FORCE PUMPS. 665 



and this reaches him at a pressure equal to that to which the 

 rest of his body is subjected, and he is therefore able to breathe 

 without effort or attention. The compression of air heats it, 

 and the breathing of air thus heated is bad for the diver 

 This has been remedied by the same gentleman, by the modifi- 

 cation of the pumps by which the air is forced in the reservoir. 

 The air is cooled by being forced to pass through two layers of 



DIVERS FINDING A BOX OF GOLD IN THE PORT OF MARSEILLES. 



water before it reaches the reservoir; and expanding in ith 

 passage into the air-chamber it becomes again cooled. 



With the use of this apparatus another advantage is gained. 

 When the diver is down the air he expires rises in bubbles to 

 the surface, and by the regularity with which they rise his 

 condition can be easily known. If they cease, it is known 

 that something must have happened, and that he should be 

 instantly hauled up. In the old diving dress the expired air 

 passed into the space between his body and the clothing and 

 out at a valve in the helmet, bat as the excess of air supplied 

 to him escaped in this way also, it could not be told from this 

 whether the diver was alive or dead. 



