In this Magazine for August last I considered the moon's multitudinous
small craters with special reference to the theory that some among those
small craters may have been produced by the downfall of aerolithic or
meteoric masses upon the moon's once plastic surface. Whether it be
considered probable that this is really the case or not with regard to
actually existent lunar craters, it cannot be doubted that during one
period of the moon's history, a period probably lasting many millions of
years, many crater-shaped depressions must have been produced in this way.
As I showed in that essay, it is absolutely certain that thousands of
meteoric masses, large enough to form visible depressions where they fell,
must have fallen during the moon's plastic era. It is certain also that
that era must have been very long-lasting. Nevertheless, it remains
possible (many will consider it extremely probable, if not absolutely
certain) that during sequent periods all such traces were removed. There
is certainly nothing in the aspect of the present lunar craters, even the
smallest and most numerous, to preclude the possibility that they, like
the larger ones, were the results of purely volcanic action; and to many
minds it seems preferable to adopt one general theory respecting all such
objects as may be classed in a regular series, than to consider that some
members of the series are to be explained in one way and others in a
different way. We can form a series extending without break or
interruption from the largest lunar craters, more than a hundred miles in
diameter, to the smallest visible craters, less than a quarter of a mile
across, or even to far smaller craters, if increase of telescopic power
should reveal such. And therefore many object to adopt any theory in
explanation of the smaller craters (or some of them) which could
manifestly not be extended to the largest. Albeit we must remember that
certainly if any small craters had been formed during the plastic era by
meteoric downfall, and had remained unchanged after the moon solidified,
it would now be quite impossible to distinguish these from craters formed
in the ordinary manner.
While we thus recognise the possibility, at any rate, that multitudes of
small lunar craters, say from a quarter of a mile to two miles in
diameter, may have been formed by falling meteoric masses hundreds of
millions of years ago, and may have remained unchanged even until now, we
perceive that on the moon later processes must have formed many small
craters, precisely as such small craters have been formed on our own
earth. I consider, at the close of the essay above mentioned, the two
stages of the moon's development which must have followed the period
during which her surface was wholly or in great part plastic. First, there
was the stage during which the crust contracted more rapidly than the
nucleus, and was rent from time to time as though the nucleus were
expanding within it. Secondly, there came the era when the nucleus, having
retained a greater share of heat, began to cool, and therefore to contract
more quickly than the crust, so that the crust became wrinkled or
corrugated, as it followed up (so to speak) the retreating nucleus.
It would be in the later part of this second great era that the moon (if
ever) would have resembled the earth. The forms of volcanic activity still
existing on the earth seem most probably referable to the gradual
contraction of the nucleus, and the steady resulting contraction of the
rocky crust. As Mallet and Dana have shown, the heat resulting from the
contraction, or in reality from the slow downfall of the crust, is amply
sufficient to account for the whole observed volcanian energy of the
earth. It has indeed been objected, that if this theory (which is
considered more fully in my "Pleasant Ways in Science") were correct, we
ought to find volcanoes occurring indifferently, or at any rate volcanic
phenomena of various kinds so occurring, in all parts of the earth's
surface, and not prevalent in special regions and scarcely ever noticed
elsewhere. But this objection is based on erroneous ideas as to the length
of time necessary for the development of subterranean changes, and also as
to the extent of regions which at present find in certain volcanic craters
a sufficient outlet for their subterranean fires. It is natural that, if a
region of wide extent has at any time been relieved at some point, that
spot should long afterwards remain as an outlet, a sort of safety-valve,
which, by yielding somewhat more quickly than any neighbouring part of the
crust, would save the whole region from destructive earthquakes; and
though in the course of time a crater which had acted such a part would
cease to do so, yet the period required for such a change would be very
long indeed compared with those periods by which men ordinarily measure
time. Moreover, it by no means follows that every part of the earth's
crust would even require an outlet for heat developed beneath it. Over
wide tracts of the earth's surface the rate of contraction may be such, or
may be so related to the thickness of the crust, that the heat developed
can find ready escape by conduction to the surface, and by radiation
thence into space. Nay, from the part which water is known to play in
producing volcanic phenomena, it may well be that in every region where
water does not find its way in large quantities to the parts in which the
subterranean heat is great, no volcanic action results. Mallet, following
other experienced vulcanologists, lays down the law, "Without water there
can be no volcano;" so that the neighbourhood of large oceans, as well as
special conditions of the crust, must be regarded as probably essential to
the existence of such outlets as Vesuvius, Etna, Hecla, and the rest.
So much premised, let us enquire whether it is antecedently likely that in
the moon volcanic action may still be in progress, and afterwards consider
the recent announcement of a lunar disturbance, which, if really volcanic,
certainly indicates volcanic action far more intense than any which is at
present taking place in our own earth. I have already, I may remark,
considered the evidence respecting this new lunar crater which some
suppose to have been formed during the last two years. But I am not here
going over the same ground as in my former paper ("Contemporary Review"
for August, 1878). Moreover, since that paper was written, new evidence
has been obtained, and I am now able to speak with considerable confidence
about points which were in some degree doubtful three months ago.
Let us consider, in the first place, what is the moon's probable age, not
in years, but in development. Here we have only probable evidence to guide
us, evidence chiefly derived from the analogy of our own earth. At least,
we have only such evidence when we are enquiring into the moon's age as a
preliminary to the consideration of her actual aspect and its meaning. No
doubt many features revealed by telescopic scrutiny are full of
significance in this respect. No one who has ever looked at the moon,
indeed, with a telescope of great power has failed to be struck by the
appearance of deadness which her surface presents, or to be impressed (at
a first view, in any case), with the idea that he is looking at a world
whose period of life must be set in a very remote antiquity. But we must
not take such considerations into account in discussing the _a priori_
probabilities that the moon is a very aged world. Thus we have only
evidence from analogy to guide us in this part of our enquiry. I note the
point at starting, because the indicative mood is so much more convenient
than the conditional, that I may frequently in this part of my enquiry use
the former where the actual nature of the evidence would only justify the
latter. Let it be understood that the force of the reasoning here depends
entirely on the weight we are disposed to allow to arguments from analogy.
Assuming the planets and satellites of the solar system to be formed in
some such manner as Laplace suggested in his "Nebular Hypothesis," the
moon, as an orb travelling round the earth, must be regarded as very much
older than she is, even in years. Even if we accept the theory of
accretion which has been recently suggested as better according with known
facts, it would still follow that probably the moon had existence, as a
globe of matter nearly of her present size, long before the earth had
gathered in the major portion of her substance. Necessarily, therefore, if
we assume as far more probable than either theory that the earth and moon
attained their present condition by combined processes of condensation and
accretion, we should infer that the moon is far the older of the two
bodies in years.
But if we even suppose that the earth and moon began their career as
companion planets at about the same epoch, we should still have reason to
believe that these planets, equal though they were in age so far as mere
years are concerned, must be very unequally advanced so far as development
is concerned, and must therefore in that respect be of very unequal age.
It was, I believe, Sir Isaac Newton who first called attention to the
circumstance that the larger a planet is, the longer will be the various
stages of its existence. He used the same reasoning which was afterwards
urged by Buffon, and suggested an experiment which Buffon was the first to
carry out. If two globes of iron, of unequal size, be heated to the same
degree, and then left to cool side by side, it will be found that the
larger glows with a ruddy light after the smaller has become quite dark,
and that the larger remains intensely hot long after the smaller has
become cool enough to be handled. The reason of the difference is very
readily recognised. Indeed, Newton perceived that there would be such a
difference before the matter had been experimentally tested. The quantity
of heat in the unequal globes is proportional to the volume, the substance
of each being the same. The heat is emitted from the surface, and at a
rate depending on the extent of surface. But the volume of the larger
exceeds that of the smaller in greater degree than the surface of the
larger exceeds the surface of the other. Suppose, for instance, the larger
has a diameter twice as great as that of the smaller, its surface is four
times as great as that of the smaller, its volume eight times as great.
Having, then, eight times as much heat as the smaller at the beginning,
and parting with that heat only four times as fast as the smaller, the
supply necessarily lasts twice as long; or, more exactly, each stage in
the cooling of the larger lasts twice as long as the corresponding stage
in the cooling of the smaller. We see that the duration of the heat is
greater for the larger in the same degree that the diameter is greater.
And we should have obtained the same result whatever diameters we had
considered. Suppose, for instance, we heat two globes of iron, one an inch
in diameter, the other seven inches, to a white heat. The surface of the
larger is forty-nine times that of the smaller, and thus it gives out at
the beginning, and at each corresponding stage of cooling, forty-nine
times as much heat as the smaller. But it possesses at the beginning three
hundred and forty-three (seven times seven times seven) times as much
heat. Consequently, the supply will last seven times as long, precisely as
a stock of three hundred and forty-three thousand pounds, expended
forty-nine times as fast as a stock of one thousand pounds only, would
last seven times as long. In every case we find that the duration of the
heat-emission for globes of the same material equally heated at the outset
is proportional to their diameters.
Now, before applying this result to the case of the moon, we must take
into account two considerations:--First, the probability that when the
moon was formed she was not nearly so hot as the earth when it first took
planetary shape; and secondly, the different densities of the earth and
moon.
The original heat of every member of the solar system, including the sun,
depended on the gravitating energy of its own mass. The greater that
energy, the greater the heat generated either by the process of steady
contraction imagined in Laplace's theory, or by the process of meteoric
indraught imagined in the aggregation theory. To show how very different
are the heat-generating powers of two very unequal masses, consider what
would happen if the earth drew down to its own surface a meteoric mass
which had approached the earth under her own attraction only. (The case is
of course purely imaginary, because no meteor can approach the earth which
has not been subjected to the far greater attractive energy of the sun,
and does not possess a velocity far greater than any which the earth
herself could impart). In this case such a mass would strike the earth
with a velocity of about seven miles per second, and the heat generated
would be that due to this velocity only. Now, when a meteor strikes the
sun full tilt after a journey from the star depths under his attraction,
it reaches his surface with a velocity of nearly three hundred and sixty
miles per second. The heat generated is nearly fifty times greater than in
the imagined case of the earth. The moon being very much less than the
earth, the velocity she can impart to meteoric bodies is still less. It
amounts, in fact, to only about a mile per second. The condensing energy
of the moon in her vaporous era was in like manner far less than that of
the earth, and consequently far less heat was then generated. Thus,
although we might well believe on _a priori_ grounds, even if not assured
by actual study of the lunar features, that the moon when first formed as
a planet had a surface far hotter than molten iron, we must yet believe
that, when first formed, the moon had a temperature very much below that
of our earth at the corresponding stage of her existence.
On this account, then, we must consider that the moon started in planetary
existence in a condition as to heat which our earth did not attain till
many millions, probably hundreds of millions of years after the epoch of
her first formation as a planet.
As regards the moon's substance, we have no means of forming a
satisfactory opinion. But we shall be safe in regarding quantity of matter
in the moon as a safer basis of calculation than volume, in comparing the
duration of her various stages of development with those of our own earth.
When, in the August number of this Magazine, I adopted a relation derived
from the latter and less correct method, it was because the more correct
method gave the result most favourable to the argument I was then
considering. The same is indeed the case now. Yet it will be better to
adopt the more exact method, because the consideration relates no longer
to a mere side issue, but belongs to the very essence of my reasoning.
The moon has a mass equal to about one eighty-first part of the earth's.
Her diameter being less than the earth's, about as two to seven, the
duration of each stage of her cooling would be in this degree less than
the corresponding duration for the earth, if her density were the same as
the earth's, in which case her mass would be only one forty-ninth part of
the earth's. But her mass being so much less, we must assume that her
amount of heat at any given stage of cooling was less in similar degree
than it would have been had her density been the same as the earth's. We
may, in fact, assume that the moon's total supply of heat would be only
one eighty-first of the earth's if the two bodies were at the same
temperature throughout.[63] But the surface of the moon is between
one-thirteenth and one-fourteenth of the earth's. Since, then, the earth
at any given stage of cooling parted with her heat between thirteen and
fourteen times as fast as the moon, but had about eighty-one times as much
heat to part with (for that stage), it follows that she would take about
six times as long (six times thirteen and a-half is equal to eighty-one)
to cool through that particular stage as the moon would.
If we take this relation as the basis of our estimate of the moon's age,
we shall find that, even if the moon's existence as a planet began
simultaneously with the earth's instead of many millions of years earlier,
even if the moon was then as hot as the earth instead of being so much
cooler that many millions of years would be required for the earth to cool
to the same temperature--making, I say, these assumptions, which probably
correspond to the omission of hundreds of millions of years in our
estimate of the moon's age, we shall still find the moon to be hundreds of
millions of years older than the earth.
Nay, we may even take a position still less favourable to my argument. Let
us overlook the long ages during which the two orbs were in the vaporous
state, and suppose the earth and moon to be simultaneously in that stage
of planetary existence when the surface has a temperature of two thousand
degrees Centigrade.
From Bischoff's experiments on the cooling of rocks, it appears to follow
that some three hundred and twenty millions of years must have elapsed
between the time when the earth's surface was at this temperature and the
time when the surface temperature was reduced to two hundred degrees
Centigrade, or one hundred and eighty degrees Fahrenheit above the
boiling point. The earth was for that enormous period a mass (in the main)
of molten rock. In the moon's case this period lasted only one-sixth of
three hundred and twenty million years, or about fifty-three million
years, leaving two hundred and sixty-seven million years' interval between
the time when the moon's surface had cooled down to two hundred degrees
Centigrade and the later epoch when the earth's surface had attained that
temperature.
I would not, however, insist on these numerical details. It has always
seemed to me unsafe to base calculations respecting suns and planets on
experiments conducted in the laboratory. The circumstances under which the
heavenly bodies exist, regarding these bodies as wholes, are utterly
unlike any which can be produced in the laboratory, no matter on what
scale the experimenter may carry on his researches. I have often been
amused to see even mathematicians of repute employing a formula based on
terrestrial experiments, physical, optical, and otherwise, as though the
formula were an eternal omnipresent reality, without noting that, if
similarly applied to obtain other determinations, the most stupendously
absurd results would be deduced. It is as though, having found that a
child grows three inches in the fifth year of his age, one should infer
not only that that person but every other person in every age and in every
planet, nay, in the whole universe, would be thirty inches taller at the
age of fifteen than at the age of five, without noticing that the same
method of computation would show everyone to be more than fifteen feet
taller at the age of sixty-five. It may well be that, instead of three
hundred and twenty millions of years, the era considered by Bischoff
lasted less than a hundred millions of years. Or quite as probably it may
have lasted five or six hundred millions of years. And again, instead of
the corresponding era of the moon's past history having lasted one sixth
of the time required to produce the same change in the earth's condition,
it may have lasted a quarter, or a third, or even half that time, though
quite as probably it may have lasted much less than a sixth. But in any
case we cannot reasonably doubt that the moon reached the stage of cooling
through which the earth is now passing many millions of years ago. We
shall not probably err very greatly in taking the interval as at least two
hundred millions of years.
But I could point out that in reality it is a matter of small importance,
so far as my present argument is concerned, whether we adopt Bischoff's
period or a period differing greatly from it. For if instead of about
three hundred millions the earth required only thirty millions of years to
cool from a surface temperature of two thousand degrees Centigrade to a
temperature of two hundred degrees, we must assume that the rate of
cooling is ten times greater than Bischoff supposed. And we must of course
extend the same assumption to the moon. Now, since the sole question
before us is to what degree the moon has cooled, it matters nothing
whether we suppose the moon has been cooling very slowly during many
millions of years since she was in the same condition as the earth at
present, or that the moon has been cooling ten times as quickly during a
tenth part of the time, or a hundred times as quickly during one-hundredth
part of the time.
We may, therefore, continue to use the numbers resulting from Bischoff's
calculation, even though we admit the probability that they differ widely
from the true values of the periods we are considering.
Setting the moon, then, as about two hundred and fifty millions of years
in advance of the earth in development, even when we overlook all the eras
preceding that considered by Bischoff, and the entire sequent interval
(which must be long, for the earth has no longer a surface one hundred
degrees Centigrade hotter than boiling water), let us consider what is
suggested by this enormous time-difference.
In the first place, it corresponds to a much greater interval in our
earth's history. During the two hundred and fifty millions of years the
moon has been cooling at her rate, not at the earth's. According to the
conclusion we deduced from the moon's relative mass and surface, she has
aged as much during those two hundred and fifty million years as the earth
will during the next fifteen hundred million years.
Now, however slowly we suppose the earth's crust to be changing, it must
be admitted that in the course of the next fifteen hundred millions of
years the earth will have parted with far the greater part, if not with
the whole, of that inherent heat on which the present movements of her
surface depend. We know that these movements at once depend upon and
indicate processes of contraction. We know that such processes cannot
continue at their present rate for many millions of years. If we assume
that the rate of contraction will steadily diminish--which is equivalent,
be it noticed, to the assumption that the earth's vulcanian or
subterranean energies will be diminished--the duration of the process will
be greater. But even on such an assumption, controlled by consideration of
the evidence we have respecting the rate at which terrestrial contraction
is diminishing, it is certain that long before a period of fifteen hundred
millions of years has elapsed, the process of contraction will to all
intents and purposes be completed.
We must assume, then, as altogether the most probable view, that the moon
has reached this stage of planetary decrepitude, even if she has not
become an absolutely dead world. We can hardly reject the reasoning which
would show that the moon is far older than has been assumed when long
stages of her history and our earth's have been neglected. Still less
reasonable would it be to reject the conclusion that at the very least she
has reached the hoar antiquity thus inferred. Assuming her to be no older,
we yet cannot escape the conviction that her state is that of utter
decrepitude. To suppose that volcanic action can now be in progress on the
moon, even to as great a degree as on the earth, would be to assume that
measurable sources of energy can produce practically immeasurable results.
But no volcanic changes now in process on the earth could possibly be
discernible at the moon's distance. How utterly unlikely does it seem,
then, that any volcanic changes can be now taking place on the moon which
could be recognized from the earth! It seems safe to assume that no
volcanic changes at all can be in progress; but most certainly the
evidence which should convince us that volcanic changes of so tremendous a
character are in progress that at a distance of two hundred and sixty
thousand miles terrestrial telescopists can discern them, must be of the
strongest and most satisfactory character.
Evidence of change may indeed be discovered which can be otherwise
explained. The moon is exposed to the action of heat other than that which
pervaded her own frame at the time of her first formation. The sun's heat
is poured upon the moon during the long lunar day of more than a
fortnight, while during the long lunar night a cold prevails which must
far exceed that of our bitterest arctic winters. We know from the
heat-measurements made by the present Lord Rosse, that any part of the
moon's surface at lunar mid-day is fully five hundred degrees Fahrenheit
hotter than the same part two weeks later at lunar midnight. The alternate
expansions and contractions resulting from these changes of temperature
cannot but produce changes, however slowly, in the contour of the moon's
surface. Professor Newcomb, indeed, considers that all such changes must
long since have been completed. But I cannot see how they can be completed
so long as the moon's surface is uneven, and at present there are regions
where that surface is altogether rugged. Mighty peaks and walls exist
which must one day be thrown down, so unstable is their form; deep ravines
can be seen which must one day be the scene of tremendous landslips, so
steep and precipitous are their sides. Changes such as these may still
occur on so vast a scale that telescopists may hope from time to time to
recognise them. But changes such as these are not volcanic; they attest no
lunar vitality. They are antecedently so probable, indeed, while volcanic
changes are antecedently so unlikely, that when any change is clearly
recognised in the moon's surface, nothing but the most convincing evidence
could be accepted as demonstrating that the change was of volcanic origin
and not due to the continued expansion and contraction of the lunar crust.
And now let us see how stands the evidence in the few cases which seem
most to favour the idea that a real change has taken place.
We may dismiss, in the first place, without any hesitation, the assertion
that regular changes take place in the floor of the great lunar crater
Plato. According to statements very confidently advanced a few years ago,
this wide circular plain, some sixty miles in diameter, grows darker and
darker as the lunar day advances there until the time corresponding to
about two o'clock in the afternoon, and then grows gradually lighter again
till eventide. The idea seems to have been at first that some sort of
vegetation exists on the floor of this mighty ring-shaped mountain, and
that, as the sun's heat falls during the long lunar day upon the great
plain, the vegetation flourishes, darkening the whole region just as we
might imagine that some far-extending forest on the earth would appear
darker as seen from the moon when fully clothed with vegetation than when
the trees were bare and the lighter tints of the ground could be seen
through them. Another idea was that the ground undergoes some change under
the sun's heat corresponding to those which are produced in certain
substances employed in photography; though it was not explained why the
solar rays should produce no permanent change, as in the terrestrial cases
adduced in illustration. Yet another and, if possible, an even stranger
explanation, suggested that, though the moon has no seas, there may be
large quantities of water beneath her crust, which may evaporate when that
crust becomes heated, rising in the form of vapour to moisten and so
darken the crust. Certainly, the idea of a moistening of the lunar crust,
or of portions thereof, as the sun's rays fall more strongly upon it, is
so daring that one could almost wish it were admissible, instead of being
altogether inconsistent, as unfortunately it is, with physical
possibilities.
But still more unfortunately, the fact supposed to have been observed, on
which these ingenious speculations were based, has not only been called in
question, but has been altogether negatived. More exact observations have
shown that the supposed darkening of the floor of Plato is a mere optical
illusion. When the sun has lately risen at that part of the moon, the
ringed wall surrounding this great plain throws long shadows across the
level surface. These shadows are absolutely black, like all the shadows on
the moon. By contrast, therefore, the unshadowed part of the floor appears
lighter than it really is; but the mountain ring which surrounds this dark
grey plain is of light tint. So soon as the sun has passed high above the
horizon of this region, the ring appears very brilliant compared with the
dark plain which it surrounds; thus the plain appears by comparison even
darker than it really is. As the long lunar afternoon advances, however,
black shadows are again thrown athwart the floor, which therefore again
appears by contrast lighter than it really is. All the apparent changes
are such as might have been anticipated by anyone who considered how
readily the eye is misled by effects of contrast.
To base any argument in favour of a regular change in the floor of Plato
on evidence such as this, would be as unwise as it would be to deduce
inferences as to changes in the heat of water from experiments in which
the heat was determined by the sensations experienced when the hands were
successively immersed, one hand having previously been in water as hot as
could be borne, the other in water as cold as could be borne. We know how
readily these sensations would deceive us (if we trusted them) into the
belief that the water had warmed notably during the short interval of time
which had elapsed between the two immersions; for we know that if both
hands were immersed at the same moment in lukewarm water, the water would
appear cold to one hand and warm to the other.
Precisely as in such a case as we have just considered, if we were obliged
to test the water by so inexact a method, we should make experiments with
one hand only, and carefully consider the condition of that hand during
the progress of the experiments, so in the case of the floor of Plato, we
must exclude as far as possible all effects due to mere contrast. We must
examine the tint of the plain, at lunar morning, mid-day, and evening,
with an eye not affected either by the darkness or brightness of adjacent
regions, or adjacent parts of the same region. This is very readily done.
All we have to do is to reduce the telescopic field of view to such an
extent that, instead of the whole floor, only a small portion can be seen.
It will then be found, as I can myself certify (the more apparently
because the experience of others confirms my own), that the supposed
change of tint does not take place. One or two who were and are strong
believers in the reality of the change do indeed assert that they have
tried this experiment, and have obtained an entirely different result. But
this may fairly be regarded as showing how apt an observer is to be
self-deceived when he is entirely persuaded of the truth of some favourite
theory. For those who carried out the experiment successfully had no views
one way or the other; those only failed who were certainly assured
beforehand that the experiment would confirm their theory.
The case of the lunar crater Linne, which somewhere about November 1865
attracted the attention of astronomers, belongs to a very different
category. In my article on the moon in the "Contemporary Review" I have
fully presented the evidence in the case of this remarkable object. I need
not therefore consider here the various arguments which have been urged
for and against the occurrence of change. I may mention, however, that, in
my anxiety to do full justice to the theory that change has really
occurred, I took Maedler's description of the crater's interior as "very
deep," to mean more than Maedler probably intended. There is now a
depression several hundred yards in depth. If Maedler's description be
interpreted, as I interpreted it for the occasion in the above article, to
mean a depth of two or three miles, it is of course certain that there has
been a very remarkable change. But some of the observers who have devoted
themselves utterly, it would seem, to the lively occupation of measuring,
counting, and describing the tens of thousands of lunar craters already
known, assert that Maedler and Lohrman (who uses the same description)
meant nothing like so great a depth. Probably Maedler only meant about half
a mile, or even less. In this case their favourite theory no longer seems
so strongly supported by the evidence. In some old drawings by the
well-known observer Schroeter, the crater is drawn very much as it now
appears. Thus, I think we must adopt as most probable the opinion which
is, I see, advanced by Prof. Newcomb in his excellent "Popular Astronomy,"
that there has been no actual change in the crater. I must indeed remark
that, after comparing several drawings of the same regions by Schroeter,
Maedler, Lohrman, and Schmidt, with each other and with the moon's surface,
I find myself by no means very strongly impressed by the artistic skill of
any of these observers. I scarcely know a single region in the moon where
change might not be inferred to have taken place if any one of the
above-named observers could be implicitly relied upon. As, fortunately,
their views differ even more widely _inter se_ than from the moon's own
surface, we are not driven to so startling a conclusion.
However, if we assume even that Linne has undergone change, we still have
no reason to believe that the change is volcanic. A steep wall, say half a
mile in height, surrounding a crater four or five miles in diameter, no
longer stands at this height above the enclosed space, if the believers in
a real change are to be trusted. But, as Dr. Huggins well remarked long
ago, if volcanic forces competent to produce disturbance of this kind are
at work in the moon, we ought more frequently to recognize signs of
change, for they could scarcely be at work in one part only of the moon's
surface, or only at long intervals of time. It is so easy to explain the
overthrow of such a wall as surrounded Linne (always assuming we can rely
upon former accounts) without imagining volcanic action, that, considering
the overwhelming weight of _a priori_ probability against such action at
the present time, it would be very rash to adopt the volcanic theory. The
expansions and contractions described above would not only be able to
throw down walls of the kind, but they would be sure to do so from time to
time. Indeed, as a mere matter of probabilities, it may be truly said that
it would be exceedingly unlikely that catastrophes such as the one which
may have occurred in this case would fail to happen at comparatively short
intervals of time. It would be so unlikely, that I am almost disposed to
adopt the theory that there really has been a change in Linne, for the
reason that on that theory we get rid of the difficulty arising from the
apparent fixity of even the steepest lunar rocks. However, after all, the
time during which men have studied the moon with the telescope--only two
hundred and sixty-nine years--is a mere instant compared with the long
periods during which the moon has been exposed to the sun's intense heat
by day and a more than arctic intensity of cold by night. It may well be
that, though lunar landslips occur at short intervals of time, these
intervals are only short when compared with those periods, hundreds of
millions of years long, of which we had to speak a little while ago.
Perhaps in a period of ten or twenty thousand years we might have a fair
chance of noting the occurrence of one or two catastrophes of the kind,
whereas we could hardly expect to note any, save by the merest accident,
in two or three hundred years.
To come now to the last, and, according to some, the most decisive piece
of evidence in favour of the theory that the moon's crust is still under
the influence of volcanic forces.
On May 19, 1877, Dr. Hermann J. Klein, of Cologne, observed a crater more
than two miles in width, where he felt sure that no crater had before
existed. It was near the centre of the moon's visible hemisphere, and not
far from a well-known crater called Hyginus. At the time of observation it
was not far from the boundary between the light and dark parts of the
moon: in fact, it was near the time of sunrise at this region. Thus the
floor of the supposed new crater was in shadow--it appeared perfectly
black. In the conventional language for such cases made and provided (it
should be stereotyped by selenographers, for it has now been used a great
many times since Schroeter first adopted the belief that the great crater
Cassini, thirty-six miles in diameter, was a new one) Dr. Klein says, "The
region having been frequently observed by myself during the last few
years, I feel certain that no such crater existed in the region at the
time of my previous observations." He communicated his discovery to Dr.
Schmidt, who also assured him that the region had been frequently observed
by himself during the last few years, and he felt certain that no such
crater, &c., &c. It is not in the maps by Lohrman and by Beer and Maedler,
or in Schroeter's drawings, and so forth. "We know more," says a recent
writer, singularly ready to believe in lunar changes; "we know that at a
later period, with the powerful Dorpat telescope, Maedler carefully
re-examined this particular region, to see if he could detect any
additional features not shown in his map. He found several smaller
craterlets _in other parts_" (the italics are mine), "but he could not
detect any other crater in the region where Dr. Klein now states there
exist a large crater, though he did find some very small hills close to
this spot." "This evidence is really conclusive," says this very confident
writer, "for it is incredible that Maedler could have seen these minute
hills and overlooked a crater so large that it is the second largest
crater of the score in this region." Then this writer comes in, of course,
in his turn, with the customary phrases. "During the six years, 1870-1876,
I most carefully examined this region, for the express purpose of
detecting any craters not shown by Maedler," and he also can certify that
no such crater existed, etc., etc. He was only waiting, when he thus
wrote, to see the crater for himself. "One suitable evening will settle
the matter. If I find a deep black crater, three miles in diameter, in the
place assigned to it by Dr. Klein, and when six years' observation
convinces me no such crater did exist, I shall know that it must be new."
Astronomers, however, require somewhat better evidence.
It might well be that a new crater-shaped depression should appear in the
moon without any volcanic action having occurred. For reasons already
adduced, indeed, I hold it to be to all intents and purposes certain that
if a new depression is really in question at all, it is in reality only an
old and formerly shallow crater, whose floor has broken up, yielding at
length to the expansive and contractive effects above described, which
would act with exceptional energy at this particular part of the moon's
surface, close as it is to the lunar equator.
But it is by no means clear that this part of the moon's surface has
undergone any change whatever. We must not be misled by the very confident
tone of selenographers. Of course they fully believe what they tell us:
but they are strongly prejudiced. Their labours, as they well know, have
now very little interest unless signs of change should be detected in the
moon. Surveyors who have done exceedingly useful work in mapping a region
would scarcely expect the public to take much interest in additional
information about every rock or pebble existing in that region, unless
they could show that something more than a mere record of rocks and
pebbles was really involved. Thus selenographers have shown, since the
days of Schroeter, an intense anxiety to prove that our moon deserves, in
another than Juliet's sense, to be called "the inconstant moon." In
another sense again they seem disposed to "swear by the inconstant moon,"
as changing yearly, if not "monthly, in her circled orb." Thus a very
little evidence satisfies them, and they are very readily persuaded in
their own mind that former researches of theirs, or of their
fellow-pebble-counters, have been so close and exact, that craters must
have been detected then which have been found subsequently to exist in the
moon. I do not in the slightest degree question their _bona fides_, but a
long experience of their ways leads me to place very little reliance on
such stereotyped phrases as I have quoted above.
Now, in my paper in the "Contemporary Review" on this particular crater, I
called attention to the fact that in the magnificent photograph of the
moon taken by Dr. Louis Rutherfurd on March 6, 1865 (note well the date)
there is a small spot of lighter colour than the surrounding region,
nearly in the place indicated in the imperfect drawing of Klein's record
which alone was then available to me. For reasons, I did not then more
closely describe this feature of the finest lunar photograph ever yet
obtained.
The writer from whom I have already quoted is naturally (being a
selenographer) altogether unwilling to accept the conclusion that this
spot is the crater floor as photographed (not as seen) under a somewhat
higher illumination than that under which the floor of the crater appears
dark. There are several white spots immediately around the dark crater, he
says: "which of these is the particular white spot which the author"
(myself) "assumes I did not see?" a question which, as I had made no
assumption whatever about this particular writer, nor mentioned him, nor
even thought of him, as I wrote the article on which he comments, I am
quite unable to answer. But he has no doubt that I have "mistaken the
white spot" (which it seems he can identify, after all) "for Klein's
crater, which is many miles farther north, and which never does appear as
a white spot: he has simply mistaken its place."
I have waited, therefore, before writing this, until from my own
observation, or from a drawing carefully executed by Dr. Klein, I might
ascertain the exact place of the new crater. I could not, as it turned
out, observe the new crater as a black spot myself, since the question was
raised; for on the only available occasion I was away from home. But I now
have before me Dr. Klein's carefully drawn map. In this I find the new
crater placed not nearly, but _exactly_ where Rutherfurd's crater appears.
I say "Rutherfurd's crater," for the white spot is manifestly not merely a
light tinted region on the darker background of the Sea of Vapours (as the
region in which the crater has been found is called): it is a circular
crater more than two miles in diameter; and the width of the crescent of
shadow surrounding its eastern side shows that in March 1865, when
Rutherfurd took that photograph, the crater was not (for its size) a
shallow one, but deep.
Now, it is quite true that, to the eye, under high illumination, the floor
of the crater does not appear lighter than the surrounding region; at
least, not markedly so, for to my eye it appears slightly lighter. But
everyone knows that a photograph does not show all objects with the same
depth of shading that they present to the naked eye. A somewhat dark green
object will appear rather light in a photograph, while a somewhat light
orange-yellow object will appear quite dark. We have only to assume that
the floor of the supposed new crater has a greenish tinge (which is by no
means uncommon) to understand why, although it is lost to ordinary vision
when the Sea of Vapours is under full illumination, it yet presents in a
photograph a decidedly lighter shade than the surrounding region.
I ought to mention that the writer from whom I have quoted says that all
the photographs were examined and the different objects in this region
identified within forty-eight hours of the time when Dr. Klein's letter
reached England. He mentions also that he has himself personally examined
them. Doubtless at that time the exact position of the supposed new crater
was not known. By the way, it is strange, considering that the name Louis
Rutherfurd is distinctly written in large letters upon the magnificent
photograph in question, that a selenographer who has carefully examined
that photograph should spell the name Rutherford. He must really not
assume, when on re-examining the picture he finds the name spelled
Rutherfurd, that there has been any change, volcanic or otherwise, in the
photograph.
In conclusion I would point out that another of these laborious
crater-counters, in a paper recently written with the express purpose of
advocating a closer and longer-continued scrutiny of the moon, makes a
statement which is full of significance in connection with the subject of
lunar changes. After quoting the opinion of a celebrated astronomer, that
one might as well attempt to catalogue the pebbles on the sea-shore as the
entire series of lunar craters down to the minutest visible with the most
powerful telescope, he states that while on the one hand, out of
thirty-two thousand eight hundred and fifty-six craters given in Schmidt's
chart, not more than two thousand objects have been entered in the
Registry he has provided for the purpose (though he has been many years
collecting materials for it from all sides); on the other hand, "on
comparing a few of these published objects with Schmidt's map, it has been
found _that some are not in it_,"--a fact to which he calls attention,
"not for the purpose of depreciating the greatest selenographical work
that has yet appeared, but for the real advancement of selenography."
Truly, the fact is as significant as it is discouraging,--unless we are
presently to be told that the craters which are not common to both series
are to be regarded as new formations.
RICHARD A. PROCTOR, _in Belgravia_.