ame time, and therefore it must
travel even faster than the head. The pace is such that no calculations
can account for it, if the tail is composed of matter in any sense as we
know it. Then when the sun is passed the comet sinks away again, and as
it goes the tail dies down and finally disappears. The comet itself
dwindles to a hairy star once more and goes--whither? Into space so
remote that we cannot even dream of it--far away into cold more
appalling than anything we could measure, the cold of absolute space.
More and more slowly it travels, always away and away, until the sun, a
short time back a huge furnace covering all the sky, is now but a faint
star. Thus on its lonely journey unseen and unknown the comet goes.

This comet which we have taken as an illustration is a typical one, but
all are not the same. Some have no tails at all, and never develop any;
some change utterly even as they are watched. The same comet is so
different at different times that the only possible way of identifying
it is by knowing its path, and even this is not a certain method, for
some comets appear to travel at intervals along the same path!

Now we come to the question that must have been in the mind of everyone
from the beginning of this chapter, What are comets? This question no
one can answer definitely, for there are many things so puzzling about
these strange appearances that it is difficult even to suggest an
explanation. Yet a good deal is known. In the first place, we are
certain that comets have very little density--that is to say, they are
indescribably thin, thinner than the thinnest kind of gas; and air,
which we always think so thin, would be almost like a blanket compared
with the material of comets. This we judge because they exercise no sort
of influence on any of the planetary bodies they draw near to, which
they certainly would do if they were made of any kind of solid matter.
They come sometimes very close to some of the planets. A comet was so
near to Jupiter that it was actually in among his moons. The comet was
violently agitated; he was pulled in fact right out of his old path, and
has been going on a new one ever since; but he did not exercise the
smallest effect on Jupiter, or even on the moons. And, as I said earlier
in this chapter, we on the earth have been actually in the folds of a
comet's tail. This astonishing fact happened in June, 1861. One evening
after the sun had set a golden-yellow disc, surrou