For every boat there is a limiting depth, beyond which she must not go. While it is quite possible to construct a boat strong enough to resist the pressure at depths of 50 fathoms and over, it will not be necessary to go deeper than 5 to 6 fathoms, or enough to clear the keel of a big ship. Still it will be well to give the boat a hull capable of resisting pressures greater than those she will normally encounter. By means of a hydrostatic valve or some similar arrangement, the submarine may be kept from diving to too great depths.
It is said that the “Goubet” boats can withstand the pressure at a depth of about 5,000 feet, or very nearly a mile beneath the surface; the “Holland” boats can navigate with safety at 150 feet, a depth quite sufficient for all practical purposes.
The French submersible Silure was recently sunk to a depth of 134½ feet, and it was found that the hull was compressed to the extent of 1–25th of an inch. No inconvenience was felt by the crew greater than that experienced at a depth of 20 feet.
Submarines possessing floatability have the power of rising to the surface should any accident happen to the motive power, steering gear, machinery, &c., and automatic arrangements are provided for working the horizontal rudders in order to keep the boat on an even keel. Some accident might, however, cause the boat to begin to sink, and the advisability of the submarine carrying a false bottom or detachable keel has been pointed out, as this could be dropped in an emergency, causing the vessel to rise at once to the surface.
The vessel Le Plongeur carried a detachable boat, and Captain Hovgaard in his design also supplied such a boat. It was made to stand the same pressure as the submarine itself, and rested on a saddle-shaped packing, against which it was tightly pressed down by means of a number of clips. Inside the packing was a circular door in the boat and a corresponding and smaller one in the ship arranged in such a way that it was possible to get up into the boat, close the lower lid in the ship, and then the lid in the boat. This done, all the handles of the clips were turned and the water would probably enter the space inside the packing, and if not it might be made to do so through a small pipe leading from the outside to the space, and provided with a stopcock. The boat would now have a certain buoyancy, but would hang on in two main clips, placed one at each end of the detachable boat on mechanical connection with each other so that they could only be let go both at the same time, thereby preventing jamming. When these clips were opened the boat would ascend to the surface; communication with the vessel, if somebody should be left behind, might be kept up by telephonic connection.
M. Goubet, M. Drzewiecki, and other inventors provided their vessels with means for being propelled by the crew, working either oars or pedals, in the event of the machinery failing to act.
Respiration, or breathing, is a part of the life of all organisms, whether animal or vegetable. Air is taken into the lungs; the oxygen is absorbed, while the carbonic acid is given back again to the atmosphere. The respiration of human beings or animals in closed chambers to which the air is denied access is not possible beyond a certain period. The oxygen is sooner or later, according to the size of the chamber, used up, and the air becomes so vitiated with the carbonic acid expelled by the lungs that the vital functions of the body are arrested.
Many of the earlier submarine boats carried no reserve of air, as the time that they were intended to remain under water was not long.
Reference is made in old writings to the “chymicall liquor” supposed to have been used by Cornelius Drebbel to restore the purity of the air in his under-water vessel, but what its composition was we shall never know.
The air required for respiration in a submarine vessel may be supplied in two ways.