different proposition. It, too, is apt to be detected coming out of
the depths of space before the world at large can get a glimpse of it,
but as it approaches the sun its aspect undergoes a marvelous change.
Agitated apparently by solar influence, it throws out a long streaming
tail of nebulous light, directed away from the sun and looking as if
blown out like a pennon by a powerful wind. Whatever may be the
position of the comet with regard to the sun, as it circles round him
it continually keeps its tail on the off side. This, as we shall soon
see, is a fact of capital importance in relation to the probable
nature of comets' tails. Almost at the same time that the formation of
the tail is observed a remarkable change takes place in the comet's
head, which, by the way, is invariably and not merely occasionally its
most important part. On approaching the sun the head usually
contracts. Coincidently with this contraction a nucleus generally
makes its appearance. This is a bright, star-like point in the head,
and it probably represents the totality of solid matter that the comet
possesses. But it is regarded as extremely unlikely that even the
nucleus consists of a uniformly solid mass. If it were such, comets
would be far more formidable visitors when they pass near the planets
than they have been found to be. The diameter of the nucleus may vary
from a few hundred up to several thousand miles; the heads, on the
average, are from twenty-five thousand to one hundred thousand miles
in diameter, although a few have greatly exceeded these dimensions;
that of the comet of 1811, one of the most stupendous ever seen, was a
million and a quarter miles in diameter! As to the tails, not
withstanding their enormous length -- some have been more than a
hundred million miles long -- there is reason to believe that they are
of extreme tenuity, ``as rare as vacuum.'' The smallest stars have
been seen shining through their most brilliant portions with
undiminished luster.