2. By the careful construction of the hull, and by strict control of all sea-valves, the admission of water may be prevented. If the boat is stove in and water enters in any quantity, she will inevitably sink. As an escape some inventors have provided their submarines with detachable boats. 3. Usually the length of the run under water will not be so great that the foul air will need to be got rid of. If necessary it can be expelled by drawing on the store of compressed air, and as the substances withdrawn will always be small, no change in longitudinal balance need be feared if precautions are taken. As all modern submarines are driven by electricity beneath the surface, the expulsion of products of combustion need not be considered. 3a. In the earliest submarine boats the torpedoes consisted of charges of explosive in cases, which were attached to the outside of the vessel to be attacked, or were towed against her sides. Those who had little faith in the future of under-water warfare declared that a torpedo could never be fired from a tube in a submerged vessel without disastrous effects. The Nordenfelt boats were certainly not successful in discharging torpedoes, for as a general rule they as nearly as possible stood up vertically on their tails and proceeded to plunge to the bottom stern first on these occasions. However, since then, submarines have fired torpedoes quite satisfactorily under water. The expulsion of a torpedo from a vessel totally submerged in the water, whether equal to or less than the weight to the water displaced, naturally reduces her weight and tends to send her up towards the surface. This tendency can best be counteracted by the admission of a certain quantity of water ballast into the boat. The method now usually followed is to allow the surrounding water to enter the tube immediately after the launch of the torpedo, and as the weight of the volume of water admitted will be about equal to that of the missile ejected, the longitudinal stability of the submarine should not be disturbed. When the second torpedo comes to be placed in the tube, the volume of water already in it must of necessity be ejected, and a compensating reservoir may be used to receive it. As each torpedo is fired a certain amount of water, corresponding to the weight of the projectile, must be allowed to enter the compensating reservoir. This may be done automatically. The _Engineer_ in a leader on January 18, 1901, said: “The discharge of a bow torpedo (by a submarine) would be instantly followed by the rise of the bow; relieved of the weight the boat would tend to stand on end. If going ahead at the time she would immediately come to the surface to be destroyed. If going astern she would plunge downwards and the consequences might be equally serious.... Torpedoes must be fired when the submarine is at rest.”