CHAPTER XII 



Effects of High Atmospheric Pressures. 



The foundations of our scientific knowledge of the physiological 

 effects of high and low atmospheric pressures were laid broad 

 and firm by the investigations of Paul Bert, collected together in 

 his book, already so often referred to, ''La Pression Barome- 

 trique," published in 1878. It will be convenient to consider first 

 the effects of high atmospheric pressures. 



Very high atmospheric pressures are met with in deep diving 

 and in engineering work under water or in water-logged strata. 



Apart from laboratory experiments on animals, the highest 

 atmospheric pressures (up to ten atmospheres) have been met 

 with in deep diving. To understand the conditions under which 

 a diver is placed, it is necessary to understand the design of the 

 ordinary diving dress, which was introduced early last century by 

 Siebe, the founder of the well-known London firm of manu- 

 facturers of diving apparatus. The dress consists of a copper 

 helmet which screws on to a metal corselet, the latter being 

 clamped water-tight to a stout waterproof dress covering the 

 whole body except the hands, which project through elastic cuffs 

 (Figures 75 and J6). Air is supplied to the diver through a non- 

 return valve at the back of the helmet from a stout flexible pipe 

 strengthened with steel wire and connected with an air pump at 

 the surface. The air escapes through an adjustable spring valve 

 at the side of the helmet (Figure Tj). The arrangement is thus 

 such that the pressure of air in the helmet is at least equal to, and 

 can, by varying the resistance of the valve, be made greater than, 

 the water pressure at the outlet valve. For every 34 feet of fresh 

 water (or 33 feet or 10 meters of sea water) the pressure in- 

 creases by one atmosphere, or nearly 15 pounds per square inch. 

 At a depth of 33 feet of sea water the diver is therefore breathings 

 air at an excess pressure of one atmosphere, or a total pressure of 

 two atmospheres. It is absolutely necessary that he should breathe 

 compressed air, otherwise his breathing would be stopped in- 

 stantly by the pressure of the water upon the abdomen ; and at a 

 greater depth blood would probably pour from his nose and mouth 

 on account of the squeezing to which all parts of his body, except 

 his head in the helmet, would be subjected. 



