at of the mechanical ones, and
apparently independent of mechanical intensity. These are _acoustic
waves_. Finally, there will doubtless be created _optical waves_, whose
velocity will exceed that of the acoustic ones. That is to say, if a
person fell into water from a great height, and all his senses were
sufficiently acute, he would first perceive a luminous sensation when
the first optical wave reached him, then he would perceive the sound
produced, and later still he would feel, through a slight tremor, the
mechanical wave.[1]

[Footnote 1: Certain persons, as well known, undergo an optical
impression under the action of certain sounds.]

[Illustration: I]

Under the action of the same mechanical energy there form, then, in a
mass of fluid, waves that vary in nature, intensity, and velocity of
propagation; and although but three modes appreciable to our senses have
been cited, it does not follow that these are the only ones possible.

We may remark, again, that if we produce a single wave upon water, it
will be propagated in a uniform motion, and will form in front of it
successive waves whose velocity of propagation is accelerated.

This may explain why sounds perceived at great distances are briefer
than at small ones. A detonation that gives a quick dead sound at a few
yards is of much longer duration, and softer at a great distance.

The laws that govern the system of wave propagation are, then, very
complex.

[Illustration: II]

II.--If an obstacle be in the way of the waves, there will occur in each
of them an _alteration_, a break, which it will carry along with it to a
greater or less distance. This succession of alterations forms a trace
behind the obstacle, and in opposition to the line of the centers.
Finally, if the obstacle itself emits waves in space that are of less
intensity then those which meet it, these little waves will extend in
the wake of the large ones, and will form a trace of parabolic form
situated upon the line of the centers.

[Illustration: III]

III.--Let us admit, then, that the sun, through the peculiar energy that
develops upon its surface or in its atmosphere, engenders in ethereal
space successive waves of varying nature and intensity, as has been said
above, and let us admit that its _mechanical_ waves are traversed
obliquely (Fig. 1) by any spherical body--by a comet, for example; then,
under the excitation of the waves that it is traversing, and through its
velocit