The extent and
depth of the water are likewise factors.
If we cause a cord to vibrate in the water, we shall obtain a succession
of waves, the velocity and size of which will be derived from the cord's
size and the intensity of its action. These waves, which are visible
upon the surface, constitute what I shall call _mechanical waves_. But
there will be created at the same time other waves, whose velocity of
propagation will be much greater than that 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.
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