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CHAPTER IX 



THE INFLUENCE OF INCREASED ATMOSPHERIC PRESSURE 



NOWHERE on the surface of the world do- we find aerial animals 

 naturally exposed to increased atmospheric pressure. The deep 

 holes in the earth's crust are filled with water, and in the valley 

 of the Dead Sea where the climate is so dry that the level of 

 this sea lies below that of the ocean in this valley the atmos- 

 pheric pressure does not exceed 825 mm. Hg. Man, in his 

 restless pursuit of a living, has driven mines into the bowels 

 of the earth, and has sought to gather the riches of the sea 

 by the invention of diving-gear, and to build harbours and 

 quays, to tunnel under rivers, and establish the foundations of 

 bridges by the construction of caissons and the employment of 

 compressed air. 



In the sixteenth century Sturmius invented the diving-bell. 

 This bell, full of air, was lowered into the water, and at 10 metres 

 the air occupied one-half and the water the other half of the bell, 

 at 20 metres the air one-third, and the water two-thirds, for 10 

 metres of water is about equal to the pressure of 1 atmosphere. 

 The men naturally worked under very bad conditions, and Halley 

 tried to improve matters by sending down casks of air, which 

 were opened in the bell, while the warm foul air escaped through 

 a valve in the top. 



In 1830, Cochrane, the famous admiral, patented a method 

 for using compressed air to keep the water out in tunnelling under 

 rivers. The method was first put into practical use by Triger 

 in 1839, who successfully sunk a caisson through a layer of 

 quicksand at Chalonnes, and thereby reached rich beds of coal. 

 A caisson is a steel cylinder, which is sunk in water, or wet soil, 

 and out of which the water may be kept by means of compressed 

 ir. The men in the caisson are thus enabled to work in dryness 

 n the bed of the sea or river. The inlet of the caisson is fitted 

 with a double set of air-tight doors opening towards the caisson 

 and forming an air-lock. The air-lock is provided with suitable 



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