In spite of this dogma submarines have fired torpedoes whilst in motion with success, and in modern submarines ample provision is made for the loss of weight occasioned by the discharge of the torpedo. 4. Reference has been made to the fact that when the Nordenfelt boat was moving along on an even keel, and a greaser walked forward a couple of feet in his engine room, her head would go down a little, the water would surge forward in the tanks, and she would plunge to the bottom, unless checked in time. It has been said that one man going forward in a submarine boat would cause her to dive to a depth of thirty-six feet in one minute. The movements of the crew may be compensated for by automatic arrangements, but the ideal method would be one in which every one remained immovable at his post during the submerged run. _Steering Below Water._ Quite early in the history of submarine navigation it was found that the compass was not so reliable when the boat was navigating under water as it was when she was on the surface. This is not to be wondered at, for the compass of a submarine is placed in the interior of a tightly-closed metallic shell and in close proximity to an electro-motor and powerful currents capable of influencing it considerably, if not of rendering it altogether useless. The principal causes of the unreliability of the compass on a submerged boat are:— 1. The currents normally produced by the electric motor. 2. The abnormal currents flowing in certain unknown parts of the hull owing to lack of proper insulation. 3. The permanent or transitory magnetisation of the hull if made of a magnetic metal. The best position for the compass on a submarine has been a much debated point, but it is now generally agreed to be in the centre of the hull. The conning tower of the first British submarine was made of steel, but it was afterwards replaced by one of brass. M. M. Gaget in his book[4] states that so entirely untrustworthy and impracticable has steering by compass in French submarines been found, that the gyroscope has been requisitioned. He inclines to the belief that this instrument is the best indicator of route that has yet been devised, yet he points out the want of some reliable method by which the distance made by a submerged boat could be gauged with accuracy. Footnote 4: “La Navigation Sous-Marin.” MOTIVE POWER. The question of the best method of propulsion for submarine boats must be considered under two heads, namely, propulsion on the surface and below the water.