t'' being the time taken to pass over aa''.
[Illustration: V]
VI.--The tail, then, is not a special matter which is transported in
space with the comet, but a disturbance in the solar waves, just as
sound is an atmospheric disturbance which is propagated with the
velocity of the sonorous wave, although the air is not transported. The
tail which we see in one position, then, is not that which we see in
another; it is constantly renewed. Consequently, it is easy to conceive
how, in as brief a time as it took the comet of 1843 to make a half
revolution round the sun, the tail which extended to so great a distance
appeared to sweep the 180° of space, while at the same time remaining in
opposition to the great luminary.
[Illustration: VI]
The spiral under consideration may be represented practically. If to a
vertical pipe we adapt a horizontal one that revolves with a certain
velocity, and throws out water horizontally, it will be understood that,
from a bird's eye view, the jet will form a spiral. Each drop of water
will recede radially in space, the spiral will keep forming at the jet,
and if, through any reason, the latter alone be visible, we shall see a
nearly rectilinear jet that will seem to revolve with the pipe.
Finally, if the jet be made to describe a curve, m n (Fig. 4), while it
is kept directed toward the opposite of a point, c, the projected water
will mark the spiral indicated, and this will continue to widen, and
each drop will recede in the direction shown by the arrows.
[Illustration: VII]
VII.--It seems to result from this explanation that all the planets and
their satellites ought to produce identical effects, and have the
appearance of comets. In order to change the conditions, it suffices to
admit that the ethereal mass revolves in space around the sun with a
velocity which is in each place that of the planets there; and this is
very reasonable if, admitting the nebular hypothesis, we draw the
deduction that the cause that has communicated the velocity to the
successive rings has communicated it to the ethereal mass.
The planets, then, have no appreciable, relative velocity in space, and
for this reason do not produce mechanical waves; and, if they become
capable of doing so through a peculiar energy developed at their
surface, as in the case of the sun, they are still too weak to give very
perceptible effects. The satellites, likewise, have relatively too
feeble velocities.
The comet, on the contrary, directly penetrates the solar waves, and
sometimes has a relatively great velocity in space. If its proper
velocity be of directly opposite direction to that of the ethereal
mass's rotation, it will then be capable of producing sufficiently
intense mechanical effects to affect our vision.
VIII.--Finally, seeing the slight distances at which these stars pass
the sun, the attraction upon the comet and its satellites may be very
different, and the velocity of rotation of the latter, being added to or
deducted from that of the forward motion, there may occur (as in the
case shown in Fig. 6) a separation of a satellite from the principal
star. The comet then appears to separate into two, and each part follows
different routes in space; or, as in Fig. 7, one of the satellites may
either fall into the sun or pursue an elliptical orbit and become
periodical, while the principal star may preserve a parabolic orbit, and
make but one appearance.--_A. Goupil._
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