We have already alluded to a little body called Encke’s comet, which
was discovered by an astronomer at Marseilles. It was in the year 1818
that he was scanning the heavens with a small telescope, when an object
attracted his attention. It was not one of those grand long-tailed
comets which every one notices; this body was so faint that it merely
appeared as a very small cloud of light, and was recognized as a comet
by the fact that it was moving. It happens that there are other bodies
in the sky very like comets; we call them nebulæ, and we shall have
something to say about them afterwards. But it is remarkable that
just as a planet is liable to be mistaken for a star, so a comet is
liable to be mistaken for a nebula. However, in each case the fact of
its movement is the test by which the planet or the comet is at once
detected. A nebula stays always in the same spot, like a star, while a
comet is incessantly moving. In fact, with a telescope you can actually
watch a comet stealing past the stars that lie near it. You know that
an object a very long way off may appear to move slowly, though in
reality it is moving very rapidly. Look at a steamer near the horizon
at sea. In the course of a minute or two it will not appear to have
shifted its position to any appreciable extent, but that is because
it is far off. If you were near the ship, you would see that it was
dashing along at the rate of perhaps fifteen or twenty miles an hour.
In a similar manner the comet seems to move slowly, because it is at
such a great distance. As a matter of fact it is moving faster at the
time we see it than any steamer, faster than any express train, faster
than any cannon-ball. There were special reasons why the movements
of Encke’s comet should be watched with peculiar care, and the track
which it pursued be ascertained. If you can observe a comet three
times and measure its position in the sky, the movement of that comet
is completely determined. Perhaps I should say would be determined if
the comet were let alone, which, unfortunately, is not often the case.
Indeed, you may remember how I told you some of the misadventures of
this very comet when we were speaking about the planet Mercury. Encke’s
comet comes round in a period of a little more than three years, and it
gives us some curious information that has been ascertained during its
journeys. One of the facts we have thus learned is so important that we
cannot omit to notice it (Fig. 73).
[Illustration: FIG. 73.--The Orbit of Encke’s Comet.]
At increasing heights above the earth’s surface there is gradually
less and less air; until at last, at about 200 or 300 miles above the
surface on which we dwell, there would be none. You might as well try
to quench your thirst by drinking out of an empty cup as attempt to
breathe in the open space which begins a few hundred miles aloft.
In open space motion could take place quite freely. Down here the
resistance of the air is a great impediment to movement, especially
when very rapid. A heavy cannon-ball is checked and robbed of its pace
by having to plough its way through our dense atmosphere. The motion
is arrested in the same way, though not of course to the same degree,
as if the cannon-ball had been fired into water. Unsubstantial objects
are, of course, impeded by the air to a far greater extent than such
heavy bodies as cannon-balls. You know that you cannot throw a handful
of feathers across the road in the same way that you could throw a
handful of gravel. The light feathers cannot force their way through
the air so well as the pebbles. A body so flimsy as a comet would
never be able to push its way through an atmosphere like ours; but out
in empty space the comet meets with no resistance during the greater
part of its path. Accordingly, though it has little more substance
than a will-o’-the-wisp, the comet pursues its journey with as much
resolute dignity as if it were made of cast iron. If in any part of
its track the body should have to pierce its way through any material
like even the thinnest possible air, then the unsubstantial nature of
the cometary materials would be at once shown. The motion would be
impeded, and the body’s path would be changed. In this way a comet may
be made very instructive, for it will show whether space is really
so empty as we sometimes suppose it to be. During the greater part
of its course the flimsy little Encke tears along with such ease and
speed that there seems to be nothing to impede it, and thus we learn
that space is generally empty. However, when the comet begins to wheel
around the sun, the freedom of its movements seems to receive a check.
The unsubstantial object has to force its way with a difficulty that
it did not experience so long as it was moving round the greater part
of its orbit. We thus learn that there is a thin diffused atmosphere
surrounding the sun. We cannot, indeed, say that it is like our air.
Its composition is quite different, and almost the only way we know
of its existence is by the evidence which this comet affords. In a
former lecture I showed how Encke’s comet told us the mass of the
planet Mercury. Now we see how the travels of the same body give us
information about the sun himself. I ought, however, to add that some
more recent observations seem not to have confirmed the belief that
there is the resistance of the kind we have just been considering.