268 POPULAR LECTURES AND ADDRESSES. 



perceive any difference in the sense of force, any 

 perception of pressure. What you do perceive 

 is this : behind the tympanum is a certain cavity 

 filled with air, and a greater pressure on one 

 side of the tympanum than on the other, gives rise 

 to a painful sensation, and sometimes produces 

 rupture of it in a person going down in a diving 

 bell suddenly. The remedy for the painful sensa- 

 tion thus experienced, or rather I should say its pre- 

 vention, is to keep chewing a piece of hard biscuit, 

 or making believe to do so. If you are chewing a 

 hard biscuit, the operation keeps open a certain 

 passage, by which the air pressure getting access 

 to the inside of the tympanum balances the out- 

 side pressure and thus prevents the painful effect. 

 This painful effect on the ear experienced by 

 going down in a diving bell, is simply because 

 a certain piece of tissue is being pressed more on 

 one side than on the other ; and when we get 

 such a tremendous force on a delicate thing like 

 the tympanum, we may experience a great deal 

 of pain, and it may be dangerous ; indeed it is 

 dangerous, and produces rupture or damage to the 





