RESPIRATION 



337 



bell in its crude original form was invented by Sturmius in the 

 sixteenth century, and further developed by Halley two centuries 

 later. 



The caisson introduced about 1840 by the French engineer 

 Triger, for sinking colliery shafts through water-logged strata 

 near the surface, is a further development of the diving bell. It 

 is now largely used for carrying the foundations of the piers of 

 bridges, etc., through soft ground on the bottom of a river or the 

 sea. The caisson (see Figure 81) is the bottom section of the 

 steel pier, and resembles a diving bell except for the fact that it 

 communicates with surface through a tube occupying the center 

 of the future pier and kept full of compressed air. This tube 

 serves for access and for removal of excavated material. The men 

 excavate the soft bottom so as to allow the caisson to sink down 

 to a secure foundation, and the sections of the pier are added 

 from above and filled with concrete as the caisson sinks. Access 

 to the central tube is through an air lock on surface. The men 

 enter the air lock, close the door, and then let the air pressure 

 rise till they can open the door into the central tube ; and in coming 

 out the reverse process is used. 



In tunneling operations in soft strata under water, the ad- 

 vancing tunnel is kept full of compressed air, so as to hinder the 

 penetration of water into the advancing end, as the steel rings 

 forming the permanent walls of the tunnel are successively put 

 in. The men thus work in an atmosphere of compressed air, to 

 which access is gained through one or more air locks. The tubes 

 and large tunnels under the Thames or deep in the water-logged 

 London clay, and under the Hudson and East Rivers at New 

 York, have been, or are being, constructed by this means. In the 

 sinking of colliery shafts through water-logged strata the freez- 

 ing or cementation processes are now generally used, as, except 

 in strata fairly near the surface, the water pressures are too high 

 for the compressed-air process. 



Various physiological disturbances are associated with ex- 

 posure to compressed air, and these must now be considered one by 

 one. As the pressure rises when a man goes below water, in a 

 diver's suit, or as compressed air enters an air lock through which 

 he is passing to a caisson or tunnel, the first trouble usually noticed 

 is a sense of pressure and pain in the ears. This is due to un- 

 balanced pressure on the memhrana tympani, owing to the fact 

 that the Eustachian duct does not open freely so as to equalize 

 the air pressure in the middle ear with the atmospheric pressure 



