_French Submarine Boat "Plongeur."_--The boat termed the _Plongeur_ was designed by Admiral Bougois and M. Brune, and was exhibited at the Paris Exhibition of 1867. She was 26' long, 9' deep, and fitted with centre and bilge keels. She carried two small tanks containing compressed air, and four large tanks were placed at the bottom of the boat for the purpose of sinking her, these latter tanks communicating with the water outside and the air tanks. She also was fitted with a compass for steering by, a water gauge to show the depth of submersion, and an air gauge to show the pressure of air in the boat. Rectangular valves were placed at the bottom of the boat for entrance or exit therefrom, for the use of divers, and to affix torpedoes to a ship's bottom. On the top a circular opening for entrance and exit was arranged, also an iron cupola fitted with bull's-eyes. She was also fitted with an apparatus for spraying water through the air in the interior of the boat on its becoming foul, and escape valves for releasing any foul air were placed at the top of the boat. The water tanks were filled by means of pumps, and emptied by means of the compressed air. She was propelled by a three-bladed screw worked by four men. Her rate of progression was about four knots per hour. The anchors consisted of two 15 inch shot, fitted with wire rope cables, working through watertight stuffing boxes. This vessel has been subjected to some experiments, but with what results is not generally known. One of the most important uses to which a submarine boat would be put in connection with torpedo operations would be "to discover the exact position and number of an enemy's submarine mines, and if necessary destroy them," the former being an operation in the present day quite impossible to perform, and the latter one rarely to be depended on. FOOTNOTES: [Footnote M: Extract from 'European Ships of War,' &c., by J. W. King, U.S.N., page 312.] [Footnote N: Extract from _Engineering_, under date April 13, 1877.] [Footnote O: Extract from letter of Captain Ericsson that appeared in _Engineer_, under date Nov. 8, 1878.] [Footnote P: Extract from the Engineering of the 10th of January, 1879.] CHAPTER VII. TORPEDO OPERATIONS. A REVIEW, however brief, of the numerous torpedo operations that have of late years been carried out in actual war, must prove not only of great interest, but of material aid to those who may be desirous of studying this branch of naval warfare, for the experience so gained ought alone to be the basis on which a system of submarine offence and defence should be constructed.