distances in all directions, and in calculable orbits; but as to
_what_ they were, this, even in the childhood of those of us who are
middle-aged, was as little known as to the centuries during which they
still from their horrid heads shook pestilence and war. We do not know
even now by any means exactly what they are, for enough yet remains to
be learned about them still to give their whole study the attraction
which belongs to the unknown; and yet we learn so much, and in a way
which to our grandfathers would have been so unexpected, connecting
together the comet, the shooting star, and the meteorite, that the
astronomer who perhaps speaks with most authority about these to-day
was able, not long ago, in beginning a lecture, to state that he held
in his hand what had been a part of a comet; and what he held was,
not something half vaporous or gaseous, as we might suppose from our
old associations, but a curious stone like this on page 203, which,
with others, had fallen from the sky in Iowa, a flashing prodigy, to
the terror of barking dogs, shying horses, and fearful men, followed
by clouds of smoke and vapor, and explosions that shook the houses
like an earthquake, and “hollow bellowings and rattling sounds mingled
with clang and clash and roar,” as an auditor described it. It is only
a fragment of a larger stone which may have weighed tons. It looks
inoffensive enough now, and its appearance affords no hint of the
commotion it caused in a peaceable neighborhood only ten years ago. But
what, it may be asked, is the connection between such things and comets?
To answer this, let us recall the statement that the orbit of the
November meteor swarm has been computed; which means that those flying
bodies have been found to come only from one particular quarter out
of all possible quarters, at one particular angle out of all possible
angles, at one particular velocity out of all possible velocities, and
so on; so that the chances are endless against mere accident producing
another body which agreed in all these particulars, and others besides.
Now, in 1867 the remarkable fact was established that a comet seen in
the previous year (Comet 1, 1866) had the same orbit as the meteoroids,
which implies, as we have just seen, that the comet and the meteors
were in some way closely related.
The paths of the August meteors and of the Lyrids also have both been
found to agree closely with those of known comets, and there is other
evidence which not only connects the comets and the shooting stars, and
makes it probable that the latter are due to some disintegration of the
former, but even looks as though the process were still going on. And
now with this in mind we may, perhaps, look at these drawings with more
interest.
[Illustration: FIG. 80.--COMET OF DONATI, SEPT. 16, 1858.[7]]
[7] The five engravings of the Comet of Donati are from “Annals
of the Astronomical Observatory of Harvard College.”
We have all seen a comet, and we have all felt, perhaps, something of
the awe which is called up by the thought of its immensity and its rush
through space like a runaway star. Its head is commonly like a small
luminous point, from which usually grows as it approaches the sun a
relatively enormous brush or tail of pale light, which has sometimes
been seen to stretch across the whole sky from zenith to horizon. It
is useless to look only along the ecliptic road for a comet’s coming;
rather may we expect to see it rushing down from above, or up from
below, sometimes with a speed which is possibly greater than it
could get from any fall--not so much, that is, the speed of a body
merely dropping toward the sun by its weight, as that of a missile
hurled into the orderly solar system from some unknown source without,
and also associated with some unknown power; for while it is doubtful
whether gravity is sufficient to account for the velocity of all
comets, it seems certain that gravity can in no way explain some of the
phenomena of their tails.
[Illustration: FIG. 81.--“A PART OF A COMET.”]
Thousands of comets have been seen since the Christian era, and the
orbits of hundreds have been calculated since the time of Newton.
Though they may describe any conic section, and though most orbits
are spoken of as parabolas, this is rather a device for the analyst’s
convenience than the exact representation of fact. Without introducing
more technical language, it will be enough to say here that we learn
in other cases from the form of the orbit whether the body is drawn
essentially by the sun’s gravity, or whether it has been thrown into
the system by some power beyond the sun’s control, to pass away again,
out of that control, never to return. It must be admitted, however,
that though several orbits are so classed, there is not any one known
to be beyond doubt of this latter kind, while we are certain that many
comets, if not all, are erratic members of the solar family, coming
back again after their excursions, at regular, though perhaps enormous,
intervals.
But what we have just been saying belongs rather to the province of
the Old Astronomy than the New, which concerns itself more with the
nature and appearance of the heavenly bodies than the paths they travel
on. Perhaps the best way for us to look at comets will be to confine
our attention at first to some single one, and to follow it from its
earliest appearance to its last, by the aid of pictures, and thus
to study, as it were, the species in the individual. The difficulty
will be one which arises from the exquisitely faint and diaphanous
appearance of the original, which no ordinary care can possibly render,
though here the reader has had done for him all that the wood-engraver
can do.
We will take as the subject of our illustration the beautiful comet
which those of us who are middle-aged can remember seeing in 1858,
and which is called Donati’s from the name of its discoverer. We
choose this one because it is the subject of an admirable monograph
by Professor Bond of the Harvard College Observatory, from which our
engravings have, by permission, been made.
Let us take the history of this comet, then, as a general type of
others; and to begin at the beginning, we must make the very essential
admission that the origin of the comet’s life is unknown to us. Where
it was born, or how it was launched on its eccentric path, we can only
guess, but do not know; and how long it has been traversing it we can
only tell later. On the 2d of June, 1858, this one was discovered
in the way most comets are found, that is, by a _comet-hunter_, who
detected it as a telescopic speck long before it became visible to the
naked eye, or put forth the tail which was destined to grow into the
beautiful object many of us can remember seeing. For over a century now
there has been probably no year in which the heavens have not been thus
searched by a class of observers who make comet-hunting a specialty.
[Illustration: FIG. 82.--COMET OF DONATI, SEPT. 24, 1858. (TELESCOPIC
VIEW OF HEAD.)]
The father of this very valuable class of observers appears to have
been Messier, a Frenchman of the last century and of the purest type of
the comet-hunters, endowed by Nature with the instinct for their search
that a terrier has for rats. In that grave book, Delambre’s “History of
Astronomy,” as we plod along its dry statements and through its long
equations, we find, unexpected as a joke in a table of logarithms, the
following piece of human nature (quoted from Messier’s contemporary, La
Harpe):--
“He [Messier] has passed his life in nosing out the tracks
of comets. He is a very worthy man, with the simplicity of a
baby. Some years ago he lost his wife, and his attention to her
prevented him from discovering a comet he was on the search for,
and which Montaigne of Limoges got away from him. He was in
despair. When he was condoled with on the loss he had met, he
replied, with his head full of the comet, ‘Oh, dear! to think
that when I had discovered twelve, this Montaigne should have got
my thirteenth.’ And his eyes filled with tears, till, remembering
what it was he ought to be weeping for, he moaned, ‘Oh, my poor
wife!’ but went on crying for his comet.”
Messier’s scientific posterity has greatly multiplied, and it is rare
now for a comet to be seen by the naked eye before it has been caught
by the telescope of one of these assiduous searchers. Donati had, as
we see, observed his some months before it became generally visible,
and accordingly the engraving on page 201 shows it as it appeared on
the evening of September 16, 1858, when the tail was already formed,
and, though small, was distinct to the naked eye, near the stars of the
Great Bear. The reader will easily recognize in the plate the familiar
“dipper,” as the American child calls it, where the leading stars are
put down with care, so that he may, if he please, identify them by
comparison with the originals in the sky, even to the little companion
to Mizar (the second in the handle of the “dipper,” and which the
Arabs say is the lost Pleiad). We would suggest that he should note
both the length of the tail on this evening as compared with the space
between any two stars of the “dipper” (for instance, the two right-hand
ones, called the “pointers”) and its distance from them, and then turn
to page 209, where we have the same comet as seen a little over a
fortnight later, on October 3d. Look first at its new place among the
stars. The “dipper” is still in view, but the comet has drifted away
from it toward the left and into other constellations. The large star
close to the left margin of the plate, with three little stars below
and to the right, is Arcturus; and the western stars of the Northern
Crown are just seen higher up. Fortunately the “pointers,” with which
we compared the comet on September 16th, are still here, and we can see
for ourselves how it has not only shifted but grown. The tail is three
times as long as before. It is rimmed with light on its upper edge,
and fades away so gradually below that one can hardly say where it
ends. But,--wonderful and incomprehensible feature!--shot out from the
head, almost as straight as a ray of light itself, but fainter than the
moonbeam, now appears an extraordinary addition, a sort of spur, which
we can hardly call a new tail, it is so unlike the old one, but which
appears to have been darted out into space as if by some mysterious
force acting through the head itself. What the spur is, what the tail
is, even what the nucleus is, we cannot be said really to know even
to-day; but of the tail and of the nucleus or speck in the very head of
the comet (too small to be visible in the engraving), we may say that
the hairy tail (_comes_) gives the comet its name, and _is_ the comet
to popular apprehension, but that it is probably the smallest part of
the whole mass, while the little shining head, which to the telescope
presents a still smaller speck called the nucleus, contains, it now
seems probable, the only element of possible danger to the earth.
While admitting our lack of absolute knowledge, we may, if we agree
that meteorites were once part of a comet, say that it now seems
probable that the nucleus is a hard, stone-like mass, or collection
of such masses, which comes from “space” (that is, from we don’t know
how far) to the vicinity of the sun, and there is broken by the heat
as a stone in a hot fire. (Sir Isaac Newton calculates, in an often
quoted passage of the Principia, that the heat which the comet of 1680
was subjected to in its passage by the sun was two thousand times
that of red-hot iron.) We have seen the way in which meteoric stones
actually do crack in pieces with heat in our own atmosphere, partly,
perhaps, from the expansion of the gases the stone contains, and it
seems entirely reasonable to suppose that they may do so from the heat
of the sun, and that the escaped gases may contribute something toward
the formation of the tail, which is always turned away from the sun,
and which always grows larger as that is approached, and smaller as
it is receded from. However this may be, there is no doubt that the
original solid which we here suppose may form the nucleus is capable
of mischief, for it is asserted that it often passes the earth’s orbit
with a velocity of as much as one hundred times that of a cannon-ball;
that is, with ten thousand times the destructive capacity of a ball of
the same weight shot from a cannon.
[Illustration: FIG. 83.--COMET OF DONATI, OCT. 3, 1858.]
One week later, October 9th, the comet had passed over Arcturus
with a motion toward our left into a new region of the sky, leaving
Arcturus, which we can recognize with the upper one of its three little
companions, on the right. Above it is the whole sickle of the Northern
Crown, and over these stars the extremity of the now lengthened tail
was seen to spread, but with so thin a veil that no art of the engraver
can here adequately represent its faintness. The tail then, as seen in
the sky, was now nearly twice its former size, though for the reason
mentioned it may not appear so in our picture. It should be understood,
too, that even the brightest parts of the original were far fainter
than they seem here in comparison with the stars, which in the sky
are brilliant points of light, which the engraver can only represent
by dots of the whiteness of the paper. This being observed, it will
be better understood that in the sky itself the faintest stars were
viewed apparently undimmed through the brighter parts of the comet,
while we can but faintly trace here another most faint but curious
feature, a division of the tail into faint cross-bands like auroral
streamers, giving a look as if it were yielding to a wind, which folded
it into faint ridges like those which may be seen in the smoke of a
steamer as it lags far behind the vessel. In fact, when we speak of
“the” tail, it must be understood, as M. Faye reminds us, to be in
the same sense that we speak of the plume of smoke that accompanies
an ocean steamer, without meaning that it is the same thing which
we are watching from night to night, more than we do that the same
smoke-particles accompany the steamer as it moves across the Atlantic.
In both cases the form alone probably remains; the thing itself is
being incessantly dissipated and renewed. There is no air here, and yet
some of these appearances in the original almost suggest the idea of
medium inappreciably thin as compared with the head of the comet, but
whose resistance is seen in the more unsubstantial tail, as that is
drawn through it and bent backward, as if by a wind blowing toward the
celestial pole.
The most notable feature, however, is the development of a second ray
or spur, which has been apparently darted through millions of miles in
the interval since we looked at it, and an almost imperceptible bending
backward in both, as if they too felt the resistance of something in
what we are accustomed to think of as an absolute and perfect void.
These tails are a peculiarly mysterious feature. They are apparently
shot out in a direction opposite to the sun (and consequently opposed
to the direction of gravity) at the rate of millions of miles a day.
[Illustration: FIG. 84.--COMET OF DONATI, OCT. 9, 1858.]
Beyond the fact that the existence of some _repulsive_ force in the
sun, a “negative gravity” actually existent, not in fancy, but in fact,
seems pointed at, astronomers can offer little but conjecture here;
and while some conceive this force as of an electrical nature, others
strenuously deny it. We ought to admit that up to the present time we
really know nothing about it, except that it exists.
At this date (October 9th) the comet had made nearly its closest
approach to the earth, and the general outline has been compared to
that of the wing of some bird, while the actual size was so vast that
even at the distance from which it was seen it filled an angle more
than half of that from the zenith to the horizon.
All the preceding drawings have been from naked-eye views; but if
the reader would like to look more closely, he can see on page 217
one taken on the night of October 5th through the great telescope
at Cambridge, Mass. We will leave this to tell its own story, only
remarking that it is not possible to reproduce the phantom-like
faintness of the original spur, here also distinctly seen, or indeed
to indicate fairly the infinite tenuity of the tail itself. Though
millions of miles thick, the faintest star is yet perceptibly undimmed
by it, and in estimating the character and quantity of matter it
contains, after noting that it is not self-luminous, but shines
only like the moon by reflected sunlight, we may recall the acute
observation of Sir Isaac Newton where he compares the brightness of a
comet’s tail with that of the light reflected from the particles in a
sunbeam an inch or two thick, in a darkened room, and, after observing
that if a little sphere of common air one inch in diameter were
rarified to the degree which must obtain at only four thousand miles
from the earth’s surface it would fill all the regions of the planets
to far beyond the orbit of Saturn, suggests the excessively small
quantity of vapor that is really requisite to create this prodigious
phantom.
The writer has had occasion for many years to make a special study of
the reflection of light from the sky; and if such studies may authorize
him to express any opinion of his own, he would give his adhesion to
the remark of Sir John Herschel, that the actual weight of matter in
such a cometary tail may be conceivably only an affair of pounds or
even ounces. But if this is true of the tail, it does not follow of
the nucleus, just seen in this picture, but of which the engraving on
page 205 gives a much more magnified view. It is a sketch of the head
alone, taken from a telescopic view on the 24th of September. Here the
direction of the comet is still toward the sun (which must be supposed
to be some indefinite distance beyond the upper part of the drawing),
and we see that the lucid matter appears to be first jetted up, and
then forced backward on either side, as if by a wind _from_ the sun,
to form the tail, presenting successive crescent-shaped envelopes of
decreasing brightness, which are not symmetrical, but one-sided, while
sometimes the appearance is that of spurts of luminous smoke, wavering
as if thrown out of particular parts of the internal nucleus “like a
squib not held fast.” Down the centre of the tail runs a wonderfully
straight black line, like a shadow cast from the nucleus. Only the
nucleus itself still evades us, and even in this, the most magnified
view which the most powerful telescope till lately in existence could
give, remains a point.
Considering the distance of the comet and the other optical conditions,
this is still perfectly consistent with the possibility that it may
have an actual diameter of a hundred miles or more. It “may” have,
observe, not it “has,” for in fact we know nothing about it; but that
it is at any rate less than some few hundred miles in diameter, and it
may, for anything we can positively say, not be more than a very large
stone, in which case our atmosphere would probably act as an efficient
buffer if it struck us; or it may have a mass which, coupled with its
terrible speed, would cause the shock of its contact not so much to
pulverize the region it struck, as dissipate it and everything on it
instantly into vapor.
[Illustration: FIG. 85.--COMET OF DONATI, OCT. 5, 1858. (TELESCOPIC
VIEW.)]
Of the remarkable investigations of the spectroscope on comets, we
have only room left to say that they inform us that the most prominent
cometary element seems to be carbon,--carbon, which Newton two hundred
years before the spectroscope, and before the term “carbonic-acid gas”
was coined, by some guess or divination had described in other words
as possibly brought to us by comets to keep up the carbonic-acid-gas
supply in our air,--carbon, which we find in our own bodies, and of
which, according to this view, the comets are original sources.
That _we_ may be partly made of old and used-up comets,--surely it
might seem that a madder fancy never came from the brain of a lunatic
at the full of the moon!
Science may easily be pardoned for not giving instant reception to such
an idea, but let us also remember, first, that it is a consequence of
that of Sir Isaac Newton, and that in the case of such a man as he
we should not be hasty to think we understand his ignorance, when we
may be “ignorant of his understanding;” and, second, that it has been
rendered at least debatable by Dr. Hunt’s recent researches whether
it is possible to account for the perennial supply of carbon from the
earth’s atmosphere, without looking to some means of renewal external
to the planet.
The old dread of comets is passing away, and all that science has
to tell us of them indicates that, though still fruitful sources of
curiosity and indeed of wonder, they need no longer be objects of
terror. Though there be, as Kepler said, more comets in the sky than
fish in the ocean, the encounter of the earth with a comet’s tail would
be like the encounter with a shadow, and the chance of a collision
with the nucleus is remote indeed. We may sleep undisturbed even if
a new comet is announced every month, though it is true that here as
elsewhere lie remote possibilities of evil.
The consideration of the unfamiliar powers certainly latent in Nature,
such as belong to a little tremor of the planet’s surface or such
as was shown in that scene I have described, when the comparatively
insignificant effect of the few tons of dynamite was to make solid
buildings unrealities, which vanished away as quickly as magic-lantern
pictures from a screen, may help us to understand that the words of
the great poet are but the possible expression of a physical fact,
and that “the cloud-capped towers, the gorgeous palaces, the solemn
temples,”--and we with them,--may indeed conceivably some day vanish
as the airy nothings at the touch of Prospero’s wand, and without the
warning to us of a single instant that the security of our ordinary
lives is about to be broken. We concede this, however, in the present
case only as an abstract possibility; for the advance of astronomical
knowledge is much more likely to show that the kernel of the comet
is but of the bigness of some large meteorite, against which our air
is an efficient shield, and the chance of evil is in any case most
remote,--in any case only such as may come in any hour of our lives
from any quarter, not alone from the earthquake or the comet, but
from “the pestilence that walketh in darkness;” from the infinitely
little below and within us, as well as from the infinite powers of the
universe without.
VIII.
THE STARS.
In the South Kensington Museum there is, as everybody knows, an immense
collection of objects, appealing to all tastes and all classes, and
we find there at the same time people belonging to the wealthy and
cultivated part of society lingering over the Louis Seize cabinets or
the old majolica, and the artisan and his wife studying the statements
as to the relative economy of baking-powders, or admiring Tippoo Saib’s
wooden tiger.
There is one shelf, however, which seems to have some attraction common
to all social grades, for its contents appear to be of equal interest
to the peer and the costermonger. It is the representation of a _man_
resolved into his chemical elements, or rather an exhibition of the
materials of which the human body is composed. There is a definite
amount of water, for instance, in our blood and tissues, and there on
the shelf are just so many gallons of water in a large vessel. Another
jar shows the exact quantity of carbon in us; smaller bottles contain
our iron and our phosphorus in just proportion, while others exhibit
still other constituents of the body, and the whole reposes on the
shelf as if ready for the coming of a new Frankenstein to re-create
the original man and make him walk about again as we do. The little
vials that contain the different elements which we all bear about in
small proportions are more numerous, and they suggest, not merely the
complexity of our constitutions, but the identity of our elements with
those we have found by the spectroscope, not alone in the sun, but even
in the distant stars and nebulæ; for this wonderful instrument of the
New Astronomy can find the traces of poison in a stomach or analyze
a star, and its conclusions lead us to think that the ancients were
nearly right when they called man a microcosm, or little universe. We
have literally within our own bodies samples of the most important
elements of which the great universe without is composed; and you and I
are not only like each other, and brothers in humanity, but children of
the sun and stars in a more literal sense, having bodies actually made
in large part of the same things that make Sirius and Aldebaran. They
and we are near relatives.
[Illustration: FIG. 86.--TYPES OF STELLAR SPECTRA.]