Very many ingenious devices have been invented to enable the crew of a stranded submarine to escape. The best-known and most widely used is some form of the air-lock or diver’s chamber, as described in the chapter on the Lake boats. Through this the crew can pass in succession to the water outside and swim to the surface. If the depth is so great that an unprotected swimmer would be crushed by the weight of water above him, there is a great variety of safety-helmets, and of jackets with mouth-pieces leading to tanks containing enough air under moderate pressure to inflate the lungs and cheeks so that the internal pressure of the body will counteract that of the water. An escaping seaman, burdened with such a device, cannot rise unaided to the surface but must climb or be hauled up by a rope let down from above. Moreover, he must not ascend too rapidly, or the pressure within his body will dangerously exceed that without, as if he had been suddenly picked up at the seashore and carried to the top of the Andes. The human body is too delicate and elaborate a structure to be carelessly turned into a compressed-air tank. The surplus oxygen forms bubbles which try to force their way out through the tissues of the body, causing intense pain, and possibly paralysis or death. To avoid this, divers are brought up from any great depth by slow and careful stages, unless they can be placed at once in specially-constructed tanks on shore, where the pressure they are under can be gradually reduced to normal. [Illustration: Courtesy of the Scientific American. One Type of Safety-jacket.] A covered lifeboat carried in a socket on the submarine’s deck, so that in case of accidental stranding the crew could get into the small boat from below, close the hatch cover, release the lifeboat from within, and rise safely and comfortably to the surface, was an attractive feature of the _Plongeur_ in 1863, and of many projected but unbuilt submarines since then. A detachable conning-tower, containing a small lifeboat that could be launched after the safety compartment had risen to the surface, has also been designed and patented more than once. Theoretically, these devices seem admirable but naval architects will have none of them. The reason for this is very simple. A submarine is primarily a warship, an instrument of destruction, and its carrying capacity is too limited to permit several hundredweight of torpedoes or supplies being crowded out by a lifeboat or a score of safety-helmets. A divers’ compartment and one or two ordinary diving-suits--for these things are of military value--and a buoy that can be sent up to mark the spot where the boat has gone down are as much as you can expect to find in the average naval submarine.