The tails
sometimes appear single, sometimes, although more rarely, double; and in the
comets of 1807 and 1843 the branches were of different lengths; in one
instance (1744) the tail had six branches, the whole forming an angle of 60
degrees. The tails have been sometimes straight, sometimes curved, either
toward both sides, or toward the side appearing to us as the exterior (as in
1811), or convex toward the direction in which the comet is moving (as in
that of 1618); and sometimes the tail has even appeared like a flame in
motion. The tails are always turned away from the sun, so that their line
of prolongation passes through its center; a fact which, according to Edward
Biot, was noticed by the Chinese astronomers as early as 837, but was first
generally made known in Europe by Fracastoro and Peter Apian in the
sixteenth century. These emanations may be regarded as conoidal envelopes
of greater of less thickness,
p 102
and, considered in this manner, they furnish a simple explanation of many of
the remarkable optical phenomena already spoken of.
Comets are not only characteristically different in form, some being
entirely without a visible tail, while others have a tail of immense length
(as in the instance of the comet of 1618, whose tail measured 104 degrees),
but we also see the same comets undergoing successive and rapidly-changing
processes of configuration.
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