the early stages of cosmical life are characterized by great heat; a
remark of the truth of which the sun itself furnishes the first and
most obvious illustration. Then come periods which we appear to have
seen exemplified in Jupiter, where the planet is surrounded by volumes
of steam-like vapor, through which we may almost believe we recognize
the dull glow of not yet extinguished fires; then times like those
which our earth passed through before it became the abode of man;
and then the times in which human history begins. But if this process
of the gradual loss of heat go on indefinitely, we must yet come to
still another era, when the planet has grown too cold to support life,
as it was before too hot; and this condition, in the light of some very
recent investigations, it seems probable we have now before us on the
moon.
We have, it is true, been taught until very lately that the side of the
moon turned sunward would grow hotter and hotter in the long lunar day,
till it reached a temperature of two hundred to three hundred degrees
Fahrenheit, and that in the equally long lunar night it would fall as
much as this below zero. But the evidence which was supposed to support
this conclusion as to the heat of the lunar day is not supported by
recent experiments of the writer; and if these be trustworthy, certain
facts appear to him to show that the temperature of the moon’s surface,
even under full perpetual sunshine, must be low,--and this because
of the absence of air there to keep the stored sun-heat from being
radiated away again into space.
As we ascend the highest terrestrial mountains, and get partly above
our own protecting blanket of air, things do not grow hotter and
hotter, but colder and colder; and it seems contrary to the teachings
of common sense to believe that if we could ascend higher yet, where
the air ceases altogether, we should not find that it grew colder
still. But this last condition (of airlessness) is the one which does
prevail beyond a doubt in the moon, on whose whole surface, then, there
must be (unless there are sources of internal heat of which we know
nothing) conditions of temperature which are an exaggeration of those
we experience on the summit of a very lofty mountain, where we have the
curious result that the skin may be burned under the solar rays, while
we are shivering at the same time in what the thermometer shows is an
arctic cold.
We have heard of this often; but a personal experience so impressed the
fact on me that I will relate it for the benefit of the reader, who
may wish to realize to himself the actual conditions which probably
exist in the airless lunar mountains and plains we are looking at.
He cannot go there; but he may go if he pleases, as I have done, to
the waterless, shadeless waste which stretches at the eastern slope
of the Sierra Nevadas (a chain almost as high and steep as the lunar
Apennines), and live some part of July and August in this desert, where
the thermometer rises occasionally to one hundred and ten degrees in
the shade, and his face is tanned till it can tan no more, and he
appears to himself to have experienced the utmost in this way that the
sun can do.
The sky is cloudless, and the air so clear that all idea of the real
distance and size of things is lost. The mountains, which rise in
tremendous precipices above him, seem like moss-covered rocks close
at hand, on the tops of which, here and there, a white cloth has been
dropped; but the “moss” is great primeval forests, and the white cloths
large isolated snow-fields, tantalizing the dweller in the burning
desert with their delusive nearness. When I climbed the mountains,
at an altitude of ten thousand feet I already found the coolness
delicious, but at the same time (by the strange effect I have been
speaking of) the skin began to burn, as though the seasoning in the
desert counted for nothing at all; and as the air grew thinner and
thinner while I mounted still higher and higher, though the thermometer
fell, every part of the person exposed to the solar rays presented the
appearance of a recent severe burn from an actual fire,--and a really
severe burn it was, as I can testify,--and yet all the while around us,
under this burning sun and cloudless sky, reigned a perpetual winter
which made it hard to believe that torrid summer still lay below. The
thinner the air, then, the colder it grows, even where we are exposed
to the sun, and the lower becomes the reading of the thermometer.
Now, by means of suitable apparatus, it was sought by the writer to
determine, while at this elevation of fifteen thousand feet, _how_
great the fall of temperature would be if the thin air there could be
removed altogether; and the result was that the thermometer would under
such circumstances fall, at any rate, below zero in the full sunshine.
[Illustration: FIG. 71.--PTOLEMY AND ARZACHEL.]
Of course, all this applies indirectly to the moon, above whose
surface (if these inferences be correct) the mercury in the bulb of a
thermometer would probably freeze and never melt again during the lunar
day (and still less during the lunar night),--a conclusion which has
been reached through other means by Mr. Ericsson,--and whose surface
itself cannot be very greatly warmer. Other and direct measures of the
lunar heat are still in progress while this is being written, but their
probable result seems to be already indicated: it is that the moon’s
surface, even in perpetual sunshine, must be forever cold. Just how
cold, is still doubtful; and it is not yet certain whether ice, if once
formed there, could ever melt.
Here (Fig. 72) is one more scene from the almost unlimited field the
lunar surface affords.
The most prominent things in the landscape before us are two fine
craters (Mercator and Campanus), each over thirty miles in diameter;
but we have chosen this scene for remark rather on account of the great
crack or rift which is seen in the upper part, and which cuts through
plain and mountain for a length of sixty miles. Such cracks are counted
by hundreds on the moon, where they are to be seen almost everywhere;
and other varieties, in fact, are visible on this same plate, but
we will not stop to describe them. This one varies in width from an
eighth of a mile to a mile; and though we cannot see to the bottom
of it, others are known to be at least eight miles deep, and may be
indefinitely deeper.
The edge of a cliff on the earth commonly gets weather-worn and
rounded; but here the edge is sharp, so that a traveller along the
lunar plains would come to the very brink of this tremendous chasm
before he had any warning of its existence. It is usually thus with all
such rifts; and the straightness and sharpness of the edge in these
cases suggest the appearance of an ice-crack to the observer. I do
not mean to assert that there is more than a superficial resemblance.
I do not write as a geologist; but in view of what we have just been
reading of the lunar cold, we may ask ourselves whether, if water
ever did exist here, we should not expect to find perpetual ice, not
necessarily glittering, but covered, perhaps, with the deposits of an
air laden with the dust-products of later volcanic eruptions, or even
covered in after ages, when the air has ceased from the moon, with the
slow deposit of meteoric dust during millions of years of windless
calm. What else can we think will become of the water on our own earth
if it be destined to pass through such an experience as we seem to see
prophesied in the condition of our dead satellite?
The reader must not understand me as saying that there is ice on the
moon,--only that there is not improbably perpetual ice there now _if_
there ever was water in past time; and he is not to suppose that to
say this is in any way to deny what seems the strong evidence of the
existence of volcanic action everywhere, for the two things may well
have existed in successive ages of our satellite’s past, or even have
both existed together, like Hecla, within our own arctic snows; and
if no sign of any still active lunar volcano has been discovered, we
appear to read the traces of their presence in the past none the less
clearly.
I remember that at one time, when living on the lonely upper
lava-wastes of Mount Etna, which are pitted with little craters, I grew
acquainted with so many a chasm and rent filled with these, that the
dreary landscape appeared from above as if a bit of the surface of
the moon I looked up at through the telescope had been brought down
beside me.
[Illustration: FIG. 72.--MERCATOR AND CAMPANUS.]
I remember, too, that as I studied the sun there, and watched the
volcanic outbursts on its surface, I felt that I possibly embraced in a
threefold picture as many stages in the history of planetary existence,
through all of which this eruptive action was an agent,--above in the
primal energies of the sun; all around me in the great volcano, black
and torn with the fires that still burn below, and whose smoke rose
over me in the plume that floated high up from the central cone; and
finally in this last stage in the moon, which hung there pale in the
daylight sky, and across whose face the vapors of the great terrestrial
volcano drifted, but on whose own surface the last fire was extinct.
We shall not get an adequate idea of it all, unless we add to our
bird’s-eye views one showing a chain of lunar mountains as they would
appear to us if we saw them, as we do our own Alps or Apennines, from
about their feet; and such a view Fig. 74 affords us. In the barren
plain on the foreground are great rifts such as we have been looking
at from above, and smaller craters, with their extinct cones; while
beyond rise the mountains, ghastly white in the cold sunshine, their
precipices crowned by no mountain fir or cedar, and softened by no
intervening air to veil their nakedness.
If the reader has ever climbed one of the highest Alpine peaks, like
those about Monte Rosa or the Matterhorn, and there waited for the
dawn, he cannot but remember the sense of desolation and strangeness
due to the utter absence of everything belonging to man or his works
or his customary abode, above all which he is lifted into an upper
world, so novel and, as it were, so unhuman in its features, that he
is not likely to have forgotten his first impression of it; and this
impression gives the nearest but still a feeble idea of what we see
with the telescope in looking down on such a colorless scene, where
too no water bubbles, no tree can sigh in the breeze, no bird can
sing,--the home of silence.
[Illustration: FIG. 73.--WITHERED HAND.]
But here, above it, hangs a world in the sky, which we should need to
call in color to depict, for it is green and yellow with the forests
and the harvest-fields that overspread its continents, with emerald
islands studding its gray oceans, over all of which sweep the clouds
that bring the life-giving rain. It is our own world, which lights up
the dreary lunar night, as the moon does ours.
[Illustration: FIG. 74.--IDEAL LUNAR LANDSCAPE AND EARTH-SHINE.]
The signs of age are on the moon. It seems pitted, torn, and rent by
the past action of long-dead fires, till its surface is like a piece
of porous cinder under the magnifying-glass,--a burnt-out cinder of a
planet, which rolls through the void like a ruin of what has been; and,
more significant still, this surface is wrinkled everywhere, till the
analogy with an old and shrivelled face or hand or fruit (Figs. 73 and
75), where the puckered skin is folded about a shrunken centre, forces
itself on our attention, and suggests a common cause,--a something
underlying the analogy, and making it more than a mere resemblance.
[Illustration: FIG. 75.--WITHERED APPLE.]
The moon, then, is dead; and if it ever was the home of a race like
ours, that race is dead too. I have said that our New Astronomy
modifies our view of the moral universe as well as of the physical one;
nor do we need a more pregnant instance than in this before us. In
these days of decay of old creeds of the eternal, it has been sought
to satisfy man’s yearning toward it by founding a new religion whose
god is Humanity, and whose hope lies in the future existence of our own
race, in whose collective being the individual who must die may fancy
his aims and purpose perpetuated in an endless progress. But, alas for
hopes looking to this alone! we are here brought to face the solemn
thought that, like the individual, though at a little further date,
Humanity itself may die!
Before we leave this dead world, let us take a last glance at one of
its fairest scenes,--that which we obtain when looking at a portion
on which the sun is rising, as in this view of Gassendi (Fig. 76),
in which the dark part on our right is still the body of the moon,
on which the sun has not yet risen. Its nearly level rays stretch
elsewhere over a surface that is, in places, of a strangely smooth
texture, contrasting with the ruggedness of the ordinary soil, which is
here gathered into low plaits, that, with the texture we have spoken
of, look
“Like marrowy crapes of China silk,
Or wrinkled skin on scalded milk,”
as they lie, soft and almost beautiful, in the growing light.
Where its first beams are kindling, the summits cast their shadows
illimitedly over the darkening plains away on the right, until they
melt away into the night,--a night which is not utterly black, for even
here a subdued radiance comes from the earth-shine of our own world in
the sky.
Let us leave here the desolation about us, happy that we can come back
at will to that world, our own familiar dwelling, where the meadows
are still green and the birds still sing, and where, better yet, still
dwells our own kind,--surely the world, of all we have found in our
wanderings, which we should ourselves have chosen to be our home.
[Illustration: FIG. 76.--GASSENDI. NOV. 7, 1867.]
VI.
METEORS.
What is truth? What is fact, and what is fancy, even with regard to
solid visible things that we may see and handle?
Among the many superstitions of the early world and credulous fancies
of the Middle Ages, was the belief that great stones sometimes fell
down out of heaven onto the earth.
Pliny has a story of such a black stone, big enough to load a chariot;
the Mussulman still adores one at Mecca; and a mediæval emperor of
Germany had a sword which was said to have been forced from one of
these bolts shot out of the blue. But with the revival of learning,
people came to know better! That stones should fall down from the sky
was clearly, they thought, an absurdity; indeed, according to the
learned opinion of that time, one would hardly ask a better instance
of the difference between the realities which science recognized and
the absurdities which it condemned than the fancy that such a thing
could be. So at least the matter looked to the philosophers of the
last century, who treated it much as they might treat certain alleged
mental phenomena, for instance, if they were alive to-day, and at first
refused to take any notice of these stories, when from time to time
they still came to hand. When induced to give the matter consideration,
they observed that all the conditions for scientific observation were
violated by these bodies, since the wonder always happened at some
far-off place or at some past time, and (suspicious circumstance!)
the stones only fell in the presence of ignorant and unscientific
witnesses, and never when scientific men were at hand to examine the
facts. That there were many worthy, if ignorant, men who asserted that
they had seen such stones fall, seen them with their very eyes, and
held them in their own hands, was accounted for by the general love of
the marvellous and by the ignorance of the common mind, unlearned in
the conditions of scientific observation, and unguided by the great
principle of the uniformity of the Laws of Nature.
Such a tone, of course, cannot be heard among us, who never hastily
pronounce anything a departure from the “Laws of Nature,” while
uncertain that these can be separated from the laws of the fallible
human mind, in which alone Nature is seen. But in the last century
philosophers had not yet become humble, or scientific men diffident
of the absoluteness of their own knowledge, and so it seemed that no
amount of evidence was enough to gain an impartial hearing in the face
of the settled belief that the atmosphere extended only a few miles
above the earth’s surface, and that the region beyond, whence alone
such things could come, was an absolute void extending to the nearest
planet.
[Illustration: FIG. 77.--THE CAMP AT MOUNT WHITNEY.
(FROM “PROFESSIONAL PAPERS OF THE SIGNAL SERVICE,” VOL. XV.)]
It used to be supposed that we were absolutely isolated, not only from
the stars but from other planets, by vast empty spaces extending from
world to world,--regions altogether vacant except for some vagrant
comet; but of late years we are growing to have new ideas on this
subject, and not only to consider space as far from void or tenantless,
but to admit, as a possibility at least, that there is a sort of
continuity between our very earth’s surface, the air above it, and
all which lies beyond the blue overarching dome of our own sky. Our
knowledge of the physical nature of the universe without has chiefly
come from what the spectroscope, overleaping the space between us
and the stars, has taught us of them; as a telegram might report to
us the existence of a race across the ocean, without telling anything
of what lay between. It would be a novel path to the stars, and to the
intermediate regions whence these once mythical stones are now actually
believed to come, if we could take the reader to them by a route which
enabled us to note each step of a continuous journey from the earth’s
surface out into the unknown; but if we undertake to start upon it, he
will understand that we must almost at the outset leave the ground of
comparative certainty on which we have hitherto rested, and need to
speak of things on this road which are still but probabilities, and
even some which are little more than conjectures, before we get to the
region of comparative certainty again,--a region which, strange to say,
exists far away from us, while that of doubt lies close at hand, for we
may be said without exaggeration to know more about Sirius than about
the atmosphere a thousand miles above the earth’s surface; indeed, it
would be more just to say that we are sure not only of the existence
but of the elements that compose a star, though a million of times as
far off as the sun, while at the near point named we are not sure of so
much as that the atmosphere exists at all.
To begin our outward journey in a literal sense, we might rise from
the earth’s surface some miles in a balloon, when we should find our
progress stayed by the rarity of the air. Below us would be a gray
cloud-ocean, through which we could see here and there the green
earth beneath, while above us there would still be something in the
apparently empty air, for if the sun has just set it will still be
_light_ all round us. Something then, in a cloudless sky, still exists
to reflect the rays towards us, and this something is made up of
separately invisible specks of dust and vapor, but very largely of
actual dust, which probably forms the nucleus of each mist-particle.
That discrete matter of some kind exists here has long been recognized
from the phenomena of twilight; but it is, I think, only recently that
we are coming to admit that a shell of actual solid particles in the
form of dust probably encloses the whole globe, up to far above the
highest clouds.
In 1881 the writer had occasion to conduct a scientific expedition to
the highest point in the territories of the United States, on one of
the summits of the Sierra Nevadas of Southern California, which rise
even above the Rocky Mountains.
The illustration on page 177 represents the camp occupied by this party
below the summit, where the tents, which look as if in the bottom of a
valley, are yet really above the highest zone of vegetation, and at an
altitude of nearly twelve thousand feet.
Still above these rise the precipices of barren rock seen in
the background, their very bases far above the highest visible
dust-clouds, which overspread like a sea the deserts at the mountain’s
foot,--precipices which when scaled lift the observer into what is,
perhaps, the clearest and purest air to be found in the world. It will
be seen from the mere looks of the landscape that we are far away here
from ordinary sources of contamination in the atmosphere. Yet even
above here on the highest peak, where we felt as if standing on the
roof of the continent and elevated into the great aerial currents of
the globe, the telescope showed particles of dust in the air, which the
geologists deemed to have probably formed part of the soil of China
and to have been borne across the Pacific, but which also, as we shall
see later, may owe something to the mysterious source of the phenomena
already alluded to.
It is far from being indifferent to us that the dust is there; for, to
mention nothing else, without it, it would be night till the sunrise,
and black night again as soon as the sun’s edge disappeared below
the horizon. The morning and the evening twilight, which in northern
latitudes increase our average time of light by some hours, and add
very materially to the actual days of man’s life, are probably due
almost wholly to particles scarcely visible in the microscope, and to
the presence of such atoms, smaller than the very motes ordinarily
seen in the sunbeam, which, as Mr. Aitken has shown, fill the air we
breathe,--so minute and remote are the causes on which the habits of
life depend.
Before we can see that a part of this impalpable, invisible dust is
also perhaps a link between our world and other members of the solar
system, we must ask how it gets into the atmosphere. Is it blown up
from the earth, or does it fall down out of the miscalled “void” of
space?
If we cast a handful of dust into the air, it will not mount far above
the hand unless we set the air in motion with it, as in ascending
smoke-currents; and the greatest explosions we can artificially
produce, hurl their finer products but a few hundred feet at most from
the soil. Utterly different are the forces of Nature. We have on page
183 a reproduction from a photograph of an eruption of Vesuvius,--a
mere toy-volcano compared to Etna or Hecla. But observe the smoke-cloud
which rises high in the sunshine, looking solid as the rounded snows of
an Alp, while the cities and the sea below are in the shadow. The smoke
that mounts from the foreground, where the burning lava-streams are
pouring over the surface and firing the woods, is of another kind from
that rolling high above. _This_ comes from within the mountain, and
is composed of clouds of steam mingled with myriads of dust-particles
from the comminuted products of the earth’s interior; and we can see
ourselves that it is borne away on a level, miles high in the upper air.