The vapoury tail often grows to an enormous length as the comet
approaches the sun. The great comet of 1843 had a tail two hundred
million miles long. It is, however, composed of the thinnest vapours
imaginable. Twice during the nineteenth century the earth passed through
the tail of a comet, and nothing was felt. The vapours of the tail are,
in fact, so attenuated that we can hardly imagine them to be white-hot.
They may be lit by some electrical force. However that may be, the comet
dashes round the sun, often at three or four hundred miles a second,
then may pass gradually out of our system once more. It may be a
thousand years, or it may be fifty years, before the monarch of the
system will summon it again to make its fiery journey round his throne.
[Illustration: _Photo: Harvard College Observatory._