got the tube empty, and can open the inner door and reload. But what of the torpedo which has been fired? It is travelling towards its mark at a speed of between thirty-five and forty knots, if we suppose the range to be an ordinary one, under 1000 yards, and the torpedo to have been ‘run hot,’ _i.e._ driven by hot air instead of cold. The compressed air is heated mechanically inside the torpedo, in the act of passing from the air chamber to the machinery, and this increases both the speed and range. But it is not always convenient or possible to start the heating apparatus, and even when ‘run cold’ the fish will do thirty knots. This speed is amazing, but it is one of the least wonderful of the torpedo’s qualities. The steering of the machine is a double miracle. One device makes it take, after the first plunge, exactly the depth you desire, and another--a gyroscope fitted inside the rudder gear--keeps it straight on its course; or makes it, if you wish, turn in a circle and strike its prey, boomerang fashion. The head of the fish can also be fitted with cutters which will cut through any torpedo-netting that a ship can afford to carry. The only thing that no ingenuity can accomplish is to make a torpedo invisible during its run. The compressed air, when it has passed through the engine, must escape, and it comes to the surface in a continuous boiling line of bubbles. This is visible at a considerable distance; and though, when the track is sighted by the look-out, the torpedo itself is of course always well ahead of the nearest spot where the bubbles are seen rising, it is surprising how often ships do succeed in avoiding a direct shot. A prompt cry from the look-out, a steersman ready to put his helm over instantly, and the torpedo goes bubbling past, a few feet ahead or astern, or comes in on a tangent and runs harmlessly along the ship’s side without exploding. Then away it goes across the open sea, until the compressed air is exhausted, the engine stops, and the mechanical sinker sends it to the ocean bed, which must be fairly strewn with dead torpedoes by this time; for as we know, to our advantage, the proportion of misses to hits is very large in the U-boat’s record.