Doctor Bushnell was also the inventor of the submarine mine, with which he blew up a schooner anchored off New London, Connecticut, and attempted to sink some British men-of-war in the Delaware River off Philadelphia by setting them adrift with the tide, expecting them to float down, strike against the sides of the ship, and then explode. Fortunately for the ships, none of them happened to strike, but the fact becoming known that torpedoes were being set adrift in the river caused great consternation among the British shipping people. When some wag set a lot of kegs adrift, which floated down the river, it caused tremendous excitement, the English crews firing at the kegs as they came floating down the river. This has been recorded in that humorous poem called "The Battle of the Kegs," by Francis Hopkinson, one of the signers of the Declaration of Independence. =Fulton's Attempt.=--Robert Fulton, the man whose genius made steam navigation a success, was the next to turn his attention to submarine boats, and submarine warfare by submerged mines. A large part of his life was devoted to the solution of this problem. He went to France with his project and interested Napoleon Bonaparte, who became his patron and who was the means of securing sufficient funds for him to build a boat which was called the _Nautilus_. With this vessel Fulton made numerous descents, and it is reported that he covered fifty yards in a submerged run of seven minutes. In the spring of 1801 he took the _Nautilus_ to Brest, and experimented with her for some time. He and three companions descended in the harbor to a depth of twenty-five feet and remained one hour, but he found the hull would not stand the pressure of a greater depth. They were in total darkness during the whole time, but afterward he fitted his craft with a glass window, one and a half inches in diameter, through which he could see to count the minutes on his watch. He also discovered during his trials that the mariner's compass pointed equally as true under water as above it. His experiments led him to believe that he could build a submarine vessel with which he could swim under the surface and destroy any man-of-war afloat. When he came before the French Admiralty, however, he was met with blunt refusal, one bluff old French admiral saying, "Thank God, France still fights her battles on the surface, not beneath it!"--a sentiment which apparently has changed since those days, as France now has a large fleet of submarines. [Illustration: ROBERT FULTON'S SUBMARINE]