s unusual that so large and interesting
a comet should return within a comparatively limited time. It is the
smaller comets, those that can only be seen telescopically, that usually
run in small orbits. The smallest orbits take about three and a half
years to traverse, and some of the largest orbits known require a period
of one hundred and ten thousand years. Between these two limits lies
every possible variety of period. One comet, seen about the time
Napoleon was born, was calculated to take two thousand years to complete
its journey, and another, a very brilliant one seen in 1882, must
journey for eight hundred years before it again comes near to the sun.
But we never know what might happen, for at any moment a comet which has
traversed a long solitary pathway in outer darkness may flash suddenly
into our ken, and be for the first time noted and recorded, before
flying off at an angle which must take it for ever further and further
from the sun.

Everything connected with comets is mysterious and most fascinating.
From out of the icy regions of space a body appears; what it is we know
not, but it is seen at first as a hairy or softly-glowing star, and it
was thus that Herschel mistook Uranus for a comet when he first
discovered it. As it draws nearer the comet sends out some fan-like
projections toward the sun, enclosing its nucleus in filmy wrappings
like a cocoon of light, and it travels faster and faster. From its head
shoots out a tail--it may be more than one--growing in splendour and
width, and always pointing away from the sun. So enormous are some of
these tails that when the comet's head is close to the sun the tail
extends far beyond the orbit of the earth. Faster still and faster flies
the comet, for as we have seen it is a consequence of the law of
gravitation that the nearer planets are to the sun the faster they move
in their orbits, and the same rule applies to comets too. As the comet
dashes up to the sun his pace becomes something indescribable; it has
been reckoned for some comets at three hundred miles a second! But
behold, as the head flies round the sun the tail is always projected
outwards. The nucleus or head may be so near to the sun that the heat it
receives would be sufficient to reduce molten iron to vapour; but this
does not seem to affect it: only the tail expands. Sometimes it becomes
two or more tails, and as it sweeps round behind the head it has to
cover a much greater space in the s