As sound is the result of the tremulous fluctuations of air, it will not surprise our reader to know, that no sound is conveyed from the percussion of bodies in a vacuum. The experiment of suspending a bell in the exhausted receiver of the air-pump, and causing the clapper to strike it as when rung, has been very often repeated; no sound is communicated to the listener, although the vibrations of the bell are clearly perceived; should a little air be now admitted, a faint tinkling is heard, which becomes stronger in proportion to the admission of air, until it is natural; on the contrary, if the receiver be filled with greatly condensed air, the tinkling of the bell is louder than it is when in an atmosphere of the ordinary degree of density. Sound, or, rather, the wavelets of the atmosphere producing this sensation, radiate from the sound-causing object in all directions; these wavelets, however, extend only to a certain distance, according to the violence of the central agitation; they become gradually feebler and feebler, and at last die away. Thunder, the firing of cannon, and the clang of trumpets, are not heard beyond a certain distance, and even at that distance, whatever it may be, the ear only faintly distinguishes the report, for the vibratory action and reaction on each other of the atmospheric particles, become feebler and feebler in proportion to the aërial distance of the listener. If we stand on an eminence commanding a view of a number of cannon, say a mile distant, which the soldiery are from time to time firing, a decided difference of time will be perceived between the flash and the report; the same observation applies to lightning, a considerable interval often occurring between the flash and the thunder. Hence, light is said to travel faster than sound. Sound travels at the ratio of about twelve miles and a half in a minute, but its velocity is greater in a denser than in a rarer medium, as is also the distance to which it is propagated. These points are illustrated by a comparison of water with atmospheric air. All aquatic animals are not constituted for hearing; myriads, indeed, have neither sight nor hearing. By the cuttle-fishes (_cephalopoda_) both these senses are enjoyed as well as by fishes, aquatic reptiles, and we need not say by such aquatic mammalia as whales, grampuses, porpoises, etc. To them the water is the medium of sound, and granting that their auditory nerves have the same sensibility as those of man, they will hear sounds not only more quickly, more distinctly, but at greater distances. This is accounted for by "the greater elasticity of the constituent particles of water, within the minute distance required for their action in propagating sound. Stones struck together under water are heard at great distances by a person under water. Franklin found, by experiment, that sound, after travelling above a mile through the water, loses but little of its intensity.