In the ear-trumpet, used by persons partially deaf, and in the stethoscope, it appears to us that we have concentrations of sound into a focus, by repeated reflexions of the aërial wavelets from side to side of the tube; and we may ask, does not the murmuring sound which we hear, when we apply a conch shell to our ear, arise from the same kind of percussion?
In echoes we have an instance of the reflection of atmospheric undulations, and this reflection, as in many natural situations, may be multiplied till the last faint repetitions die away; and when a succession of echoes is very rapid, a loud exclamation of the monosyllable, Hah! may be thrown back upon the ear like a boisterous laugh. The echoes of old halls and galleries, of vaults and dungeons, of caverns, rocks, and grottoes, have served as subject-matter for the writers of imaginative poetry and romance. It is not only to old romantic halls and galleries that echoes are confined; they are not uncommon in modern public buildings, so that the speaker hears his words repeated. "The distribution of sound in public edifices, so that the echoes (or reflections of aërial waves from the walls, etc.) may be most advantageously brought to strengthen the original sound, is a subject deserving of much attention." For some observations on the errors of architects on this point we may refer to sir J. Herschel's Treatise on Sound.
When our auditory nerves receive a succession of uniform impulses at equal intervals of time, the intervals being at the same time very small, the impressions become so interblended with each other as to produce an apparently continuous sound, even and equal, except as to variation in loudness. This sound we call a musical note, produced by the string of the violin, or piano; if the intervals between the vibrations be, comparatively speaking, long, the note is grave; but if within the same given time the vibrations be very numerous, the note is in proportion acute. Some notes harmonize with each, and thereby please the mind through the ear; others are discords, and offend. Grave or acute notes depend upon the length and the tension of the strings of an instrument, or the length of a wind instrument. In the piano, the low notes are produced by long, thick strings, the high notes by slender, short wires, in which the vibrations are excessively rapid. In the violin, which has strings of equal length, the thin strings for producing the high tones are screwed to a far greater degree of tension than those of the lower tones; and each string, as its length (that is, vibratory length) is shortened by the pressure of the finger at due intervals, produces high and higher tones in proportion.
We have here said enough (for we aim not at a treatise on acoustics) to show what sound really is, and what are the principal laws by which it is governed. We have also shown, that it is our mind alone that takes cognizance of the impulses given by vibratory wavelets of the atmosphere to the auditory nerves; consequently, that as the mind sees, so the mind alone hears, for vibration cannot in and of itself be that sensation which we call sound.
What is the knowledge gained through the ear by the appreciation of sound, or in common language, by hearing? Let us answer:—We commonly judge of the distance and position of objects producing sound by the sense of hearing, but in these points our accuracy greatly depends upon our attention and our experience. Yet are we liable to deception; we are thoroughly deceived by a skilful ventriloquist, who can so modulate his voice as to make it seem distant or near, or to proceed from one quarter of the room or house, or from another. The voice of the corn-crake (Crax pratensis) is, from some cause or other, very perplexing; sometimes it will appear as if the bird were only a few yards distant from us, and the next instant in some far part of the field. Perhaps instinct leads it to alter the pitch of its monotonous cry, in order to deceive the intruder. Again, echoes deceive the ear; not only may we mistake echo for the actual voice of the speaker, but it may appear as if close or distant, faint or loud, and thus in every point lead to erroneous conclusions.
That the ordinary animals around hear will not be denied, nor can we blind ourselves as to the fact, that they acquire a limited share of information, bearing upon their corporeal necessities, through this sense. Animals have a natural language, expressive of pain or pleasure, of surprise, of fear, of anger; and this language consists of cries or tones, variously modulated, each species having its own range of vocal intonations. Every species understands the simple instinctive language of its own species. Some birds, for example, as the wild geese, have sentinels around the flock while feeding, and rise en masse at the warning cry of their guards. Who has not marked the distress and agitation of the ewe, when she hears the plaintive tremulous bleat of her lamb, forcibly separated from her? The warning note of the cock, his cluck of invitation, his scream of surprise or fear, his cackle of agitation, his crow of defiance, are well understood by his train. Quadrupeds, birds, and even some insects call to each other, and are answered again—
"Steed answers steed with loud and boastful neighing."
The bird invites his mate by a sweet strain, and is answered by her low chirp. The call of the young in their nests is responded to by the parents; the parents call to each other, as if to assure themselves of each other's safety, or to find out where they mutually are, and what they are doing.
Imitation is an instinct; we see it powerful in its dominion over children; it is in exercise before reason assumes a definite sway; it prompts to the acquisition of the mother-tongue, till, by repetition, the true pronunciation is acquired, and the names of all common articles known: then reason steps in. But imitation is not confined to the human race; monkeys imitate, and the grotesqueness of their imitations renders them ludicrous; they do not imitate voice, but action. Some birds, however, imitate the human voice, and are easily taught to utter words or sentences, (the parrot, the mino-bird, the magpie, are examples,) nay, even to sing musical airs with great accuracy. But, by all this acquisition through the organ of hearing, they gain no information, nor add to the limited number of their ideas. The parrot may be, indeed, made by practice to associate the sound of a word, say bread, with the article thus denominated, and to call for it by name when wanted; but reason steps not in to give further assistance in the building up of idea upon idea, without which there can be no definite language—no real speech. Though the dog and the horse cannot utter the sound, they know what bread is quite as well as the parrot, and, indeed, the ear of the dog is the inlet to an instinctive mind of no mean order, for the Almighty created the dog to be man's assistant and humble friend.