A comet is made of very unsubstantial material. This we can show in a
very interesting manner, when we see it moving over the sky between
the earth and the stars. Sometimes a comet will pass over a cluster of
very small stars, so faint that the very lightest cloud that is ever
in the sky would be quite sufficient to hide them. Yet the stars are
distinctly visible right through the comet, notwithstanding that it may
be hundreds of thousands of miles thick. This shows us how excessively
flimsy is the substance of a comet, for while a few feet of haze or
mist suffice to extinguish the brightest of stars, this immense curtain
of comet stuff, whatever it may be made of, is practically transparent.
I have often told you that we are able to weigh the heavenly bodies,
but a comet gives us a great deal of trouble. You see that the weighing
machine must be of a very delicate kind if you are going to weigh a
very light object. Take, for example, a little lock of golden hair,
which no doubt has generally a value quite independent of the number of
grains that it contains. Suppose, however, that we are so curious as to
desire to know its weight, then one of those beautiful balances in our
laboratories will tell us. In fact, if you snipped a little fragment
from a single hair, the balance would be sensitive enough to weigh it.
If, however, you were only provided with a common pair of scales like
those which are suited for the parcel post, then you could never weigh
anything so light as a lock of hair. You have not small enough weights
to begin with, and even if you had they would be of no use, for the
scale is too coarse to estimate such a trifle. This is precisely the
sort of difficulty we experience when we try to weigh a comet. The
body, though so big, is very light, and our scales are so cumbersome
that we are in a position of one who would try to weigh a lock of hair
with a parcel-post balance. We cannot always find suitable scales for
weighing celestial bodies. We have to use for the purpose whatever
methods of discovering the weights happen to be available. So far, the
methods I have mentioned are of the rudest description; they serve well
enough for weighing heavy masses like planets, but they will not do for
such unsubstantial bodies as comets.
But, though we fail in this endeavor, _i.e._ to weigh comets, yet
skilful astronomers have succeeded in something which at first you
might think to be almost impossible. They have actually been able to
discover some of the ingredients of which a comet is made. This is so
important a subject that I must explain it fully.
The most instructive comet which we have seen in modern days is that
which appeared in the year 1882. It was an object so great that its
tail alone was double as long as from the earth to the sun. It was
discovered at the observatories in the southern hemisphere early in
September of that year. A little later it was observed in the northern
hemisphere in extraordinary circumstances. It must be remembered that
a comet is generally a faint object, and that even those comets which
are large enough and bright enough to form glorious spectacles in the
sky at night are usually invisible during the brightness of day. For a
comet to be seen in daylight was indeed an unusual occurrence; but on
the forenoon of Sunday, September 17, Mr. Common at Ealing saw a great
comet close to the sun. Unfortunately clouds intervened, and he was
prevented from observing the critical occurrence just approaching. An
astronomer at the Cape of Good Hope--Mr. Finlay--who had also been one
of the earliest discoverers of the comet, was watching the body on the
same day. He followed it as it advanced close up to the sun; bright
indeed must that comet have been which permitted such a wonderful
observation. At the sun’s edge the comet disappeared instantly; in
fact, the observers thought that it must have gone behind the sun. They
could not otherwise account for the suddenness with which it vanished.
This was not what really happened. It was afterwards ascertained that
the comet had not passed behind the sun; it had, indeed, come between
us and our luminary. In its further progress this body showed in a
striking degree the incoherent nature of the materials of which a comet
is composed. It seemed to throw off portions of its mass along its
track, each of which continued an independent journey. Even the central
part in the head of the comet--the nucleus, as it is called--showed
itself to be of a widely different nature from a substantial planetary
body. The nucleus divided into two, three, four, or even five distinct
parts, which seemed, in the words of one observer, to be connected
together like pearls on a string.
The comet of 1882 was also very instructive with regard to the actual
materials from which such bodies are made. Astronomers have a beautiful
method by which they find out the substances present in a heavenly
body, even though they never can get a specimen of the body into their
hands. We know at least three materials which were present in this
comet. The first of them is an ingredient which is very commonly found
in comets--a chemist calls it carbon. It is an extremely familiar
material on the earth; for instance, coal is chiefly composed of
carbon. Charcoal when burned leaves only a few ashes. All the substance
that has vanished during combustion is carbon; in fact, it is not too
much to say that carbon is found abundantly not only in wood, but
in almost every form of vegetable matter. The food we eat contains
abundant carbon, and it is an important constituent in the building up
of our own bodies. Generally speaking, carbon is not found in a pure
state--it is associated with other substances. Soot and lampblack are
largely composed of it; but the purest form of this element carbon that
we know is the diamond.
It is interesting to note that carbon is certainly found as a frequent
constituent of comets. The great comet of 1882 undoubtedly contained
it, as well as certain other substances. Of these we know two: the
first is the element sodium, an extremely abundant material on earth,
inasmuch as the salt in the sea is mainly composed of it. It was also
discovered that the same great comet contained another substance very
common here and extremely useful to mankind. Dr. Copeland and Dr. Lohse
at Dunecht showed that iron was present in this body which had come in
to visit us from the depths of space.
These discoveries are especially interesting because they seem to show
the uniformity of material composing our system. We already knew that
sodium and iron abounded in the sun, and now we have learned that
these bodies and carbon as well are present in the comets. In the
next chapter we shall learn that the very same materials--sodium and
iron--are met with in bodies far more remote from us than any bodies of
our own system.
Comets have such a capricious habit of dashing into the solar system
at any time and from any direction, that it is worth while asking
whether a comet might not sometimes happen to come into collision with
the earth. There is nothing impossible in such an occurrence. There
is, however, no reason to apprehend that any disastrous consequences
would ensue to the earth. Man has lived on this globe for many, many
thousands of years, and the rocks are full of the remains of fossil
animals which have flourished during past ages; indeed, we cannot
possibly estimate the number of millions of years that have elapsed
since living things first crawled about this globe. There has never
been any complete break in the succession of life, consequently during
all those millions of years we are certain that no such dire calamity
has happened to the earth as a frightful collision would have produced,
and we need not apprehend any such catastrophe in the future.
I do not mean, however, that harmless collisions with comets may not
have occasionally happened; in fact, there is good reason for knowing
that they have actually taken place. In the year 1861 a fine comet
appeared; and it is not so well remembered as its merits deserve,
because it happened, unfortunately for its own renown, to appear
just three years after the comet of 1858, which was one of the most
gorgeous objects of this kind in modern times. But in 1861 we had a
novel experience. On a Sunday evening in midsummer of that year, we
dashed into the comet, or it dashed into us. We were not, it is true,
in collision with its densest part; it was rather the end of the tail
which we encountered. There were, fortunately, no very serious results.
Indeed, most of us never knew that anything had happened at all, and
the rest only learned of the accident long after it was all over.
For a couple of hours that night it would seem that we were actually
in the tail of the comet, but so far as I know no one was injured or
experienced any alarming inconvenience. Indeed, I have only heard of
one calamity arising from the collision. A clergyman tells us that at
midsummer he was always able in ordinary years to read his sermon at
evening service without artificial light. On this particular occasion,
however, the sky was overcast with a peculiar glow, while the ordinary
light was so much interfered with that the sexton had to provide a pair
of candles to enable him to get through the sermon. The expense of
those candles was, I believe, the only loss to the earth in consequence
of its collision with the comet of 1861.
[Illustration: FIG. 76.--How the Tail of a Comet arises.]
The tail of a comet appears to develop under the influence of the sun.
As the wandering body approaches the source of central heat it grows
warm, and as it gets closer and closer to the sun, the fervor becomes
greater and greater, until sometimes the comet experiences a heat more
violent than any we could produce in our furnaces. The most infusible
substances, such as stones or earth, would be heated white-hot and
melted, and even driven off into vapor, under the intense heat to which
a comet is sometimes exposed. Comets, indeed, have been known to sweep
round the sun so closely as to pass within a seventh part of its radius
from the surface. It seems that certain materials present in the comet,
when heated to this extraordinary temperature, are driven away from
the head, and thus form the tail (Fig. 76). Hence we see that the tail
consists of a stream of vaporous particles, dashing away from the sun
as if the heat which had called them into being was a torment from
which they were endeavoring to escape.
The tail of a comet is, therefore, not a permanent part of the body.
It is more like the smoke from a great chimney. The smoke is being
incessantly renewed at one end as the column gets dispersed into the
air at the other. As the comet retreats, the sun’s heat loses its
power. In the chills of space there is, therefore, no tail-making
in progress, while the small mass of the comet renders it unable to
gather back again by its attraction the materials which have been
expelled. Should it happen that the comet moves in an elliptic orbit,
and thus comes back time after time to be invigorated by a good
roasting from the sun, it will, of course, endeavor to manufacture a
tail each time that it approaches the source of heat. The quantity
of material available for the formation of tails is limited to the
amount with which the comet originally started; no fresh supply can be
added. If, therefore, the comet expends a portion of this every time
it comes round, an inevitable consequence seems to follow. Suppose a
boy receives a sovereign when he goes back to school, and that every
time he passes the pastry-cook’s shop some of his money disappears in
a manner that I dare say you can conjecture, I need not tell you that
before long the sovereign will have totally vanished. In a similar way
comets cannot escape the natural consequences of their extravagance;
their store of tail-making substance must, therefore, gradually
diminish. At each successive return the tails produced must generally
decline in size and magnificence, until at last the necessary
materials have been all squandered, and we have the pitiful spectacle
of a comet without any tail at all.
The gigantic size of comets must excite our astonishment. A pebble
tossed into a river would not be more completely engulfed than is our
whole earth when it enters the tail of one of these bodies. But we now
pass by a sudden transition to speak of the very smallest bodies, of
little objects so minute that you could carry them in your waistcoat
pocket. You will perhaps be surprised that such things can play an
important part in our system and have a momentous connection with
mighty comets.