bration. The ear can not
perceive a sound originating in less than thirty-two vibrations in a
second, nor one of more than 24,000. The actual number of vibrations in
a given note was counted by means of revolving wheels and other
contrivances. I have not space to relate the investigation of many other
acoustic facts, the reference of sounds to phases of condensation, and
rarefaction in the elastic medium taking place in a normal direction;
the affections of note, intensity, quality; the passage in curved lines
and around obstacles; the production of sympathetic sounds; nodal
points; the effect of reeds; the phenomena of pipes and flutes, and
other wind instruments; the various vibrations of solids, as bells; or
of membranes, as drums; visible acoustic lines; the reflexion of
undulations by surfaces of various forms; their interferences, so that,
no matter how intense they may be individually, they can be caused to
produce silence; nor of whispering galleries, echoes, the nature of
articulate sounds, the physiology of the vocal and auditory organs of
man, and the construction of speaking machines.

[Sidenote: The ocean; its size.] Like the air, the ocean, which covers
three-fourths of the earth's surface, when reduced to a proper standard
of measure, loses very much of its imposing aspect. The varnish that
covers a twelve-inch globe represents its relative dimension not
inadequately.

[Sidenote: Tides and currents.] On the theory of gravitation, the tides
of the ocean were explained as depending on the attractive force of the
sun and moon. Its currents, in a general manner, are analogous to those
of the air. They originate in the disturbing action of solar heat, the
temperature of the sea varying from 85 deg. in the torrid zone to the
freezing-point as the poles are approached. Its specific gravity at the
equator is estimated at 1.028; but this density necessarily varies with
the rate at which superficial evaporation takes place; the pure vapour
rising, leaves a more concentrated salt solution. The effect is
therefore, in some degree, to counteract the expansion of the water by
warmth, for the sun-rays, being able to penetrate several feet below the
surface, correspondingly raise the temperature of that portion, which
expands and becomes lighter; but, simultaneously, surface evaporation
tends to make the water heavier. Notwithstanding this, currents are
established through the preponderance of the dilatation, and of the