DANGERS AND PRECAUTIONS 



concerting, to say the least. After that had happened in our Navy twice, 

 even to men who had been instructed in the use of 'lungs,' a long medical 

 investigation was carried out, ending with a careful series of experiments 

 on animals in the Harvard Medical School. This demonstrated con- 

 clusively the cause and the cure. 



"The cause was the natural reaction of a man under water, when he 

 is frightened, in holding his breath in an involuntary attempt to avoid 

 swallowing or breathing water. But it turned out that there is no surer 

 way for a man to kill himself promptly when using a 'lung' than to do 

 this apparently natural thing. For a man rising from the bottom starts 

 with his lungs full of air at a pressure corresponding to the water pres- 

 sure on the bottom. As he rises, the water pressure decreases. If he keeps 

 on breathing naturally as he goes up, the pressure in his lungs keeps 

 decreasing to correspond, with the excess air escaping continuously from 

 the 'lung' through an automatic exhaust valve. 



"If, however, he becomes frightened or hysterical at finding himself 

 completely immersed in the ocean and perhaps a long way from the sur- 

 face (a mental condition not unnatural in a person not trained as a diver) 

 and does what seems indicated as the normal thing, that is, holds his 

 breath, he practically seals his death warrant. For as he rises further, 

 probably faster now in his haste to get to the surface, the water pressure 

 surrounding his chest decreases, but the air pressure in his lungs, with 

 breathing suspended, does not. The result is that the air trapped in his 

 lungs expands and in so doing distends the lungs abnormally. It takes a 

 rise of not over six feet in the water under such circumstances for the 

 distention of the lungs to cause myriad ruptures in their walls through 

 which air in considerable quantity is forced directly into the blood in 

 large bubbles. 



"Nothing further happens until about a minute after breathing is re- 



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