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THE POPULAR SCIENCE MONTHLY.

ments, he could not wander away, and this they have done by strapping 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 b├ęton 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 employed 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 mechanical explanation: "When a man enters the tube," says one author, "he is flattened 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 when you leave." It is decompression and not compression that does the mischief.

But how does decompression act? Very simply indeed. Here in this glass jar is a rat subjected to ten atmospheres. I now turn a