SUPPLY OF AIR. 249 



of the action of the pump, have considerably improved 

 the latter apparatus by constructing pumps in such a 

 manner that the leakage decreases, until, as the pres- 

 sure increases, it can scarcely be appreciate 1. 



It is well known that air becomes hot simply by 

 being strongly compressed. The supply of air in the 

 heated state is injurious to the divers. The pumps 

 of which we speak, and in which the air traverses two 

 layers of water before entering the steel reservoir, 

 remedy this inconvenience. Moreover, the air, in 

 passing from the steel reservoir into the air-chamber, 

 from whence the workman is supplied, expands again 

 and becomes still cooler. Another important advan- 

 tage connected with this apparatus is that the expired 

 air rises in bubbles to the surface. So long as the 

 diver breathes regularly, the intervals which separate 

 the appearance of the bubbles are sensibly equal. It' 

 they come more rapidly or more slowly than usual, it 

 IS a sign that something abnormal is going on. It 

 they cease altogether, the diver must have ceased 

 breathing, and should be hauled up immediately. 



In the old diving-dress the air filled the space be- 

 twixt the body of the diver and his impervious 

 clothing, the expired air escaping by a little valve 

 fitted into the helmet. But the excess of air trans- 

 mitted by the pump also escaped by this valve. Ir- 

 regularity in working the pump would therefore 



