vibrating membrane, but provides a safety-valve, to a certain extent, against the injurious influence of loud sounds.
The chief use of the Eustachian tube is to allow a free interchange of air between the ear and the throat, and this is exceedingly important; and it is very important also that its use in this respect should be understood. Persons who go down in diving-bells soon begin to feel a great pressure in the ears, and, if the depth is great, the feeling becomes extremely painful. This arises from the fact that in the diving-bell the pressure of the air is very much increased, in order to balance the weight of the water above; and thus it presses with great force upon the membrane of the drum, which, if the Eustachian tube has been kept closed, has only the ordinary uncompressed air on the inner side to sustain it. It is therefore forced inward and put upon the stretch, and might be even broken. Many cases, indeed, have occurred of injury to the ear, producing permanent deafness, from descents in diving-bells, undertaken by persons ignorant of the way in which the ear is made; though the simple precaution of frequent swallowing suffices to ward off all mischief. For, if the Eustachian tube is thus opened, again and again, as the pressure of the outside air increases, the same compressed air that exists outside passes also into the inside of the drum, and the membrane is equally pressed upon from both sides by the air, and so is free from strain. The same precaution is necessary in ascending mountains that are lofty, for then there is the same effect of stretching produced upon the membrane, though in the opposite way. The outside air becoming less and less condensed as a greater height is gained, the ordinary air contained within the drum presses upon the membrane, which is thus insufficiently supported on the outside, and a similar feeling of weight and stretching is produced. The conjurer's trick of breaking a vase by a word rests on the same principle. The air is exhausted from within, and the thin, though massive-looking sides of the vase collapse by the pressure of the air outside; and, just as ever so small a hole, made at the right moment in the side of the vase, would prevent the whole effect, so does swallowing, which makes a little hole, as it were, for the moment in the drum of the ear, prevent the in-pressing or out-pressing of the membrane. Mr. Tyndall, in his interesting book "On Sound," tells us how he employed this precaution of swallowing, and with entire success, when, in one of his mountain excursions, the pressure on his ears became severely painful.
Deafness during colds arises very often, though not always, from a similar cause. For, when, owing to swelling of the throat, the Eustachian tube cannot be opened by its muscle, and so the air in the drum is not renewed, the air that is contained in it soon diminishes, and the outer air presses the membrane in, so that it cannot vibrate as it should. This is what has been sometimes called "throat-deafness."
There are several things very commonly done which are extremely
