The acquired perceptions of hearing are less numerous 'han those of sight, and are principally useful as enabling us to supply the deficiencies of the latter. That this is the case will be obvious, when we observe what takes place Nvith respect to the blind, who learn to substitute audible for visible impressions in a variety of instances, where it would previously have been conceived to be impossible. In this way they not only acquire correct ideas of the po sition of the bodies which immediately surround them, but they judge of distances which are far beyond their reach, as of the size of rooms and buildings, the vicinity and approach of bodies, and of other circumstances for which we exclusively employ the sight. This they ac complish by the associations which they form with certain undulations of the air that strike upon the nerves of the ear, and their attention being exclusively directed to these impressions, they learn to distinguish them with much more accuracy than the generality of mankind. But we must be aware that a great part of the knowledge which the blind thus acquire depends upon their intercourse with those who can see, as without this they would not be able to form those associations between audible and tan gible impressions upon which their knowledge depends.
Among the acquired perceptions of hearing, some of the most useful are those by which we judge of the posi tion of sounding bodies—a subject which appears to have been but little attended to until it was examined by Mr. Gough. He observes, that it cannot depend upon the mode in which the vibrations strike the auditory nerve, because, before they reach the internal cavity of the ear, they must have been reflertprt nut of their original direc tion. He conceives that the bones of the skull, in the neighbourhood of the ear, are capable of receiving the vibrations of sound, and that we judge from the part of the head which is thus affected, and especially upon the comparative effect produced upon the two sides of the head. It is on this account that persons who have lost the use of one ear are less able to judge accurately of the position of sounding bodies ; presenting a kind of analogy to the defect which is produced by the loss of one eye.
There are two kinds of sounds, those that merely pro duce a perception of sound generally, and those that pro duce what are termed musical tones. The difference
between simple sounds and musical tones is supposed to depend upon the regularity of the vibrations of the sound ing body When the body is irregular in its texture or its figure, so that its vibrations or oscillations are not iso chronous in all its parts, the effect is simply a noise when, on the contrary, the vibrations all coincide, we have a musical tone. Musical tones are generated by metallic wires, rods, or plates, by membranous cords or expansions, and by air, when confined in tubes—the two former con stituting stringed, the latter wind instruments. Sounds of all kinds differ from each other according as they are strong or weak, a difference which probably depends upon the force of the vibrations ; but musical sounds have a specific difference, independent of strength, by which they are denominated high or low, acute or grave—a property which is supposed to depend upon the rapidity of the vibrations of the minute particles of the body, those which are the must rapid constituting the high or acute sounds.
The mental feelings that are associated with certain sounds are very powerful, and often appear to depend en tirely upon accidental circumstances ; but we learn from experience, that there are certain combinations of sounds which are naturally agreeable to the ear, and others, on the contrary, that are naturally harsh and unpleasant. The effect of musical sounds of the same description appears to depend principally upon the number of vibrations which occur in a given time, and the proportion which the vi brations of the different tones bear to each other. The proper combination of these tones constitutes the science a harmony, one of the most curious branches of mechani cal philosophy, and which, when united with the different kinds of tones, and their adjustment to each other, com bined with certain mental impressions, gives rise to one of the most refined and elegant of the polite arts. The power of distinguishing musical tones is a distinct faculty from that by which sound in general is perceived ; we frequently observe deaf persons who possess an accurate knowledge of music, and, on the contrary, we find other persons who have a quick perception of sound, but who are entirely des titute of a musical ear. On what this difference depends, or by what part of the ear this faculty is exercised, is a matter of mere conjecture.