One may suppose the external and internal ear to be so formed, as that all the differences in the vibrations of the air, which arrive at the ear, may affect the auditory nerves with corresponding differences. Let us therefore first consider in what manner different sounds impress different vibrations upon the air.
First, then, Since not only the parts about the throat, but those of the mouth, cheeks, and even of the whole body, especially of the bones, vibrate in speaking, the figure of the vibrations impressed upon the air by the human voice will be different from that of the vibrations proceeding from a violin, flute, &c. provided the distance be not too great. This therefore may be considered as one help for distinguishing articulate sounds from all others.
Secondly, Articulation consists in breaking out from a whisper into sound, or closing the sound in different manners, the organs of speech being put also into different shapes, so as to join the differences mentioned in the last paragraph with various eruptions and interruptions, ascents and descents of sound. And thus each letter may be distinguished from every other by hearing.
Thirdly, It is agreeable to all this, that it is difficult at great distances to distinguish the tone of one musical instrument from another, or of any from the tone of a human voice, cæteris paribus; or to distinguish articulate sounds from one another. For at great distances the vibrations of the air are circular to sense, and all the ascents, descents, eruptions and interruptions of sound, which distinguish one compound sound from another, are confounded by numberless reflections from the intermediate bodies.
Fourthly, We may observe, that as the preserving the distinction of place is the chief end of the coats and humours in the organ of sight, so the distinction of time is of the greatest importance in hearing. It seems probable therefore, that the membrana tympani, small bones, and their muscles, are so contrived, as by their actions to preserve the distinction of time, i.e. to extinguish strong sounds, and to keep up weak ones, so as that the last may not be too much overpowered by the continuance of the first; just as the treble notes of a harpsichord would be by the bass ones, did not the bits of cloth affixed to the jacks check the vibration of the strings in due time.
Having now shown how articulate sounds may be distinguished from one another, and from all other sounds; I next observe, that, in fact, the speakers do not pronounce so articulately and distinctly in common conversation, as to furnish the hearers with the requisite criterions according to the foregoing theory; but that we arrive at a facility of understanding one another’s discourse, chiefly by the power of association.
