"No man in particular," he said. "It came into existence almost spontaneously, in response to a pressing need. Gunfire can destroy submarines when they are on the surface, but you know it can accomplish nothing against them when they are submerged. This fact made it extremely difficult to sink them in the early days of the war. One day, when the Grand Fleet was cruising in the North Sea, a submarine fired a torpedo at one of the cruisers. The cruiser saw the periscope and the wake of the torpedo, and had little difficulty in so manoeuvring as to avoid being struck. She then went full speed to the spot from which the submarine had fired its torpedo, in the hope of ramming it. But by the time she arrived the submarine had submerged so deeply that the cruiser passed over her without doing her any harm. Yet the officers and crew could see the submerged hull; there the enemy lay in full view of her pursuers, yet perfectly safe! The officers reported this incident to me in the presence of Admiral Madden, second in command. "'Wouldn't it have been fine,' said Madden, 'if they had had on board a mine so designed that, when dropped overboard, it would have exploded when it reached the depth at which the submarine was lying?'" "That remark," continued Admiral Jellicoe, "gave us the germinal idea of the depth charge. I asked the Admiralty to get to work and produce a 'mine' that would act in the way that Admiral Madden had suggested. It proved to be very simple to construct--an ordinary steel cylinder filled with TNT; this was fitted with a simple firing appliance which was set off by the pressure of the water, and could be so adjusted that it would explode the charge at any depth desired. This apparatus was so simple and so necessary that we at once began to manufacture it."