The crucial feature of diving operations lies in the time required in decompression, which, if held within the limits given by Fleet Surgeon Mourilyan, would practically limit diving operations to half the present depth of submergence and greatly increase the cost and the time demanded for such undertakings. Strange as it may seem, the human body will stand an immense amount of compression, but the greatest care must be taken to make the recovery to normal a very slow and deliberate process. Doctors Leonard Hill and Greenwood, of the London Hospital Medicine College, have conducted a series of scientific investigations regarding the physical limits of a normal man to compression without risk of strain or ultimate injury. Remarkable as it seems, they have shown that it is possible to submit to a pressure of seven atmospheres--the equivalent of a submergence to a depth of two hundred and ten feet, a depth considerably in excess of the best diving records up to the time of their experiments. These gentlemen proved conclusively that immunity from serious consequences could be assured, provided the period of decompression was sufficiently long. The experiments were not made under water, but were made in an experimental air-chamber especially fitted up for them by one of the big English submarine engineering companies. [Illustration: SKETCH DRAWING ILLUSTRATING A METHOD OF TRANSFERRING CARGOES FROM SUNKEN VESSELS TO SUBMERGED FREIGHT CARGO-CARRYING SUBMARINES Demonstrated as practical in 1900 by the combined use of the "Argonaut" and the demonstrating freight-carrying submarine shown above.] Under the conditions usually prevailing in the fields wherein divers are employed, it is not possible, with the systems of working generally adopted, to provide this period of decompression nor to work with this studied deliberation when descending from or when ascending to the ordinary surface vessel. The suit of a diver weighs over two hundred pounds, and when inflated the bulk is considerable. A diver being lowered from a vessel is swung to and fro like a pendulum, and if there is any sea on--the open sea is never entirely still--the surge naturally affects the diver so that it is beyond human possibility to limit his descent to a nicety or to take the time either in going down or coming up that science has proved necessary to his physical well-being in the most generous sense. The greater the depth the greater the difficulties, and to reach a submergence of one hundred and fifty feet is now practically prohibitive except under ideal conditions. The semi-submergible boat has, however, met the problem squarely and has overcome many of the difficulties heretofore deemed insuperable. The simplicity and the practicability of the working principle involved are graphically shown by the accompanying drawings.