Besides igniting explosive oil vapors with their sparks, the old-fashioned sulphuric acid and lead storage batteries still used in many submarines are a great source of danger in themselves. The jars are too easily broken, and the leaking acid eats into the steel plating of the boat, weakening it if not actually letting in the sea water. And if salt water comes in contact with a battery of this type, then chlorin gas--the same poisonous gas that the Germans use against the Allies’ trenches--is generated and the crew are in very great danger of suffocation. The new Edison alkali storage battery, besides being lighter and more durable, uses no acid and cannot give off chlorin when saturated with sea water. [Illustration: Redrawn from the London Sphere. Side-elevation of a Modern Submarine, A, Running on the surface; B, In awash condition; C, Submerging; D, Exposing periscope; E, Fully submerged; F, Resting on the bottom.] The remaining great danger is that a submarine may get out of control and submerge too quickly, so that it either strikes the bottom, at the risk of being crushed in or entangled, or descends to so great a depth that its sides are forced in by the pressure of the water outside, which also prevents the submarine from discharging the water in its ballast tanks and escaping to the surface. Detachable safety weights and keels to lighten the boat in such an emergency date back to the time of Bushnell and J. Day. A more modern device is to have a hydrostatic valve (see page 51) set to correspond with the pressure of a certain depth of water, so that if the submarine goes below this the valve will be forced in and automatically “blow the tanks.” A submarine that sank too deep was the _No. 6_, of the Imperial Japanese navy, which disappeared while manœuvering in Hiroshima Bay, on April 15, 1910. When she was found, her entire crew lay dead at their stations, and in the conning-tower, beside the body of the commander, was the following letter written by that officer, Lieutenant Takuma Faotomu: “Although there is indeed no excuse to make for the sinking of his Imperial Majesty’s boat, and for the doing away of subordinates through my heedlessness, all on board the boat have discharged their duties well and in everything acted calmly until death. Although we are dying in the pursuance of our duty to the State, the only regret we have is due to anxiety lest the men of the world misunderstand the matter, and that thereby a blow may be given to the future development of the submarine. “Gentlemen, we hope you will be increasingly diligent and not fail to appreciate the cause of the accident, and that you will devote your entire energy to investigate everything and so secure the future development of submarines. If this be done we have nothing to regret. “While going through gasoline submerged exercises we submerged too far, and when we attempted to shut the sluice-valve, the chain broke.