336 RESPIRATION 



labored by the time he gets first under water. The reason of this is 

 that the pressure in his lungs is that of the water at the valve 

 outlet, whereas the pressure on his chest and abdomen is greater 

 by something like a foot of water. He is thus inspiring against 

 pressure, and if he has to breathe deeply, as during exertion, the 

 breathing is apt to become fatigued in the manner described in 

 Chapter III. With another foot of adverse pressure the fatigue is 

 very rapid. One of the first things which a diver has to learn is 

 to avoid the adverse pressure by regulating the spring on the 

 outlet valve, so that the breathing is always easy. The spring 

 regulates at the same time the amount of air in the dress, and 

 therefore the buoyancy of the diver. A practiced diver can thus 

 slip easily, and without exertion, up or down the rope. A pres- 

 sure gauge attached to the air pipe where it leaves the pump 

 indicates the depth of the diver at any moment. 



The breathing is of course easiest when the dress is full of air 

 down to the level of the diaphragm, but when this is so the diver 

 is in danger of being "blown up" ; for if he is crawling on the 

 ground, it may easily happen that the air gets into the legs of 

 his dress. His head goes down so that the excess of air can- 

 not escape readily. He is then blown helplessly to the surface, 

 while his arms are fixed in an outstretched position (see Figure 

 78). His air pipe may be caught by a rope or other obstruction, 

 so that he is hung up in a helpless position with his legs upwards, 

 the excess of air being unable to escape at the valve since it is 

 downwards. In very deep diving there is considerable risk of 

 being blown up ; and to avoid this risk the arrangement for lacing 

 up the legs, shown in Figure 76, was introduced (see also Fig- 

 ure 79). 



In the Denayrouze apparatus, extensively used on the Conti- 

 nent, the air is pumped into a steel reservoir on the diver's back. 

 By means of a reducing valve his air is supplied from the reser- 

 voir according to his requirements. The arrangement is a beauti- 

 ful piece of mechanism, but an encumbrance which gives rise to 

 various inconveniences and dangers, one being that the depth of 

 the diver cannot be read off at the surface, and another that he 

 cannot regulate the pressure in his helmet. 



For engineering work in preparing foundations, etc., on the 

 sea bottom, a diving bell is sometimes employed. This is a heavy 

 metal box, open below, and supplied with compressed air by a 

 pipe (Figure 80). It is lowered to the bottom with the workmen 

 sitting in it, and they can work dry on the bottom. The diving 



