Ships are not built so that they can be picked up by the ends. Such treatment would be liable to break their backs in the middle. Were they built more like a bridge truss, the salvor's difficulties would be materially lessened. It would be a much simpler matter to raise a vessel with pontoons were it so constructed that the chains of the pontoon could be attached to each end of the hull. But because a ship is built to be supported by the water uniformly throughout its length, the salvor must use a large number of chains, properly spaced along the hull, so as to distribute the load uniformly and see that too much weight does not fall on this or that pontoon. The main problem, however, is to get hold of the wreck and this requires the services of divers, so that if there were no other limiting factor, the depth to which a diver may penetrate and perform his duties sets the mark beyond which salvage as now conducted is impossible. [Illustration: (C) International Film Service The Diving Sphere built for Deep Sea Salvage Operations] A common diver's suit does not protect the diver from hydraulic pressure. Only a flexible suit and a thin layer of air separates him from the surrounding water. This air must necessarily be of the same pressure as the surrounding water. The air that is pumped down to the diver not only serves to supply his lungs, but by entering his blood transmits its pressure to every part of his anatomy. As long as the external pressure is equalized by a corresponding pressure within him, the diver experiences no serious discomfort. In fact, when the pressure is not excessively high he finds it rather exhilarating to work under such conditions; for, with every breath, he takes in an abnormal amount of oxygen. When he returns to the surface he realizes that he has been working under forced draft. He is very much exhausted and he is very hungry. It takes a comparatively short time to build up the high internal pressure, which the diver must have in order to withstand the pressure of the water outside, but it is the decompression when he returns to the surface that is attended with great discomfort and positive danger. If the decompression is not properly effected, the diver will suffer agonies and even death from the so-called "Caisson Disease." [Illustration: The Pneumatic Breakwater--Submerged Air Tubes protecting a California Pier from Ocean Storms] A HUMAN SODA-WATER BOTTLE