It must be a nerve-racking job at the best of times, for the submarine is a very vulnerable craft. A member of the crew of a German submarine captured during the war is reported to have said that out of ten submarines attacked, nine were sunk. That may or may not be true, but it is certain that a very little damage, which would hardly affect an ordinary craft, is enough to sink a submarine. That is because, in order to be able to sink at will, the reserve of buoyancy has to be very low. An ordinary surface ship has at least as much of its bulk above water as below: hence it can take on board a weight of water almost equal to, if not exceeding its own weight before it sinks. At the best a submarine has not more than 30 per cent of excess and so it sinks if water amounting to only 30 per cent of its weight gets into it. In other words, the reserve in one case is at least 100 per cent: in the other at most 30 per cent. During the war a submarine saw and tried to track down, somewhat after the manner described, a slow, steady-going collier which plies between London and the north carrying coal for a London gas-works. Having, as it thought, got into position for discharging its torpedo it rose for a final look when (it must have been to the amazement of the crew) the collier was seen making straight for them. What they really thought no one will ever know, for the collier had the best of the encounter, the submarine was crushed beneath her blunt bows and sank, no doubt, for ever. The mere fact that a slow, clumsy, heavily-laden collier could ever thus vanquish an up-to-date submarine is eloquent testimony to their vulnerability. Many a submarine, too, has fallen to the shells of an armed fishing trawler simply because the shells of the latter were so much quicker in action than a torpedo, coupled with the fact that one well-placed shot, by preventing a submarine from diving, renders it almost helpless. Some submarines, however, have a gun on the deck, so that when light they can fight like a destroyer or other lightly-armed vessel. The gun shuts down into a cavity when the vessel goes below.