the state of a person who wants the sense of hearing; or the disease of the ear, which prevents its due reception of sounds. See MEDICINE Index.

Deafness generally arises either from an obstruction or a compression of the auditory nerve; or from some collection of matter in the cavities of the inner ear; or from the auditory 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 language, at least in the common way. However, 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. Digby gives us another instance of the same within his own knowledge; and there was a Swiss physician in Amsterdam, one John Conrad Amman, who reduced the thing to a fixed art or method, which he published in his Surdus Loquens, Amstelod. 1692, and de Loquela, ibid. 1700. See an account of methods employed in the education of this class of persons, in the article DEAF and DUMB, in the Supplement.

In the Phil. Trans. No. 312. we have an account by Mr Waller, R. S. Secr. 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 hearing afterwards: whence they retained their speech, which, though uncooth, was yet intelligible.

Such another instance is that of Mr Goddy's daughter, minister of St Gervais in Geneva, related by Bishop Burnet. "At two years old they perceived she had lost her hearing; and ever since, though she hears great noises, yet hears nothing of what is said to her. Deafness. 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 nothing 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 sister's mouth, she can perceive by that what she says, and so can discourse with her in the dark." Burnet. Let. IV. p. 248.

It is observable, that deaf persons, and several others thereto the thick of hearing, hear better 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.

DEALS, a thin kind of fir-planks, of great use in carpentry. They are formed by sawing the trunk of a tree into a great many longitudinal divisions, of more or less thickness according to the purposes they are intended to serve.

A very good method of seasoning planks of deal and fir is to throw them into salt water as soon as they are sawed, and keep them there three or four days, frequently turning them: in this case they will be rendered much harder, by drying afterwards in the air and sun: but neither this, nor any other method yet known, will prevent them from shrinking.

Rods of deal expand gradually, or cross the grain, in moist weather, and contract again in dry; and thence have been found to make an useful hygrometer.