140 THE POPULAR SCIENCE MONTHLY. 



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 impor- 

 tant ; 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 swal- 

 lowing suffices to ward off all mischief. For, if the Eustachian tube is 

 thus opened, again and again, as the pressure of the outside air in- 

 creases, 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 sup- 

 ported 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 Soifhd," tells us 

 how he employed this precaution of swallowing, and with entire suc- 

 cess, 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 Eu- 

 stachian 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 



