Their speed, never very fast upon the surface, is reduced by submersion to that of the oldest and slowest gunboats. Their radius of action is also circumscribed--that is, they cannot carry supplies sufficient to go a long distance, deal with a hostile fleet, and then return to headquarters without replenishment. Furthermore, there arise the nice questions of buoyancy combined with stability when afloat, of sinking quickly out of sight, and of keeping a correct balance under water. The equilibrium of such small vessels navigating between the surface and the bottom is extremely sensitive; even the movements to and fro of the crew are enough to imperil them. To meet this difficulty the big water-ballast tanks, engines and accumulators are necessarily arranged at the bottom of the hull, and a pendulum working a helm automatically is introduced to keep it longitudinally stable. To sink the boat, which is done by changing the angle of the propeller in the _Goubet_ and some others, and by means of horizontal rudders and vanes in the _Nordenfelt_ and _Holland_, it must first be most accurately balanced, bow and stern exactly in trim. Then the boat must be put into precise equilibrium with the water--_i.e._ must weigh just the amount of water displaced. For this its specific gravity must be nearly the same as that of the water (whether salt or fresh), and a small accident might upset all calculations. Collision, even with a large fish, could destroy the steering-gear, and a dent in the side would also tend to plunge it at once to destruction. Did it escape these dangers and succeed in steering an accurate course to its goal, we have up to now little practical proof that the mere act of discharging its torpedo--though the weight of the missile is intended to be automatically replaced immediately it drops from the tube--may not suffice to send the vessel either to bottom or top of the sea. In the latter case it would be within the danger zone of its alarmed enemy and at his mercy, its slow speed (even if uninjured) leaving it little chance of successful flight. But whatever the final result, one thing is certain, that--untried as it is--the possible contingency of a submarine attack is likely to shake the _morale_ of an aggressive fleet. “When the first submarine torpedo-boat goes into action,” says Mr. Holland, “she will bring us face to face with the most perplexing problem ever met in warfare. She will present the unique spectacle, when used in attack, of a weapon against which there is no defence.... You can send nothing against the submarine boat, not even itself.... You cannot see under water, hence you cannot fight under water. Hence you cannot defend yourself against an attack under water except by running away.”