y, the comet will itself enter into action, and produce
mechanical waves in its turn. As the trace produced in the solar waves
consists of an agitation of the ether on such trace, it will become
apparent, if we admit that every luminous effect is produced by an
excitation--a setting of the ether in vibration. The mechanical waves
engender of themselves, then, an emission of optical waves that render
perceptible the alteration which they create in each other.

Let a be the position of the comet. The altered wave, a, will carry
along the mark of such alteration in the direction a b, while at the
same time extending transversely the waves emitted by the comet. During
this time the comet will advance to a', and the wave will be altered in
its turn, and carry such alteration in the direction, a' b'.

The succession of all these alterations will be found, then, upon a
curve a'' d' d, whose first elements, on coming from the comet, will be
upon the resultant of the comet's velocity, and of the propagation of
the solar waves. Consequently, the slower the motion of the comet, with
respect to the velocity of the solar waves, the closer will such
resultant approach the line of centers, and the more rectilinear will
appear the trace or tail of the comet.

[Illustration: IV]

IV.--If the comet have satellites, we shall see, according to the
relative position of these, several tails appear, and these will seem to
form at different epochs. If c and s be the positions of a comet and a
satellite, it will be seen that if, while the comet is proceeding to c',
the satellite, through its revolution around it, goes to s', the traces
formed at c and s will be extended to d and d', and that we shall have
two tails, c' d and s' d', which will be separated at d and d' and seem
to be confounded toward c' s'.

V.--When the comet recedes from the sun, the same effect will occur--the
tail will precede it, and will be so much the more in a line with the
sun in proportion as the velocity of the solar waves exceeds that of the
comet.

If we draw a complete diagram (Fig. 4), and admit that the alteration of
the solar waves persists indefinitely, we shall see (supposing the
phenomenon to begin at a) that when the comet is at a 1, the tail will
and be at a 1 b; when it is a 2 the tail will be at a 2 b'; and when it
is at a 4, the tail will have become an immense spiral, a 4 b'''. As in
reality the trace is extinguished in space, we never see but the