ded on the great length of the tail and the velocity of
light, is sufficient to prove that these corruscations are not actually
in the tail. Now, it is undoubtedly true, that as light travels less
than two hundred thousand miles in a second, and a comet's tail is
frequently one hundred millions long, it is impossible to see an
instantaneous motion along the whole line of the tail; but granting that
there are such flickerings in the tail as are described by so many, it
must necessarily be, that these flickerings will be _visible_. It would
be wonderful indeed, if a series of waves passing from the comet to the
extremity of the tail, should have their phases so exactly harmonizing
with their respective distances as to produce a uniform steady light
from a light in rapid motion. The argument, therefore, proves too much,
and as it is in the very nature of electric light thus to corruscate, as
we see frequently in the northern lights, we must be permitted still to
believe that not only the tails, but also the heads of comets do really
corruscate as described.

With respect to the direction of the tail, astronomers have been forced
to abandon the antiquated notion, that the tail always pointed directly
from the sun; yet they still pertinaciously cling to the idea, that
although this is not always the case, the tail only deviates from this
direction _in the plane of the orbit_. As this is a most important
question, it is necessary formally to protest against such a conclusion.
If the earth should happen to be in the plane of the comet's orbit and
the tail appears in that plane, it must of course be in that plane
_really_; but if the earth is not in the plane of the comet's orbit, the
tail is not _necessarily_ in the same plane, whatever its apparent
direction may indicate. It is true there is a tendency of every particle
of the tail, moving under the restraining influence of the sun's
attraction, to continue in the plane of the orbit; and in certain
positions there is no oblique action arising from the force of the
radial stream to cause it to deviate from that plane; yet in other
positions of the comet, the action of the radial stream may be oblique,
forcing it out of that plane, and still such a direction might be
assigned to it as to make it conform. In the comet of 1843, P. Smythe
observed a forked tail 25d long on March 3d, and from the end of the
forked tail, and from its _north_ side, a streamer diverged at an angle
of