326 THE POPULAR SCIENCE MONTHLY. 



ments, he could not wander away, and this they have done by strap- 

 ping upon his back a compressed-air receiver which works very 

 ingeniously. Divers who fish in this way for coral, pearls, sponges, 

 etc., descend to the depth of forty metres, and the air they breathe is 

 under a pressure of five atmospheres. 



The apparatus used in erecting bridge-piers is a great improvement 

 on the old diving-bell. The discovery of the principle involved in 

 these is due to M, Triger, who, in 1841, applied it to the construction 

 of mine-galleries under the Loire. Nothing can be simpler than this 

 principle : it is employed by children when they amuse themselves by 

 blowing into a half-submerged tube, and causing the air to issue in 

 bubbles. The apparatus, reduced to its simplest expression, may be 

 described as follows : A tube of the length proposed for the pier is let 

 down to the bottom of the river. It is capped with a chamber, into 

 which is forced compressed air. This air expels the water from the 

 lower end of the tube, and passes out just as in the child's play. The 

 workmen can then, by means of a system of doors, as seen in Fig. 5, 

 descend to the bottom, and there dig for the foundation of the pier. 

 As the soil is removed, the tube descends by its own weight ; it is 

 lengthened by the addition of successive sections, till the solid rock 

 is reached. The cylinder is now filled with beton and the pier is 

 complete. 



In these apparatus workmen have also been subjected to pressures 

 as high as five atmospheres. 



Now, both among divers and workmen in these tubes, symptoms 

 have been noted often so serious as to terminate fatally. First there 

 is an intolerable itching, called by the workmen " the fleas " {puces) ; 

 then violent pains in the muscles and joints which have done most 

 work ; paralytic symptoms, particularly in the lower limbs, which 

 often are persistent and fatal; finally, sudden death. Of 160 men em- 

 ployed on the foundations of the St. Louis (Missouri) Bridge, thirty 

 were seriously attacked, and twelve died. 



You are welcome to all the hypotheses invented by the fertile 

 minds of physicians to explain these redoubtable troubles. Quite as 

 a matter of course, we find here, first of all, the njechanical explana- 

 tion: "When a man enters the tube," says one author, "he \& flat- 

 tened out!"" {aplati). Very likely, indeed, if we admit that at the 

 pressure of three atmospheres 4,500 kilometres more weighs upon 

 our body. But, happily, we are saved from this fate by elementary 

 physics. 



The workmen in the tube at Kehl had a saying, as is usual among 

 workmen everywhere; it is one full of acuteness and depth: "You 

 pay wlien you leave." It is decompression and not compression that 

 does the mischief. 



But how does decompression act? Very simply indeed. Herein 

 this glass jar is a rat subjected to ten atmosphei'es. I now turn a 



