24 De Vi Physica 



them to do this with impunity. Submerge 

 an ox or a sheep, and it will speedily cease 

 to be an -animal at all. We find, accord- 

 ingly, that all such animals as the whale, 

 seal, crocodile, walrus, hippopotamus, &c., 

 possess apparatus guarding their interior 

 against the fatal consequences that would 

 otherwise instantly result from going under 

 water. " When the crocodile dives, the nos- 

 " trils are closed by valves, a transparent 

 " membrane is drawn over the eyes, and 

 " the ear, which is a horizontal slit, is shut 

 " up by a moveable flap of the skin." 



Go to the Zoological Gardens, and watch 

 the organisation * hippopotamus ' at work. 

 Possibly on arriving at his cage, you will 

 find him * at home,' under water. Presently 

 you will see his huge head rise to the sur- 

 face and emerge : at that moment the two 

 great nostrils (great, but small relatively to 

 his bulk) open, and discharge a blast of 



