hich is well known to all who
are conversant with such matters. Now, that is 25,600. Only imagine a
glare 25,600 times fiercer than that of the equatorial sunshine at noon
day with the sun vertical. In such a heat there is no substance we know
of which would not run like water, boil, and be converted into smoke or
vapour. No wonder the comet gave evidence of violent excitement, coming
from the cold region outside the planetary system torpid and ice-bound.
Already when arrived even in our temperate region it began to show signs
of internal activity; the head had begun to develop, and the tail to
elongate, till the comet was for a time lost sight of--not for days
afterwards was it seen; and its tail, whose direction was reversed, and
which could not possibly be the same tail it had before, had already
lengthened to an extent of about ninety millions of miles, so that it
must have been shot out with immense force in a direction away from the
sun.'

We remember that comets have sometimes more than one tail, and a theory
has been advanced to account for this too. It is supposed that perhaps
different elements are thrust away by the sun at different angles, and
one tail may be due to one element and another to another. But if the
comet goes on tail-making to a large extent every time it returns to the
sun, what happens eventually? Do the tails fall back again into the head
when out of reach of the sun's action? Such an idea is inconceivable;
but if not, then every time a comet approaches the sun he loses
something, and that something is made up of the elements which were
formerly in the head and have been violently ejected. If this be so we
may well expect to see comets which have returned many times to the sun
without tails at all, for all the tail-making stuff that was in the head
will have been used up, and as this is exactly what we do see, the
theory is probably true.

Where do the comets come from? That also is a very large question. It
used to be supposed they were merely wanderers in space who happened to
have been attracted by our sun and drawn into his system, but there are
facts which go very strongly against this, and astronomers now generally
believe that comets really belong to the solar system, that their proper
orbits are ellipses, and that in the case of those which fly off at such
an angle that they can never return they must at some time have been
pulled out of their original orbit by the influence of one