But even a light field-piece on shore can shell and sink the sort of small, unarmored craft that must be used for mine-sweeping. When a fleet attacks a channel or harbor entrance properly defended by both mine-fields and batteries, each supporting the other, there comes a time when the naval forces must wait till troops can be landed to drive away the forces protecting the rear of the batteries, so that the mine-sweepers can advance and clear a channel for the superdreadnoughts. The most striking example of this is the holding of the Allied fleet by the Turks at the Dardanelles. There, too, effective use is being made of the latest, which is an adaptation of the oldest type of torpedo: the drifting mine.[23] This twentieth-century improvement on Bushnell’s “kegs charged with powder” floats upright, with a vertical-acting propeller on top and another on its bottom, and a hydrostatic valve set to maintain it at any desired depth. Should it rise or sink, the change in pressure will cause the valve to act on the principle already explained in connection with the Whitehead torpedo (see page 44). Controlled by the valve, the little compressed-air motor attached to the vertical propellers will cause them to make a few revolutions, just enough to keep the mine at a constant depth beneath the surface of the Dardanelles, as the four-mile-an-hour current carries it down against the Anglo-French fleet. Within a few hours of each other, during the furious bombardment of the forts on March 18, 1915, the French battleship _Bouvet_ was struck by one of these drifting mines and went down stern-foremost, then H.M.S. _Ocean_ was sunk by another, and the _Irresistible_ forced to run ashore to escape sinking, only to be pounded to pieces by the guns of the forts. A feature of this type of mine is that its size and shape enable it to be launched through a torpedo tube, either from a surface craft or from a submarine. Ordinary contact-mines, without anchors and attached to floats that held them a few feet below the surface of the water, are sometimes dropped overboard from a vessel closely pursued by an enemy. A small mine so dropped by a German light cruiser returning from an attempted raid on the English coast, early in the war, was struck by the pursuing British submarine _D-5_ and sent her to the bottom. The _D-5_ was running awash at the time and only two officers and two seamen were saved. CHAPTER XII THE SUBMARINE IN ACTION “Hit and hard hit! The blow went home The muffled knocking stroke, The steam that overrides the foam, The foam that thins to smoke, The smoke that cloaks the deep aboil, The deep that chokes her throes, Till, streaked with ash and sleeked with oil, The lukewarm whirlpools close!” --KIPLING.