DEAFNESS, the state of a person who wants the sense of hearing; or the dis ease of the ear, which prevents its due reception of sounds. Deafness generally arises, either from an obstruction or a compression of the auditory nerve ; or from some collection of matter in the ca vities of the inner ear ; or from the audi tory passage being stopped up by some hardened excrement ; or, lastly, from some excrescence, a swelling of the glands, or some foreign body introduced within it. Those born deaf are also dumb, as not being able to learn any lan guage, at least in the common way. How ever, as the eyes in some measure serve them for ears, they may understand what is said by the motion of the lips, tongue, &c. of the speaker ; and even accustom themselves to move their own, as they see other people do, and by this means learn to speak. Thus it was that Dr. Wallis taught two young gentlemen, born deaf, to know what was said to them, and to return pertinent answers. Rigby gives us another instance of the same, within his own knowledge ; and there was a Swiss physician lately living in Amsterdam, one John Conrad Amman, who effected the same in several children born deaf with surprising success.
In the "Phil. Trans." No. 312, we have an account by Mr. Waller, R. S. Se cretary, of a man and his sister, each about 50 years old, born in the same town with Mr. Waller, who had neither of them the least sense of hearing ; yet both of them knew, by the motion of the lips only, whatever was said to them, and would answer pertinently to the question proposed. It seems they could both hear and speak when children, but lost their sense afterwards ; whence they retained their speech, which, though uncouth, was yet intelligible. Such another instance is
related by Bishop Burnet of a young wo man. " At two years old, they perceiv ed she had lost her hearing; and ever since, though she hears great noises, yet hears nothing of what is said to her : but by observing the motions of the mouth and lips of others, she acquired so many words, that out of these she has formed a sort of jargon, in which she can hold conversation whole days with those that can speak her language. She knows no thing that is said to her, unless she see the motion of their mouths that speak to her, so that in the night they are obliged to light candles to speak to her. One thing will appear the strangest part of the whole narration : she has a sister, with whom she has practised her language more than with any body else ; and in the night, by laying her hand on her sis ter's mouth, she can perceive by that what she says, and so can discourse with her in the dark.
It is observable, that deaf persons, and several others thick of hearing, hear bet ter, and more easily, if a loud noise be raised at the time when you speak to them ; which is owing, no doubt, to the greater tension of the ear-drum on that occasion. Dr. Wallis mentions a deaf woman, who, if a drum were beat in the room, could hear any thing very clearly ; so that her husband hired a drummer for a servant, that by this means he might hold conversation' with his wife. The same author mentions another, who, living near a steeple, could always hear very well if there was a ringing of three or four bells, but never else. See DUMBNESS.