Compression is rather annoying to a man who has not experienced the sensation, but the principal danger comes in decompression, particularly after a person has been in the working chamber for a long time. When breathing compressed air much larger volumes of oxygen are taken into the lungs at each breath than in the ordinary atmosphere, and one feels decidedly exhilarated by this unusual supply of stimulating oxygen. But with the oxygen large volumes of nitrogen are taken into the system as well. The nitrogen permeates the blood while the oxygen is consumed and exhaled in the form of carbon dioxide. The longer a man is exposed to the pressure the more nitrogen does he absorb. When the pressure is released suddenly the nitrogen begins to froth, just as a bottle of soda water does when the stopper is removed. The nitrogen bubbles in the small veins stop the circulation and produce the dreaded “caisson disease” which makes itself felt in the form of severe cramps and excruciating pains. Not infrequently this disease ends in death. By reducing the pressure gradually the nitrogen is enabled to pass off completely without bubbling or frothing, just as it is possible to let out the gas from a soda bottle without frothing by permitting it to escape through a pin hole in the stopper. The surest cure for a victim of the caisson disease is to put him immediately into what is known as a “hospital tank” and build up the pressure in the tank equal to that which he has just been subjected to in the caisson, after which the air in the tank is let out so slowly that there is plenty of time for the nitrogen to pass off without forming bubbles. A pressure of forty-five pounds per square inch above that of the atmosphere is considered a severe pressure for excavation work. But work has been carried on in pressures up to fifty-two pounds, corresponding to a depth of 120 feet below water level. Now that the cause of the caisson disease is understood formulas have been worked out to insure proper decompression. On one occasion a diver descended 306 feet into the ocean where the water pressure was 133 pounds on every square inch of his body. This meant that the air which was pumped down to him had to be compressed to the same pressure. The diver actually remained at the bottom only a very short time, but it took two hours and three-quarters to bring him to the surface. He came up half the distance very quickly and then had to rest on the bottom rung of a Jacob’s ladder. The rungs on this ladder were ten feet apart and he was instructed to rest on each rung a certain specified time. When he was ten feet below the surface he had to wait three-quarters of an hour for the last trace of nitrogen absorbed by his blood to pass off. BORING TUNNELS THROUGH RIVER BEDS