ge below the horizon; it may stream down from the head of
the comet, as if the body had been shot up from below; it may slope to
the right or to the left. Amid all this variety and seeming caprice, can
we discover any feature common to the different phenomena? We shall find
that there is a very remarkable law which the tails of comets obey--a
law so true and satisfactory, that if we are given the place of a comet
in the heavens, it is possible at once to point out in what direction
the tail will lie.

A beautiful comet appears in summer in the northern sky. It is near
midnight; we are gazing on the faintly luminous tail, which stands up
straight and points towards the zenith; perhaps it may be curved a
little or possibly curved a good deal, but still, on the whole, it is
directed from the horizon to the zenith. We are not here referring to
any particular comet. Every comet, large or small, that appears in the
north must at midnight have its tail pointed up in a nearly vertical
direction. This fact, which has been verified on numerous occasions, is
a striking illustration of the law of direction of comets' tails. Think
for one moment of the facts of the case. It is summer; the twilight at
the north shows the position of the sun, and the tail of the comet
points directly away from the twilight and away from the sun. Take
another case. It is evening; the sun has set, the stars have begun to
shine, and a long-tailed comet is seen. Let that comet be high or low,
north or south, east or west, its tail invariably points _away_ from
that point in the west where the departing sunlight still lingers.
Again, a comet is watched in the early morning, and if the eye be moved
from the place where the first streak of dawn is appearing to the head
of the comet, then along that direction, streaming away from the sun, is
found the tail of the comet. This law is of still more general
application. At any season, at any hour of the night, the tail of a
comet is directed away from the sun.

More than three hundred years ago this fact in the movement of comets
arrested the attention of those who pondered on the movements of the
heavenly bodies. It is a fact patent to ordinary observation, it gives
some degree of consistency to the multitudinous phenomena of comets, and
it must be made the basis of our enquiries into the structure of the
tails.

In the adjoining figure, Fig. 71, we show a portion of the parabolic
orbit of a comet, and we als