The Senses and the Mind

by Anonymous

[13] Psa. xciv. 9.

[14] Psa. xix. 14.

[15] Psa. cxliii. 2.

2. Hearing.—What we call sound, noise, or tone, is a mental appreciation of the effects of the tremulous or vibratory motions of the particles of an elastic fluid, such as air or water, on the auditory or acoustic nerves of the internal ear. Sonorous vibrations are produced by the concussion of hard bodies, by the bolt of lightning, by the vibrations of stringed instruments of music, by the forcible current of air thrown into the tubes of wind instruments, and by the air driven from the lungs through the organs of voice in reptiles, birds, and mammalia. There are, indeed, few movements, few collisions, few atmospheric disturbances, which do not excite those tremulous wavelets of atmosphere, those oscillations, which produce in us the sensation of sound. In fact, during the day, the sense of hearing is as perpetually employed as is that of sight; yet, strange to say, from the impressions made through the medium of both these senses, we can so far abstract the mind, and throw it upon its own internal reflections and operations, as to see without discrimination, and to hear without attention or notice.

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. According to Chladini, the velocity of water is about four thousand nine hundred feet in a second, or between four and five times as great as it is in air."

Different bodies conduct sound to the ear with more or less distinctness, according to their susceptibility for vibrating, and also with more or less rapidity. Ice, for example, conveys sound more speedily than water, and far more rapidly than air; thus, if a cannon on the edge of a frozen lake be fired, a person on the opposite side will find the flash to be followed by two reports, the first conducted to him by the ice, the second by the air. If a long steel rod be applied to the orifice of the ear, the gentlest jar given to its further extremity, and which, but for the intervention of the metal, would not have been audible, produces a strong and distinct impression. Cotton wool, sheeps' wool, and soft stuffs, generally are bad conductors of sound. The frozen earth conducts sound from great distances; the approach of a horseman in clear, frosty weather, will be heard at a far greater distance than when the ground is moist, or not hardened; in the former case, indeed, a double sound may sometimes be heard, one produced by the vibration of the ice-bound earth, the other, if distance allow, by that of the air.

The distance to which sound can be conveyed through the speaking-trumpet is very surprising. The invention of the modern speaking-trumpet is generally accorded to sir Samuel Moreland, (1670,) and some of the large brass trumpets, made under his direction, carried the human voice from a mile and a half to between two and three miles distance. The efficiency of this instrument has been attributed to the repeated reflection of the sound, or waves of air, (like rays of light,) from side to side along the course of the tube, their ultimate efflux from the mouth of the instrument being in such a way, as either to cause the rays of sound to be collected into a focus at a distance, or to be projected forward in parallel lines, instead of allowing them to diverge in all directions, and on these principles various modifications in the form of the speaking-trumpet have been suggested.

The views, however, of professor Leslie do not coincide with this theory. "The performance of the speaking-trumpet," he says, "does certainly not depend upon any supposed repercussion of sound; repeated echoes might divide, but could not augment the quantity of impulse." His idea is, that "the tube, by its length and narrowness, detains the efflux of air, and has the same effect as if it diminished the volubility of that fluid, or increased its density." "The organs of articulation," he adds, "strike with concentrated force; and the pulses thus so vigorously excited are, from the reflected form of the aperture, finally enabled to escape and spread themselves along the atmosphere." The experiments of Hassenfratz, a French philosopher, are cited in support of this theory. He tried the power of a speaking-trumpet, by measuring the distance at which the ticking of a watch could be heard through it, and found the effect the same, whether the metal tube were used simply, or wrapped round in such a way as to prevent vibration. It was also heard at the same distance when the inner surface was lined with linen, or woollen cloth, to diminish reflection; and the range of a cylindrical trumpet was the same as that of a conical one.