The crack sea-diver is almost as difficult to find as the swimmer who can conquer the English Channel. When it comes to doing actual work at depths of 100 feet and over, the strain on the diver’s body is indeed very much greater, for his exertions use up so much oxygen that his heart is called upon to pump at an increased speed in order to replace it. All the time, of course, the diver is breathing compressed air, thus the pressure of the sea on the outside of his body is practically counterbalanced by the pressure of the air inside his body. While the weight of the sea is trying to crush him inwards, the compressed air is pushing outwards, so the air pressure within equalizes the water pressure without, and the diver is enabled to work in perfect safety under a mass of water that would crush an unprotected man flat. We might liken the water pressure to six men who are pushing hard against a door and striving to open it, while the air pressure resembles six men pushing against the other side of the door to keep it closed. With both teams equally matched in strength, the door remains quite unaffected by the contest if it be solidly built of oak. But if it is a weak door, the strain of the men pushing against it will probably break it. Breathing compressed air not only places a strain on the lungs, but it tends to fill the body with an excess of nitrogen. This nitrogen may easily form tiny bubbles of gas, and these bubbles, if they reach the heart, might cause the death of the diver or bring on that dread paralysis known as diver’s palsy, a disease which renders the lower part of the diver’s body quite useless. Strangely enough, it is not in going down that this danger threatens the diver, but only in coming up. If he comes up too suddenly, the excess of nitrogen in the blood bubbles like the tiny bubbles in a siphon of soda and at once his life is threatened. The bubbles are due to the pressure of the water on the outside of the body growing suddenly less than the pressure of air inside the body, consequently the nitrogen seeks to escape in bubbles just as the soda-water seeks to escape when the key of the siphon is depressed. The pressure inside the body cannot adjust itself quickly enough to the lessening pressure outside, and these bubbles are the result.