274 THE BOTTOM OF THE Sua 



the blowiiig-macliine is then gradually lessened, so 

 as to diminish by degrees the pressure of the air they 

 breathe. The water rises at the same time in the 

 caissons and lateral shafts until it has attained the 

 level of the river. The dour is then opened and the 

 men leave their prison, which may be regarded as 

 a species of diving-bell, but of a form devise 1 for a 

 special service. 



8. Payerne's Submarine Hydrostat 



The diving-bell proper has been much improved by 

 M. Payerne. His *' Submarine Hydrostat " possesses 

 the immense advantage of being capable, at the will 

 of those enclosed in it, either of floating on the sur- 

 face, or of sinking or rising, as may be desired. 

 Thirty men may work in it with ease for a number 

 of hours without inconvenience. It is therefore oj 

 great service in clearing ports, and in facilitating the 

 execution of other submarine work. 



The principle of the machine is very ingenious. 

 Kxternally, it has the appearance of one large 

 rectangular box, surmounted by another smaller 

 one, completely closed in except at the bottom. 



The interior of the hydrost.-it consists of three 

 principal compartments. Tiie lower, or hold, is open 



