whole effect of them, we may
conclude that our view of the moon’s surface, instead of being confined
to one half, is extended really to about four-sevenths of the whole area
of the lunar globe. The remaining three-sevenths must for ever remain a
_terra incognita_ to the habitants of this earth, unless, indeed, from
some catastrophe which it would be wild fancy to anticipate, a period of
rotation should be given to the moon different from that which it at
present possesses. Some highly fanciful theorists have speculated upon
the possible condition of the invisible hemisphere, and have propounded
the absurd notion that the opposite side of the moon is hollow, or that
the moon is a mere shell; others again have urged that the hidden half
is more or less covered with water, and others again that it is peopled
with inhabitants. There is, however, no good reason for supposing that
what we may call the back of the moon has a physical structure
essentially different from the face presented towards us. So far as can
be judged from the peeps that libration enables us to obtain, the same
characteristic features (though of course with different details)
prevail over the whole lunar surface.

The speculative ideas held by the philosophers of the pre-telescopic
age, touching the causes which produced the inequalities of light and
shade upon the moon, received their _coup de grâce_ from the revelations
of Galileo’s glasses. Our satellite was one of the earliest objects, if
not actually the first, upon which the Florentine turned his telescope;
and he found that the inequalities upon her surface were due to
differences in its configuration analogous to the continents and
islands, and (as might then have been thought) the seas of our globe. He
could trace, even with his moderate means, the semblance of
mountain-tops upon which the sun shone while their lower parts were in
shadow, of hills that were brightly illuminated upon their sides towards
the sun, of brightly shining elevations, and deeply shadowed
depressions, of smooth plains, and regions of mountainous ruggedness. He
saw that the boundary of sunlight upon the moon was not a clearly
defined line, as it would be if the lunar globe were a smooth sphere, as
the Aristotelians had asserted, but that the terminator was uneven and
broken into an irregular outline. From these observations the Florentine
astronomer concluded that the lunar world was covered not only with
mountains like our globe, but with mountains whose heights far surpassed
those existing upon the earth, and whose forms were strangely limited to
circularity.

Galileo’s best telescopes magnified only some thirty times, and the
views which he thus obtained, must have been similar to those exhibited
by the smaller photographs of the moon produced in late years by Mr. De
la Rue and now familiar to the scientific public. Of course there is in
the natural moon as viewed with a small telescope a vivid brilliancy
which no art can imitate, and in photographs especially there is a
tendency to exaggeration of the depths of shade in a lunar picture. This
arises from the circumstance that various regions of the moon do not
impress a chemically sensitized plate as they impress the retina of the
eye. Some portions, notably the so-called “seas” of the moon, which to
the eye appear but slightly duller than the brighter parts, give off so
little _actinic_ light that they appear as nearly black patches upon a
photograph, and thus give an undue impression of the relative brightness
of various parts of the lunar surface. Doubtless by sufficient exposure
of the plate in the camera-telescope the dark patches might be rendered
lighter, but in that case the more strongly illuminated portions, which
after all are those most desirable to be preserved, would be lost by the
effect which photographers understand as “solarization.”

In speaking of a view of the moon with a magnifying power of thirty, it
is necessary to bear in mind that the visible features will differ
considerably with the diameter of the object-glass of the telescope to
which this power is applied. The same details would not be seen alike
with the same power upon an object-glass of 10 inches diameter and one
of 2 inches. The superior illumination of the image in the former case
would bring into view minute details that could not be perceived with
the smaller aperture. He who would for curiosity wish to see the moon,
or any other object, as Galileo saw it, must use a telescope of the same
size and character in all respects as Galileo’s: it will not do to put
his magnifying power upon a larger telescope. With large telescopes, and
low powers used upon bright objects like the moon, there is a blinding
flood of light which tends to contract the pupil of the eye and prevent
the passage of the whole of the pencil of rays coming through the
eye-piece. Although this last result may be productive of no
inconvenience, it is clearly a waste of light, and it points to a rule
that the lowest power that a telescope should bear is that which gives a
pencil of light equal in diameter to the pupil of the eye under the
circumstances of brightness attendant upon the object viewed. In
observing faint objects this point assumes more importance, since it is
then necessary that all available light should enter the pupil. The
thought suggests itself that an artificial enlargement of the pupil, as
by a dose of belladonna, might be of assistance in searching for faint
objects, such as nebulæ and comets: but we prefer to leave the
experiment for those to try who pursue that branch of astronomical
observation.

A merely cursory examination of the moon with the low power to which we
have alluded is sufficient to show us the more salient features. In the
first place we cannot help being struck with the immense preponderance
of circular or craterform asperities, and with the general tendency to
circular shape which is apparent in nearly all the lunar surface
markings; for even the larger regions known as the “seas” and the
smaller patches of the same character seem to repeat in their outlines
the round form of the craters. It is at the boundary of sunlight on the
lunar globe that we see these craterform spots to the best advantage, as
it is there that the rising or setting sun casts long shadows over the
lunar landscape, and brings elevations and asperities into bold relief.
They vary greatly in size, some are so large as to bear an estimable
proportion to the moon’s diameter, and the smallest are so minute as to
need the most powerful telescopes and the finest conditions of
atmosphere to perceive them. It is doubtful whether the smallest of them
have ever been seen, for there is no reason to doubt that there exist
countless numbers that are beyond the revealing powers of our finest
telescopes.

From the great number and persistent character of these
circumvallations, Kepler was led to think that they were of artificial
construction. He regarded them as pits excavated by the supposed
habitants of the moon to shelter themselves from the long and intense
action of the sun. Had he known their real dimensions, of which we shall
have to speak when we come to describe them more in detail, he would
have hesitated in propounding such a hypothesis; nevertheless it was, to
a certain extent, justified by the regular and seemingly unnatural
recurrence of one particular form of structure, the like of which is,
too, so seldom met with as a structural feature of the surface of our
own globe.

The next most striking features, revealed by a low telescopic power upon
the moon, are the seemingly smooth plains that have the appearance of
dusky spots, and that collectively cover a considerable portion—about
two-thirds—of the entire disc. The larger of these spots retain the name
of _seas_, the term having been given when they were supposed to be
watery expanses, and having been retained, possibly to avoid the
confusion inevitable from a change of name, after the existence of water
upon the moon was disproved. Following the same order of nomenclature,
the smaller spots have received the appellations of _lakes_, _bays_ and
_fens_. We see that many of these “seas” are partially surrounded by
ramparts or bulwarks which, under closer examination, and having regard
to their real magnitude, resolve themselves into immense mountain
chains. The general resemblance in form which the bulwarked plains thus
exhibit to the circular craters of large size, would lead us to suppose
that the two classes of objects had the same formative origin, but when
we take into account the immense size of the former, and the process by
which we infer the latter to have been developed, the supposition
becomes untenable.

Another of the prominent features which we notice as highly curious, and
in some phases of the moon—at about the time of full—the most remarkable
of all, are certain bright lines that appear to radiate from some of the
more conspicuous craters, and extend for hundreds of miles around. No
selenological formations have so sorely puzzled observers as these
peculiar streaks, and a great deal of fanciful theorizing has been
bestowed upon them. As we are now only glancing at the moon, we do not
enter upon explanations concerning them or any other class of details;
all such will receive due consideration in their proper order in
succeeding chapters.

We thus see that the classes of features observable upon the moon are
not great in number: they may be summed up as _craters_ and their
central cones, _mountain chains_, with occasional isolated peaks,
_smooth plains_, with more or less of irregularity of surface, and
_bright radiating streaks_. But when we come to study with higher powers
the individual examples of each class we meet with considerable
diversity. This is especially the case with the craters, which appear
under very numerous variations of the one order of structure, viz., the
ring-form. A higher telescopic power shows us that not only do these
craters exist of all magnitudes within a limit of largeness, but
seemingly with no limit of smallness, but that in their structure and
arrangement they present a great variety of points of difference. Some
are seen to be considerably elevated above the surrounding surface,
others are basins hollowed out of that surface and with low surrounding
ramparts; some are merely like walled plains or amphitheatres with flat
plateaux, while the majority have their lowest point of hollowness
considerably below the general level of the surrounding surface; some
are isolated upon the plains, others are aggregated into a thick crowd,
and overlapping and intruding upon each other; some have elevated peaks
or cones in their centres, and some are without these central cones,
while the plateaux of others again contain several minute craters
instead; some have their ramparts whole and perfect, others have them
breached or malformed, and many have them divided into terraces,
especially on their inner sides.

In the plains, what with a low power appeared smooth as a water surface
becomes, under greater magnification, a rough and furrowed area, here
gently undulated and there broken into ridges and declivities, with now
and then deep rents or cracks extending for miles and spreading like
river-beds into numerous ramifications. Craters of all sizes and classes
are scattered over the plains; these appear generally of a different
tint to the surrounding surface, for the light reflected from the plains
has been observed to be slightly tinged with colour, The tint is not the
same in all cases: one large sea has a dingy greenish tinge, others are
merely grey and some others present a pale reddish hue. The cause of
this diversity of colour is mysterious; it has been supposed to indicate
the existence of vegetation of some sort; but this involves conditions
that we know do not exist.

The mountains, under higher magnification, do not present such diversity
of formation as the craters, or at least the points of difference are
not so apparent; but they exhibit a plentiful variety of combinations.
There are a few perfectly isolated examples that cast long shadows over
the plains on which they stand like those of a towering cathedral in the
rising or setting sun. Sometimes they are collected into groups, but
mostly they are connected into stupendous chains. In one of the grandest
of these chains, it has been estimated that a good telescope will show
3000 mountains clustered together, without approach to symmetrical
order. The scenery which they would present, could we get any other than
the “bird’s eye view” to which we are confined, must be imposing in the
extreme, far exceeding in sublime grandeur anything that the Alps or the
Himalayas offer; for while on the one hand the lunar mountains equal
those of the earth in altitude, the absence of an atmosphere, and
consequently of the effects produced thereby, must give rise to
alternations of dazzling light and black depths of shade combining to
form panoramas of wild scenery that, for want of a parallel on earth, we
may well call unearthly. But we are debarred the pleasure of actually
contemplating such pictures by the circumstance that we look _down_ upon
the mountain tops and into the valleys, so that the great height and
close aggregation of the peaks and hills are not so apparent. To compare
the lunar and terrestrial mountain scenery would be “to compare the
different views of a town seen from the car of a balloon, with the more
interesting prospects by a progress through the streets.” Some of the
peculiarities of the lunar scenery we have, however, endeavoured to
realize in a subsequent Chapter.

A high power gives us little more evidence than a low one upon the
nature of the long bright streaks that radiate from some of the more
conspicuous craters, but it enables us to see that those streaks do not
arise from any perceptible difference of level of the surface—that they
have no very definite outline, and that they do not present any sloping
sides to catch more sunlight, and thus shine brighter, than the general
surface. Indeed, one great peculiarity of them is that they come out
most forcibly where the sun is shining perpendicularly upon them; hence
they are best seen where the moon is at full, and they are not visible
at all at those regions upon which the sun is rising or setting. We also
see that they are not diverted by elevations in their path, as they
traverse in their course craters, mountains, and plains alike, giving a
slight additional brightness to all objects over which they pass, but
producing no other effect upon them. To employ a commonplace simile,
they look as though, after the whole surface of the moon had assumed its
final configuration, a vast brush charged with a whitish pigment had
been drawn over the globe in straight lines radiating from a central
point, leaving its trail upon everything it touched, but obscuring
nothing.

Whatever may be the cause that produces this brightness of certain parts
of the moon without reference to configuration of surface, this cause
has not been confined to the formation of the radiating lines, for we
meet with many isolated spots, streaks and patches of the same bright
character. Upon some of the plains there are small areas and lines of
luminous matter possessing peculiarities similar to those of the
radiating streaks, as regards visibility with the high sun, and
invisibility when the solar rays fall upon them horizontally. Some of
the craters also are surrounded by a kind of aureole of this highly
reflective matter. A notable specimen is that called _Linné_, concerning
which a great hue and cry about change of appearance and inferred
continuance of volcanic action on the moon was raised some years ago.
This object is an insignificant little crater of about a mile or two in
diameter, in the centre of an ill-defined spot of the character referred
to, and about eight or ten miles in diameter. With a low sun the crater
alone is visible by its shadow; but as the luminary rises the shadow
shortens and becomes all but invisible, and then the white spot shines
forth. These alternations, complicated by variations of atmospheric
condition, and by the interpretations of different observers, gave rise
to statements of somewhat exaggerated character to the effect that
considerable changes, of the nature of volcanic eruptions, were in
progress in that particular region of the moon.

In the foregoing remarks we have alluded somewhat indefinitely to high
powers; and an enquiring but unastronomical reader may reasonably demand
some information upon this point. It might have been instructive to have
cited the various details that may be said to come into view with
progressive increases of magnification. But this would be an all but
impossible task, on account of the varying conditions under which all
astronomical observations must necessarily be made. When we come to
delicate tests, there are no standards of telescopic power and
definition. Assuming the instrument to be of good size and high optical
character, there is yet a powerful influant of astronomical definition
in the atmosphere and its variable state. Upon two-thirds of the clear
nights of a year the finest telescopes cannot be used to their full
advantage, because the minute flutterings resulting from the passage of
the rays of light through moving strata of air of different densities
are magnified just as the image in the telescope is magnified, till all
minute details are blurred and confused, and only the grosser features
are left visible. And supposing the telescope and atmosphere in good
state, there is still an important point, the state of the observer’s
eye, to be considered. After all it is the eye that sees, and the best
telescopic assistance to an untrained eye is of small avail. The eye is
as susceptible of education and development as any other organ; a
skilful and acute observer is to a mere casual gazer, what a watchmaker
would be to a ploughman, a miniature painter to a whitewasher. This fact
is not generally recognized; no man would think of taking in hand an
engraver’s burin, and expecting on the instant to use it like an adept,
or of going to a smithy and without previous preparation trying to forge
a horse-shoe. Yet do folks enter observatories with uneducated eyes, and
expect at once to realize all the wonderful things that their minds have
pictured to themselves from the perusal of astronomical books. We have
over and over again remarked the dissatisfaction which attends the first
looks of novices through a powerful telescope. They anticipate
immediately beholding wonders, and they are disappointed at finding how
little they can see, and how far short the sight falls of what they had
expected. Courtesy at times leads them to express wonder and surprise,
which it is easy to see is not really felt, but sometimes honesty
compels them to give expression to their disappointment. This arises
from the simple fact that their eyes are not fit for the work which is
for the moment imposed upon them; they know not what to look for, or how
to look for it. The first essay at telescopic gazing, like first essays
generally, serves but to teach us our incapability.

To a tutored eye a great deal is visible with a comparatively low power,
and practised observers strive to use magnifying powers as low as
possible, so as to diminish, as far as may be, the evils arising from an
untranquil atmosphere. With a power so small as 30 or 40, many
exceedingly delicate details on the moon are visible to an eye that is
familiar with them under higher powers. With 200 we may say that every
ordinary detail will come out under favourable conditions; but when
minute points of structure, mere nooks and corners as it were, are to be
scrutinised, 300 may be used with advantage. Another hundred diameters
almost passes the practical limit. Unless the air be not merely fine,
but superfine, the details become “clothy” and tremulous; the extra
points brought out by the increased power are then only caught by
momentary glimpses, of which but a very few are obtained during a
lengthy period of persistent scrutiny. We may set down 250 as the most
useful, and 350 the utmost effective power that can be employed upon the
particular work of which we are treating. Could every detail on the moon
be thoroughly and reliably represented as this amount of magnification
shows it, the result would leave little to be wished for.

But it may be asked by some, what is the absolute effect of such powers
as those we have spoken of, in bringing the moon apparently nearer to
our eyes? and what is the actual size of the smallest object visible
under the most favourable circumstances? A linear mile upon the moon
corresponds to an angular interval of 0·87 of a second; this refers to
regions about the centre of the disc; near the circumference the
foreshortening makes a difference, very great as the edge is approached.
Perhaps the smallest angle that the eye can without assistance
appreciate is half a minute; that is to say, an object that subtends to
the eye an arc of less than a half a minute can scarcely be seen.[7]
Since there are 60 seconds in a minute, it follows that we must magnify
a spot a second in diameter upon the moon thirty times before we can see
it; and since a second represents rather more than a mile, really about
2000 yards, on the moon, as seen from the earth, the smallest object
visible with a power of 30 will be this number of yards in diameter or
breadth. To see an object 200 yards across, we should require to magnify
it 300 times, and this would only bring it into view as a point; 20
yards would require a power of 3000, and 1 yard 60,000 to effect the
same thing. Since, as we have said, the highest practicable power with
our present telescopes, and at ordinary terrestrial elevations, is 350,
or for an extreme say 400, it is evident that the minutest lunar object
or detail of which we can perceive as a point must measure about 150
yards: to see the form of an object, so as to discriminate whether it be
round or square, it would require to be probably twice this size; for it
may be safely assumed that we cannot perceive the outline of an object
whose average breadth subtends a less angle than a minute.

Arago put this question into another shape:—The moon is distant from us
237,000 miles (mean). A magnifying power of a thousand would show us the
moon as if she were distant 237 miles from the naked eye.

2000 would bring her within 118 miles.
4000 ” ” ” 59 ”
6000 ” ” ” 39 ”

Mont Blanc is visible to the naked eye from Lyons, at the distance of
about 100 miles; so that to see the mountains of the moon as Mont Blanc
is seen from Lyons would require the impracticable power of 2500.

It is scarcely necessary to seek the reasons which prompted astronomers,
soon after the invention of the telescope, to map the surface features
of the moon. They may have considered it desirable to record the
positions of the spots upon her disc, for the purpose of facilitating
observations of the passage of the earth’s shadow over them in lunar
eclipses; or they may have been actuated by a desire to register
appearances then existing, in order that if changes took place in after
years these might be readily detected. Scheiner was one of the earliest
of lunar cartographers; he worked about the middle of the seventeenth
century; but his delineations were very rough and exaggerated. Better
maps—the best of the time, according to an old authority—were engraved
by one Mellan, about the years 1634 or 1635. At about the same epoch,
Langreen and Hevelius were working upon the same subject. Langreen
executed some thirty maps of portions of the moon, and introduced the
practice of naming the spots after philosophers and eminent men.
Hevelius spent several years upon his task, the results of which he
published in a bulky volume containing some 50 maps of the moon in
various phases, and accompanied by 500 pages of letter-press. He
rejected Langreen’s system of nomenclature, and called the spots after
the seas and continents of the earth to which he conceived they bore
resemblance. Riccioli, another selenographer, whose map was compiled
from observations made by Grimaldi, restored Langreen’s nomenclature,
but he confined himself to the names of eminent astronomers, and his
system has gained the adhesion of the map-makers of later times. Cassini
prepared a large map from his own observations, and it was engraved
about the year 1692. It appears to have been regarded as a standard
work, for a reduced copy of it was repeatedly issued with the yearly
volumes of the _Connaissance des Temps_, (the “Nautical Almanac” of
France) some time after its publication. These small copies have no
great merit: the large copper plate of the original was, we are told by
Arago, who received the statement from Bouvard, sold to a brazier by a
director of the French Government Printing-Office, who thought proper to
disembarrass the stores of that establishment, by ridding them of what
he considered lumber! La Hire, Mayer, and Lambert, followed during the
succeeding century, in this branch of astronomical delineation. At the
commencement of the present century, the subject was very earnestly
taken up by the indefatigable Schroeter, who, although he does not
appear to have produced a complete map, produced a topograph of the moon
in a large series of partial maps and drawings of special features.
Schroeter was a fine observer, but his delineations show him to have
been an indifferent draughtsman. Some of his drawings are but the rudest
representations of the objects he intended to depict; many of the bolder
features of conspicuous objects are scarcely recognizable in them. A bad
artist is as likely to mislead posterity as a bad historian, and it
cannot be surprising if observers of this or future generations,
accepting Schroeter’s drawings as faithful representations, should infer
from them remarkable changes in the lunar details. It is much to be
regretted that Schroeter’s work should be thus depreciated. Lohrman of
Dresden, was the next cartographer of the moon; in 1824 he put forth a
small but very excellent map of 15 inches diameter, and published a book
of descriptive text, accompanied by sectional charts of particular
areas. His work, however, was eclipsed by the great one which we owe to
the joint energy of MM. Beer and Maedler, and which represents a
stupendous amount of observing work carried on during several years
prior to 1836, the date of their publication. The long and patient
labour bestowed upon their map and upon the measures on which it
depends, deserve the highest praise which those conversant with the
subject can bestow, and it must be very long before their efforts can be
superseded.

Beer and Maedler’s map has a diameter of 37 inches: it represents the
phase of the moon visible in the condition of mean libration. The
details were charted by a careful process of triangulation. The disc was
first divided into “triangles of the first order,” the points of which
(conspicuous craters) were accurately laid down by reference to the
edges of the disc: one hundred and seventy-six of these triangles,
plotted accurately upon an orthographic projection of the hemisphere,
formed the reliable basis for their charting work. From these a great
number of “points of the second order” were laid down, by measuring
their distance and angle of position with regard to points first
established. The skeleton map thus obtained was filled up by drawings
made at the telescope: the diameters of the measureable craters being
determined by the micrometer.

Beer and Maedler also measured the heights of one thousand and
ninety-five lunar mountains and crater-summits: the resulting measures
are given in a table contained in the comprehensive text-book which
accompanies their map. These heights are found by one of two methods,
either by measuring the length of the shadow which the object casts
under a known elevation of the sun above its horizon, or by measuring
the distance between the illuminated point of the mountain and the
“terminator” in the following manner. In the annexed figure (Fig. 15)
let the circle represent the moon and M a mountain upon it: let S A be
the line of direction of the sun’s rays, passing the normal surface of
the moon at A and just tipping the mountain top. A will be the
terminator, and there will be darkness between it and the star-like
mountain summit M. The distance between A and M is measured: the
distance A B is known, for it is the moon’s radius. And since the line S
M is a tangent to the circle the angle B A M is a right angle. We know
the length of its two sides AB, AM, and we can therefore by the known
properties of the right-angled triangle find the length of the
hypothenuse BM: and since BM is made up of the radius BA plus the
mountain height, we have only to subtract the moon’s radius from the
ascertained whole length of the hypothenuse and we have the height of
the mountain. MM. Beer and Maedler exhibited their measures in French
toises: in the heights we shall have occasion to quote, these have been
turned into English feet, upon the assumption that the toise is equal to
6·39 English feet. The nomenclature of lunar features adopted by Beer
and Maedler is that introduced by Riccioli: mountains and features
hitherto undistinguished were named by them after ancient and modern
philosophers, in continuance of Riccioli’s system, and occasionally
after terrestrial features. Some minute objects in the neighbourhood of
large and named ones were included under the name of the large one and
distinguished by Greek or Roman letters.

[Illustration: Fig. 15.]

[Illustration: PLATE IV.
PICTURE MAP OF THE MOON.]

[Illustration: PLATE V.
Skeleton Map of Moon
To Accompany Picture Map, Chap. VII]

The excellent map resulting from the arduous labours of these
astronomers is simply a map: it does not pretend to be a picture. The
asperities and depressions are symbolized by a conventional system of
shading and no attempt is made to exhibit objects as they actually
appear in the telescope. A casual observer comparing details on the map
with the same details on the moon itself would fail to identify or
recognize them except where the features are very conspicuous. Such an
observer would be struck by the shadows by which the lunar objects
reveal themselves: he would get to know them mostly by their shadows,
since it is mainly by those that their forms are revealed to a
terrestrial observer. But such a map as that under notice indicates no
shadows, and objects have to be identified upon it rather by their
positions with regard to one another or to the borders of the moon than
by any notable features they actually present to view. This
inconvenience occurred to us in our early use of Beer and Maedler’s
chart, and we were induced to prepare for ourselves a map in which every
object is shown somewhat, if imperfectly, as it actually appears at some
period of a lunation. This was done by copying Beer and Maedler’s
outlines and filling them up by appropriate shading. To do justice to
our task we enlarged our map to a diameter of six feet. Upon a circle of
this diameter the positions and dimensions of all objects were laid down
from the German original. Then from our own observations we depicted the
general aspect of each object: and we so adjusted the shading that all
objects should be shown under about the same angle of illumination—a
condition which is never fulfilled upon the moon itself, but which we
consider ourselves justified in exhibiting for the purpose of conveying
a fair impression of how the various lunar objects actually appear at
some one or other part of a lunation.

The picture-map thus produced has been photographed to the size
convenient for this work: and in order to make it available for the
identification of such objects as we may have occasion to refer to, we
have placed around it a co-ordinate scale of arbitrary divisions by
which any object can be found as by the latitude and longitude divisions
upon a common geographical map. We have also prepared a skeleton map
which includes the more conspicuous objects, and which faces the picture
map (Plates IV. and V.) The numbers on the skeleton map are those given
in the second column of the accompanying table. The table also gives the
co-ordinate positions of the various craters, the names of which are,
for convenience of reference, printed in alphabetical order.

Name. Number. Map Ordinates.

Abulfeda 107 30·0 120·7
Agrippa 151 31·2 110·0
Airy 93 34·7 123·0
Albategnius 109 35·5 119·7
Aliacensis 61 35·8 131·0
Almanon 94 29·0 122·3
Alpetragius 92 40·8 122·4
Alphonsus 110 39·6 120·9
Apianus 62 33·6 129·3
Apollonius 154 6·5 109·5
Arago 152 24·7 108·7
Archimedes 191 40·3 95·8
Aristarchus 176 62·3 99·2
Aristillus 190 37·0 93·3
Aristotle 209 30·0 84·6
Arzachael 84 39·5 124·0
Atlas 228 20·7 86·6
Autolycus 189 36·8 95·5
Azophi 76 30·7 126·8
Bacon 17 32·5 142·0
Baily 207 26·0 85·4
Barocius 34 31·8 138·5
Bessel 179 27·4 100·1
Bettinus 11 48·8 144·9
Bianchini 215 51·6 86·3
Billy 121 64·3 121·4
Blancanus 12 43·7 144·8
Bonpland 110 48·5 117·6
Borda 56 15·2 131·0
Boscovich 160 31·1 106·8
Bouvard 40 66·6 134·3
Briggs 196 68·0 97·2
Bullialdus 86 50·1 125·5
Burg 206 25·5 87·5
Calippus 199 32·4 90·3
Campanus 71 52·3 129·0
Capella 104 17·8 118·0
Capuanus 43 50·5 132·8
Casatus 7 43·7 147·0
Cassini 200 35·5 89·7
Catherina 95 24·7 124·0
Cavalerius 144 71·2 109·5
Cavendish 88 63·5 127·4
Cichus 44 47·3 132·8
Clavius 13 41·8 143·5
Cleomides 183 10·7 97·0
Colombo 98 12·8 122·7
Condamine 214 48·7 84·2
Condorcet 164 4·5 104·7
Copernicus 147 49·8 107·0
Cyrillus 96 23·5 121·3
Damoiseau 124 69·2 117·2
Davy 113 43·2 119·8
Deambrel 129 26·8 113·5
Delisle 195 55·7 95·2
Descartes 106 28·5 119·3
Diophantus 194 55·5 96·3
Doppelmayer 70 58·6 129·6
Encke 140 59·7 110·6
Endymion 227 20·6 83·8
Epigenes 223 39·0 79·5
Erastothenes 168 44·6 104·0
Eudoxus 208 29·7 88·0
Fabricius 35 20·0 136·8
Fernelius 37 35·1 134·8
Firmicus 156 5·8 107·7
Flamsteed 126 62·8 114·5
Fontana 122 65·9 123·0
Fontenelle 221 43·0 81·3
Fourier 67 62·5 130·7
Fracastorius 78 20·5 127·0
Furnerius 52 11·7 133·0
Gambart 138 47·2 112·2
Gartner 224 26·5 82·3
Gassendi 90 59·7 123·3
Gauricus 46 43·5 132·5
Gauss 201 10·3 90·3
Gay Lussac 169 50·1 103·8
Geber 83 29·6 124·8
Geminus 187 13·0 93·0
Gérard 218 63·7 88·8
Goclenius 101 11·8 118·5
Godin 135 31·3 111·7
Grimaldi 125 70·8 116·3
Gruemberger 6 41·4 145·8
Gueriké 114 46·5 119·6
Guttemberg 102 13·9 118·3
Inghirami 27 61·3 138·9
Isidorus 103 16·7 118·0
Kant 105 25·8 118·5
Kepler 146 60·0 108·0
Kies 72 49·7 128·8
Kircher 10 47·5 145·8
Klaproth 8 43·5 146·7
La Caille 74 37·5 126·8
Lagrange 68 67·0 131·3
La Hire 177 54·3 99·3
Lalande 117 43·4 115·3
Lambert 193 49·6 97·8
Landsberg 127 54·0 113·0
Langreen 100 6·3 117·7
Letronne 120 62·0 119·0
Licetus 21 34·1 139·6
Lichtenberg 197 66·5 94·9
Linnæus 188 31·7 95·7
Littrow 185 20·5 99·4
Lohrman 143 71·3 112·8
Longomontanus 23 45·7 140·6
Lubiniezky 91 51·3 123·5
Macrobius 182 13·7 100·2
Maginus 22 40·0 140·4
Mairan 217 56·7 89·5
Manilius 167 32·2 103·9
Manzinus 4 31·3 146·0
Maraldi 181 18·6 100·8
Marius 171 65·0 105·5
Maskelyne 132 19·5 111·0
Mason 204 23·7 88·8
Maupertius 213 48·7 85·8
Maurolycus 33 31·8 137·0
Menelaus 165 28·3 103·0
Mercator 65 51·4 130·2
Mersenius 89 61·7 125·7
Messala 202 14·0 90·5
Messier 131 10·8 114·0
Metius 36 18·8 105·9
Moretus 5 39·5 146·5
Moesting 128 41·6 113·2
Neander 57 18·7 131·0
Nearchus 18 26·8 142·0
Newton 1 41·0 147·7
Nonius 49 36·5 133·2
Olbers 172 73·0 107·7
Pallas 149 38·6 109·5
Parrot 108 35·8 121·6
Petavius 80 9·5 127·5
Phocylides 25 55·5 141·6
Piazzi 41 65·0 133·5
Picard 163 8·3 104·7
Piccolomini 58 21·7 131·0
Pico 211 41·9 87·3
Pitatus 63 44·1 130·2
Plana 205 24·8 88·8
Plato 210 41·8 84·8
Playfair 75 33·5 127·5
Pliny 165 24·2 103·4
Poisson 60 32·8 131·0
Polybius 82 24·5 125·6
Pontanus 59 29·0 130·2
Posidonius 186 22·2 94·3
Proclus 162 11·4 104·5
Ptolemy 111 39·5 118·2
Purbach 73 38·7 128·4
Pythagoras 220 53·0 81·2
Pytheas 178 49·7 100·4
Ramsden 42 52·9 132·5
Reamur 118 37·3 114·6
Reiner 145 67·3 108·5
Reinhold 139 51·5 111·2
Repsold 219 60·2 85·7
Rheita 51 16·1 134·2
Riccioli 142 72·7 113·8
Riccius 50 23·7 133·5
Ritter 134 26·0 111·6
Roemer 184 18·3 97·6
Ross 161 25·0 105·3
Sabine 133 25·0 112·0
Sacrobosco 77 27·5 127·7
Santbech 79 15·7 126·8
Saussure 31 39·6 137·7
Scheiner 14 45·5 143·5
Schickard 28 59·0 137·5
Schiller 24 51·3 141·0
Schroeter 137 42·3 110·7
Schubert 155 2·3 110·8
Segner 16 51·3 143·5
Seleucus 174 69·0 99·8
Sharp 216 54·2 87·7
Short 2 39·7 147·4
Silberschlag 157 32·0 108·1
Simpelius 3 35·8 147·7
Snell 55 11·3 129·6
Soemmering 136 42·8 112·2
Stadius 148 45·6 107·0
Stevinus 53 11·9 130·7
Stoefler 32 35·6 136·8
Strabo 226 23·2 81·6
Struve 203 18·3 88·7
Taruntius 153 11·7 109·0
Taylor 130 27·6 116·2
Thales 225 24·3 81·8
Thebit 85 40·8 126·8
Theophilus 97 22·3 120·0
Timæus 222 38·3 80·8
Timocharis 192 45·1 97·0
Tobias Mayer 170 54·5 103·0
Triesnecker 150 35·5 109·8
Tycho 30 43·0 142·3
Ukert 159 37·1 107·5
Vasco de Gama 173 72·8 104·9
Vendelinus 99 6·8 121·6
Vieta 69 64·3 129·7
Vitello 66 55·8 130·7
Vitruvius 180 20·1 102·0
Vlacq 19 25·0 140·1
Zuchius 15 50·7 144·2

The strong family likeness pervading the craters of the moon renders it
unnecessary that we should attempt a description of each one of them or
even of one in twenty. We have, however, thought that a few remarks upon
the salient features of a few of the most important may be acceptable in
explanation of our illustrative plates; and what we have to say of the
few may be taken as representative of the many.

COPERNICUS, 147—(49·8—107·0). Plate VIII.

This may deservedly be considered as one of the grandest and most
instructive of lunar craters. Although its vast diameter (46 miles) is
exceeded by others, yet, taken as a whole, it forms one of the most
impressive and interesting objects of its class. Its situation, near the
centre of the lunar disc, renders all its wonderful details, as well as
those of its immediately surrounding objects, so conspicuous as to
establish it as a very favourite object. Its vast rampart rises to
upwards of 12,000 feet above the level of the plateau, nearly in the
centre of which stands a magnificent group of cones, three of them
attaining the height of upwards of 2400 feet.

The rampart is divided by concentric segmental terraced ridges, which
present every appearance of being enormous landslips, resulting from the
crushing of their over-loaded summits, which have slid down in vast
segments and scattered their débris on to the plateau. Corresponding
vacancies in the rampart may be observed from whence these prodigious
masses have broken away. The same may be noticed, although in a somewhat
modified degree, around the exterior of the rampart. In order to
approach a realization of the sublimity and grandeur of this magnificent
example of a lunar volcanic crater, our reader would do well to
endeavour to fix his attention on its enormous magnitude and attempt to
establish in his mind’s eye a correct conception of the scale of its
details as well as its general dimensions, which, as they so
prodigiously transcend those of the largest terrestrial volcanic
craters, require that our ideas as to magnitude of such objects should
be, so to speak, educated upon a special standard. It is for this reason
we are anxious our reader, when examining our illustrations, should
constantly refer the objects represented in them to the scale of miles
appended to each plate, otherwise a just and true conception of the
grandeur of the objects will escape him.

Copernicus is specially interesting, as being evidently the result of a
vast discharge of molten matter which has been ejected at the focus or
centre of disruption of an extensively upheaved portion of the lunar
crust. A careful examination of the crater and the district around it,
even to the distance of more than 100 miles on every side, will supply
unmistakable evidence of the vast extent and force of the original
disruption, manifested by a wonderfully complex reticulation of bright
streaks which diverge in every direction from the crater as their common
centre. These streaks do not appear on our plate, nor are they seen upon
the moon except at and near the full phase. They show conspicuously,
however, by their united lustre on the full moon, Plate III. Every one
of those bright streaks, we conceive, is a record of what was originally
a crack or chasm in the solid crust of the moon, resulting from some
vastly powerful upheaving agency over the site of whose focus of energy
Copernicus stands. The cracking of the crust must have been followed by
the ejection of subjacent molten matter up through the reticulated
cracks; this, spreading somewhat on either side of them, has left these
bright streaks as a visible record of the force and extent of the
upheaval; while at the focus of disruption from whence the cracks
diverge, the grand outburst appears to have taken place, leaving
Copernicus as its record and result.

Many somewhat radial ridges or spurs may be observed leading away from
the exterior banks of the great rampart. These appear to be due to the
more free egress which the extruded matter would find near the focus of
disruption. The spur-ridges may be traced fining away for fully 100
miles on all sides, until they become such delicate objects as to
approach invisibility. Several vast open chasms or cracks may be
observed around the exterior of the rampart. They appear to be due to
some action subsequent to the formation of the great crater—probably the
result of contraction on the cooling of the crust, or of a deep-seated
upheaval long subsequent to that which resulted in the formation of
Copernicus itself, as they intersect objects of evidently prior
formation.

Under circumstances specially favourable for “fine vision,” for upwards
of 70 miles on all sides around Copernicus, myriads of comparatively
minute but perfectly-formed craters may be observed. The district on the
south-east side is specially rich in these wonderfully thickly-scattered
craters, which we have reason to suppose stand over or upon the
reticulated bright streaks; but, as the circumstances of illumination
which are requisite to enable us to detect the minute craters are widely
adverse to those which render the bright streaks visible, namely, nearly
full moon for the one and gibbous for the other, it is next to
impossible to establish the fact of coincidence of the sites of the two
by actual simultaneous observation.

At the east side of the rampart, multitudes of these comparatively
minute craters may also be detected, although not so closely crowded
together as those on the west side; but among those on the east may be
seen myriads of minute prominences roughening the surface; on close
scrutiny these are seen to be small mounds of extruded matter which, not
having been ejected with sufficient energy to cause the erupted material
to assume the crater form around the vent of ejection, have simply
assumed the mound form so well known to be the result of volcanic
ejection of moderate force.

Were we to select a comparatively limited portion of the lunar surface
abounding in the most unmistakable evidence of volcanic action in every
variety that can characterize its several phases, we could not choose
one yielding in all respects such instructive examples as Copernicus and
its immediate surroundings.

GASSENDI, 90—(59·7—123·3). Frontispiece.

An interesting crater about 54 miles diameter; the height of the most
elevated portion of the surrounding wall from the plateau being about
9600 feet. The centre is occupied by a group of conical mountains, three
of which are most conspicuous objects and rise to nearly 7000 feet above
the level of the plateau. As in other similar cases, these central
mountains are doubtless the result of the expiring effort of the
eruption which had formed the great circular wall of the crater. The
plateau is traversed by several deep cracks or chasms nearly one mile
wide.

Both the interior and exterior of the wall of the crater are terraced
with the usual segmental ridges or landslips. A remarkable detached
portion of the interior bank is to be seen on the east side, while on
the west exterior of the wall may be seen an equally remarkable example
of an outburst of lava subsequent to the formation of the wall or bank
of the crater; it is of conical form and cannot fail to secure the
attention of a careful observer.

Interpolated on the north wall of the crater may be seen a crater of
about 18 miles diameter which has burst its bank in towards the great
crater, upon whose plateau the lava appears to have discharged itself.

The neighbourhood of Gassendi is diversified by a vast number of mounds
and long ridges of exudated matter, and also traversed by enormous
chasms and cracks, several of which exceed one mile wide and are fully
100 miles in length, and, as is usual with such cracks, traverse plain
and mountain alike, disregarding all surface inequalities.

Numbers of small craters are scattered around; the whole forming an
interesting and instructive portion of the lunar surface.

EUDOXUS, 208 (29·7—88·0), and ARISTOTLE, 209 (30·0—84·6). Plate X.

Two gigantic craters, Eudoxus being nearly 35 miles in diameter and
upwards of 11,000 feet deep, while Aristotle is about 48 miles in
diameter, and about 10,000 feet deep (measuring from the summit of the
rampart to the plateau). These two magnificent craters present all the
true volcanic characteristics in a remarkable degree. The outsides as
well as the insides of their vast surrounding walls or banks display on
the grandest scale the landslip feature, the result of the over-piling
of the ejected material, and the consequent crushing down and crumbling
of the substructure. The true eruptive character of the action which
formed the craters is well evinced by the existence of the groups of
conical mountains which occupy the centres of their circular plateaux,
since these conical mountains, there can be little doubt, stand over
what were once the vents from whence the ejected matter of the craters
was discharged.

On the west side of these grand craters may be seen myriads of
comparatively minute ones (we use the expression “comparatively minute,”
although most of them are fully a mile in diameter). So thickly are
these small craters crowded together, that counting them is totally out
of the question; in our original notes we have termed them “Froth
craters” as the most characteristic description of their aspect.

The exterior banks of Aristotle are characterized by radial ridges or
spurs: these are most probably the result of the flowing down of great
currents of very fluid lava. To the east of the craters some very lofty
mountains of exudation may be seen, and immediately beyond them an
extensive district of smaller mountains of the same class, so thickly
crowded together as under favourable illumination to present a multitude
of brilliant points of light contrasted by intervening deep shade. On
the west bank of Aristotle a very perfect crater may be seen, 27 miles
in diameter, having all the usual characteristic features.

About 40 miles to the east of Eudoxus there is a fine example of a crack
or fissure extending fully 50 miles—30 miles through a plain, and the
remaining 20 miles cutting through a group of very lofty mountains. This
great crack is worthy of attention, as giving evidence of the
deep-seated nature of the force which occasioned it, inasmuch as it
disregards all surface impediments, traversing plain and group of
mountains alike.

There are several other features in and around these two magnificent
craters well worthy of careful observation and scrutiny, all of them
excellent types of their respective classes.

TRIESNEKER, 150 (35·5—109·8). Plate XI.

A fine example of a normal lunar volcanic crater, having all the usual
characteristic features in great perfection. Its diameter is about 20
miles, and it possesses a good example of the central cone and also of
interior terracing.

The most notable feature, however, in connection with this crater, and
on account of which we have chosen it as a subject for one of our
illustrations, is the very remarkable display of chasms or cracks which
may be seen to the west side of it. Several of these great cracks
obviously diverge from a small crater near the west external bank of the
great one, and they subdivide or branch out, as they extend from the
apparent point of divergence, while they are crossed or intersected by
others. These cracks or chasms (for their width merits the latter
appellation) are nearly one mile broad at the widest part, and after
extending to fully 100 miles, taper away till they become invisible.
Although they are not test objects of the highest order of difficulty,
yet to see them with perfect distinctness requires an instrument of some
perfection and all the conditions of good vision. When such are present,
a keen and practised eye will find many details to rivet its attention,
among which are certain portions of the edges of these cracks or chasms
which have fallen in and caused interruptions to their continuity.

THEOPHILUS, 97 (22·3—120·0). CYRILLUS, 96 (23·5—121·3). CATHARINA, 95
(24·7—124·0). Plate XII.

These three magnificent craters form a very conspicuous group near the
middle of the south-east quarter of the lunar disc.

Their respective diameters and depths are as follows:—

Theophilus, 64 miles diameter; depth of plateau from summit of crater
wall, 16,000 feet; central cone, 5200 feet high.

Cyrillus, 60 miles diameter; depth of plateau from summit of crater
wall, 15,000 feet; central cone, 5800 feet high.

Catharina, 65 miles diameter; depth of plateau from summit of crater
wall, 13,000 feet; centre of plateau occupied by a confused group of
minor craters and débris.

Each of these three grand craters is full of interesting details,
presenting in every variety the characteristic features which so
fascinate the attention of the careful observer of the moon’s wonderful
surface, and affording unmistakable evidence of the tremendous energy of
the volcanic forces which at some inconceivably remote period piled up
such gigantic formations.

Theophilus by its intrusion within the area of Cyrillus shows in a very
striking manner that it is of comparatively more recent formation than
the latter crater. There are many such examples in other parts of the
lunar disc, but few of so very distinct and marked a character.

The flanks or exterior banks of Theophilus, especially those on the west
side, are studded with apparently minute craters, all of which when
carefully scrutinized are found to be of the true volcanic type of
structure; and minute as they are, by comparison, they would to a
beholder close to them appear as very imposing objects; but so gigantic
are the more notable craters in the neighbourhood, that we are apt to
overlook what are in themselves really large objects. It is only by duly
training the mind, as we have previously urged, so as ever to keep
before us the vast scale on which the volcanic formations of the lunar
surface are displayed, that we can do them the justice which their
intrinsic grandeur demands. We trust that our illustrations may in some
measure tend to educate the mind’s eye, so as to derive to the full the
tranquil enjoyment which results from the study of the manifestation of
one of the Creator’s most potent agencies in dealing with the materials
of his worlds, namely, volcanic force. So rich in wonderful features and
characteristic details is this magnificent group and its neighbourhood,
that a volume might be filled in the attempt to do justice, by
description, to objects so full of suggestive subject for study.

THEBIT, 85—(40·8—126·8).

A crater about 32 miles in diameter and about 9700 feet deep, devoid of
a central cone. It appears on the upper part and near the middle of
Plate XIII. The plateau has five minute craters upon it. On the east
outside are two small craters, the lesser of which, about 2·75 miles
diameter, has a central cone. We specially note this fact, because it is
the smallest crater but one in which we have detected a central cone: no
doubt, however, many smaller craters possess this unmistakable stamp of
true volcanic origin, but so minute are the specks of light which the
central cones of such very small craters reflect, that they fail to be
visible to us.

East of Thebit is a very remarkable straight cliff 60 miles long by
about 1000 feet high, called by some observers the “Railway,” and
apparently the result either of an upheaval or of a down-sinking of the
surface of the circular area across whose diameter it stretches.

Under moderate magnifying power, this cliff appears straight, but with
higher power and under favourable conditions, its face is seen to be
serrated, and along the upper edge may be detected several very minute
craters. A more conspicuous small crater is seen at the north end of the
cliff. To the east of the cliff nearly opposite the centre are two
craters, from the east side of the larger of which proceeds a fine crack
parallel to the cliff and passing through a dome-shaped hill of low
eminence.

PLATO, 210 (41·8—81·8). Plate XIV.

This crater, besides being a conspicuous object on account of its great
diameter, has many interesting details in and around it requiring a fine
instrument and favourable circumstances to render them distinctly
visible. The diameter of the crater is 70 miles; the surrounding wall or
rampart varies in height from 4000 to upwards of 8000 feet, and is
serrated with noble peaks which cast their black shadows across the
plateau in a most picturesque manner, like the towers and spires of a
great cathedral. Reference to our illustration will convey a very fair
idea of this interesting appearance. On the north-east inside of the
circular wall or rampart may be observed a fine example of landslip, or
sliding down of a considerable mass of the interior side of the crater’s
wall. The landslip nature of this remarkable detail is clearly
established by the fact of the bottom edge of the downslipped mass
projecting in towards the centre of the plateau to a considerable
extent. Other smaller landslip features may be seen, but none on so
grand and striking a scale as the one referred to. A number of
exceedingly minute craters may be detected on the surface of the
plateau. The plateau itself is remarkable for its low reflective power,
which causes it to look like a dingy spot when Plato is viewed with a
small magnifying power. The exterior of the crater wall is remarkable
for the rugged character of its formation, and forms a great contrast in
that respect to the comparatively smooth unbroken surface of the
plateau, which by the way is devoid of a central cone. The surrounding
features and objects indicated in our illustration are of the highest
interest, and a few of them demand special description.

THE VALLEY OF THE ALPS (37·0—86·0). Plate XIV.

This remarkable object lays somewhat diagonally to the west of Plato;
when seen with a low magnifying power (80 or 100), it appears as a rut
or groove tapering towards each extremity. It measures upwards of 75
miles long by about six miles wide at the broadest part. When examined
under favourable circumstances, with a magnifying power of from 200 to
300, it is seen to be a vast flat-bottomed valley bordered by gigantic
mountains, some of which attain heights upwards of 10,000 feet; towards
the south-east of this remarkable valley, and on both sides of it, are
groups of isolated mountains, several of which are fully 8000 feet high.
This flat-bottomed valley, which has retained the integrity of its form
amid such disturbing forces as its immediate surroundings indicate, is
one of the many structural enigmas with which the lunar surface abounds.
To the north-west of the valley a vast number of isolated mounds or
small mountains of exudation may be seen; so numerous are they as to
defy all attempts to count them with anything like exactness; and among
them, a power of 200 to 300 will enable an observer, under favourable
circumstances, to detect vast numbers of small but perfectly-formed
craters.

PICO, 211 (41·9—87·3). Plate XIV.

This is one of the most interesting examples of an isolated volcanic
“mountain of exudation,” and it forms a very striking object when seen
under favourable circumstances. Its height is upwards of 8000 feet, and
it is about three times as long at the base as it is broad. The summit
is cleft into three peaks, as may be ascertained by the three-peaked
shadow it casts on the plain. Five or six minute craters of very perfect
form may be detected close to the base of this magnificent mountain.
There are several other isolated peaks or mountains of the same class
within 30 or 40 miles of it which are well worthy of careful scrutiny,
but Pico is the master of the situation, and offers a glorious subject
for realizing a lunar day-dream in the mind’s eye, if we can only by an
effort of imagination conceive its aspect under the fiercely brilliant
sunshine by which it is illuminated, contrasted with the intensely black
lunar heavens studded with stars shining with a steady brightness of
which, by reason of _our_ atmosphere intervening, we can have no
adequate conception save by the aid of a well-directed imagination.

TYCHO, 30 (43·0—142·3). Plate XVI.

This magnificent crater, which occupies the centre of the crowded group
in our Plate, is 54 miles in diameter, and upwards of 16,000 feet deep,
from the highest ridge of the rampart to the surface of the plateau,
whence rises a grand central cone 5000 feet high. It is one of the most
conspicuous of all the lunar craters, not so much on account of its
dimensions as from its occupying the great focus of disruption from
whence diverge those remarkable bright streaks, many of which may be
traced over 1000 miles of the moon’s surface, disregarding in their
course all interposing obstacles. There is every reason to conclude that
Tycho is an instance of a vast disruptive action which rent the solid
crust of the moon into radiating fissures, which were subsequently
occupied by extruded molten matter, whose superior luminosity marks the
course of the cracks in all directions from the crater as their common
centre of divergence. So numerous are these bright streaks when examined
by the aid of the telescope, and they give to this region of the moon’s
surface such an extra degree of luminosity, that, when viewed as a
whole, their locality can be distinctly seen at full moon by the
unassisted eye as a bright patch of light on the southern portion of the
disc. (See Plate III.) The causative origin of the streaks is discussed
and illustrated in Chapter XI.

The interior of this fine crater presents striking examples of the
concentric terrace-like formations that we have elsewhere assigned to
vast landslip actions. Somewhat similar concentric terraces may be
observed in other lunar craters; some of these, however, appear to be
the results of some temporary modification of the ejective force, which
has caused the formation of more or less perfect inner ramparts: what we
conceive to be true landslip terraces are always distinguished from
these by their more or less fragmentary character.

On reference to Plate III., showing the full moon, a very remarkable and
special appearance will be observed in a dingy district or zone
immediately surrounding the exterior of the rampart of Tycho, and of
which we venture to hazard what appears to us a rational explanation:
namely, that as Tycho may be considered to have acted as a sort of
safety-valve to the rending and ejective force which caused, in the
first instance, the cracking of this vast portion of the moon’s
crust—the molten matter that appears to have been forced up through
these cracks, on finding a comparatively free exit by the vent of Tycho,
so relieved the district immediately around him as to have thereby
reduced, in amount, the exit of the molten matter, and so left a zone
comparatively free from the extruded lava which, according to our view
of the subject, came up simultaneously through the innumerable fissures,
and, spreading sideways along their courses, left everlasting records of
the original positions of the radiating cracks in the form of the bright
streaks which we now behold.

“WARGENTIN,” 26 (57·5—140·2). Plate XVII.

This object is quite unique of its kind—a crater about 53 miles across
that to all appearance has been filled to the brim with lava that has
been left to consolidate. There are evidences of the remains of a
rampart, especially on the south-west portion of the rim. The general
aspect of this extraordinary object has been not unaptly compared to a
“thin cheese.” The terraced and rutted exterior of the rampart has all
the usual characteristic details of the true crater. The surface of the
high plateau is marked by a few ridges branching from a point nearly in
its centre, together with some other slight elevations and depressions;
these, however, can only be detected when the sun’s rays fall nearly
parallel to the surface of the plateau.

To the north of this interesting object is the magnificent ring
formation Schickard, whose vast diameter of 123 miles contrasts
strikingly with that of the sixteen small craters within his rampart,
and equally so with a multitude of small craters scattered around. There
are many objects of interest on the portion of the lunar surface
included within our illustration, but as they are all of the usual type,
we shall not fatigue the attention of our readers by special
descriptions of them.

ARISTARCHUS, 176 (6·3—99·2), and HERODOTUS, 175 (63·2—99·6). Plate XVIII.

These two fine examples of lunar volcanic craters are conspicuously
situated in the north-east quarter of the moon’s disc. Aristarchus has a
circular rampart nearly 28 miles diameter, the summit of which is about
7500 feet above the surface of the plateau, while its height above the
general surface of the moon is 2600 feet. A central cone having several
subordinate peaks completes the true volcanic character of this crater:
its rampart banks, both outside and inside, have fine examples of the
segmental crescent-shaped ridges or landslips, which form so constant
and characteristic a feature in the structure of lunar craters. Several
very notable cracks or chasms may be seen to the north of these two
craters. They are contorted in a very unusual and remarkable manner, the
result probably of the force which formed them having to encounter very
varying resistance near the surface.

Some parts of these chasms gape to the width of two to three miles, and
when closely scrutinized are seen to be here and there partly filled by
masses which have fallen inward from their sides. Several smaller
craters are scattered around, which, together with the great chasms and
neighbouring ridges, give evidence of varied volcanic activity in this
locality. We must not omit to draw attention to the parallelism or
general similarity of “strike” in the ridges of extruded matter; this
appearance has special interest in the eyes of geologists, and is well
represented in our illustration.

Aristarchus is specially remarkable for the extraordinary capability
which the material forming its interior and rampart banks has of
reflecting light. Although there are many portions of the lunar surface
which possess the same property, yet few so remarkably as in the case of
Aristarchus, which shines with such brightness, as compared with its
immediate surroundings, as to attract the attention of the most
unpractized observer. Some have supposed this appearance to be due to
active volcanic discharge still lingering on the lunar surface, an idea
in which, for reasons to be duly adduced, we have no faith. Copernicus,
in the remarkable bright streaks which radiate from it, and Tycho also,
as well as several other spots, are apparently composed of material very
nearly as highly reflective as that of Aristarchus. But the comparative
isolation of Aristarchus, as well as the extraordinary light-reflecting
property of its material, renders it especially noticeable, so much so
as to make it quite a conspicuous object when illuminated only by
earth-light, when but a slender crescent of the lunar disc is
illuminated, or when, as during a lunar eclipse, the disc of the moon is
within the shadow of the earth, and is lighted only by the rays
refracted through the earth’s atmosphere.

There are no features about Herodotus of any such speciality as to call
for remark, except it be the breach of the north side of its rampart by
the southern extremity of a very remarkable contorted crack or chasm,
which to all appearance owes its existence to some great disruptive
action subsequent to the formation of the crater.

WALTER, 48 (37·8—131·9), and adjacent Intrusive Craters. Plate XX.

This Plate represents a southern portion of the moon’s surface measuring
170 by 230 miles. It includes upwards of 200 craters of all dimensions,
from Walter, whose rampart measures nearly 70 miles across, down to
those of such small apparent diameter as to require a well practized eye
to detect them. In the interior of the great crater Walter a remarkable
group of small craters may be observed surrounding his central cone,
which in this instance is not so perfectly in the centre of the rampart
as is usually the case. The number of small craters which we have
observed within the rampart is 20, exclusive of those on the rampart
itself. The entire group represented in the Plate suggests in a striking
manner the wild scenery which must characterize many portions of the
lunar surface; the more so if we keep in mind the vast proportions of
the objects which they comprise, upon which point we may remark that the
smallest crater represented in this Plate is considerably larger than
that of Vesuvius.

ARCHIMEDES, 191 (40·3—95·8), AUTOLYCUS, 189 (36·8—95·5), ARISTILLUS,
190 (37·0—93·3), and the APENNINES. Plate IX.

This group of three magnificent craters, together with their remarkable
surroundings, especially including the noble range of mountains termed
the Apennines, forms on the whole one of the most striking and
interesting portions of the lunar surface. If the reader is not
acquainted with what the telescope can reveal as to the grandeur of the
effect of sunrise on this very remarkable portion of the moon’s surface,
he should carefully inspect and study our illustration of it; and if he
will pay due regard to our previously repeated suggestion concerning the
attached scale of miles, he will, should he have the good fortune to
study the actual objects by the aid of a telescope, be well prepared to
realize and duly appreciate the magnificence of the scene which will be
presented to his sight.

Were we to attempt an adequate detail description of all the interesting
features comprised within our illustration, it would, of itself, fill a
goodly volume; as there is included within the space represented every
variety of feature which so interestingly characterizes the lunar
surface. All the more prominent details are types of their class; and
are so favourably situated in respect to almost direct vision, as to
render their nature, forms, and altitudes above and depths below the
average surface of the moon most distinctly and impressively cognizable.

Archimedes is the largest crater in the group; it has a diameter of
upwards of 52 miles, measuring from summit to summit of its vast
circular rampart or crater wall, the average height of which, above the
plateau, is about 4300 feet; but some parts of it rise considerably
higher, and, in consequence, cast steeple-like shadows across the
plateau when the sun’s rays are intercepted by them at a low angle. The
plateau of this grand crater is devoid of the usual central cone. Two
comparatively minute but beautifully-formed craters may be detected
close to the north-east interior side of the surrounding wall of the
great crater. Both outside and inside of the crater wall may be seen
magnificent examples of the landslip subsidence of its overloaded banks;
these landslips form vast concentric segments of the outer and inner
circumference of the great circular rampart, and doubtless belong to its
era of formation. Two very fine examples of cracks, or chasms, may be
observed proceeding from the opposite external sides of the crater, and
extending upwards of 100 miles in each direction; these cracks, or
chasms, are fully a mile wide at their commencement next the crater, and
narrow away to invisibility at their further extremity. Their course is
considerably crooked, and in some parts they are partially filled by
masses of the material of their sides, which have fallen inward and
partially choked them. The depths of these enormous chasms must be very
great, as they probably owe their existence to some mighty upheaving
action, which there is every reason to suppose originated at a profound
depth, since the general surface on each side of the crater does not
appear to be disturbed as to altitude, which would have been the case
had the upheaving action been at a moderate depth beneath. We would
venture to ascribe a depth of not less than ten miles as the most
moderate estimate of the profundity of these terrible chasms. If the
reader would realize the scale of them, let him for a moment imagine
himself a traveller on the surface of the moon coming upon one of them,
and finding his onward progress arrested by the sudden appearance of its
vast black yawning depths; for by reason of the angle of his vision
being almost parallel to the surface, no appearance of so profound a
chasm would break upon his sight until he came comparatively close to
its fearful edge. Our imaginary lunar traveller would have to make a
very long détour, ere he circumvented this terrible interruption to his
progress. If the reader will only endeavour to realize in his mind’s eye
the terrific grandeur of a chasm a mile wide and of such dark profundity
as to be, to all appearance, fathomless—portions of its rugged sides
fallen in wild confusion into the jaws of the tortuous abyss, and
catching here and there a ray of the sun sufficient only to render the
darkness of the chasm more impressive as to its profundity—he will, by
so doing, learn to appreciate the romantic grandeur of this, one of the
many features which the study of the lunar surface presents to the
careful observer, and which exceed in sublimity the wildest efforts of
poetic and romantic imagination. The contemplation of these views of the
lunar world are, moreover, vastly enhanced by special circumstances
which add greatly to the impressiveness of lunar scenery, such as the
unchanging pitchy-black aspect of the heavens and the death-like silence
which reigns unbroken there.

These digressions are, in some respects, a forestallment of what we have
to say by-and-by, and so far they are out of place; but with the
illustration to which the above remarks refer placed before the reader,
they may, in some respects, enhance the interest of its examination.

The upper portion of our illustration is occupied by the magnificent
range of volcanic mountains named after our Apennines, extending to a
length of upwards of 450 miles. This mountain group rises gradually from
a comparatively level surface towards the south-west, in the form of
innumerable comparatively small mountains of exudation, which increase
in number and altitude towards the north-east, where they culminate and
suddenly terminate in a sublime range of peaks, whose altitude and
rugged aspect must form one of the most terribly grand and romantic
scenes which imagination can conceive. The north-east face of the range
terminates abruptly in an almost vertical precipitous face, and over the
plain beneath intense black steeple or spire-like shadows are cast, some
of which at sunrise extend fully 90 miles, till they lose themselves in
the general shading due to the curvature of the lunar surface. Nothing
can exceed the sublimity of such a range of mountains, many of which
rise to heights of 18,000 to 20,000 feet at one bound from the plane at
their north-east base. The most favourable time to examine the details
of this magnificent range is from about a day before first quarter to a
day after, as it is then that the general structure of the range as well
as the character of the contour of each member of the group can, from
the circumstances of illumination then obtaining, be most distinctly
inferred.

Several comparatively small perfectly-formed craters are seen
interspersed among the mountains, giving evidence of the truly volcanic
character of the surrounding region, which, as before said, comprises in
a comparatively limited space the most perfect and striking examples of
nearly every class of lunar volcanic phenomena.

We have endeavoured on Plate XXIII. to give some idea of a landscape
view of a small portion of this mountain range.

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