More often, however, this difficulty of depth is overcome by anchoring the mine at the depth most suitable for striking the bottom of a passing ship. But here again there seem to be insuperable difficulties, for the depth of the sea varies and so the length of the anchor rope must be varied with almost every mine that is laid. It has been found possible, however, to make the mines automatically adjust the length of their own anchor ropes so that the desired result is attained without difficulty no matter how deep the sea may be. Let me describe how it is done in the Elia mines used by Great Britain. The inventor, Captain Elia, was an officer in the Italian Navy. The mine consists of three parts: (1) the mine proper, a case containing the explosive, gun-cotton and the firing mechanism; (2) the anchor; and (3) the weight, all of which are connected together by suitable wire ropes. The mine is lighter than water and so floats: the anchor, which bears no resemblance to the ordinary anchor but which is an iron case containing mechanism, only able to act as an anchor by virtue of its weight, is heavier than water and so sinks, while the weight of solid cast iron sinks more readily still. The anchor is often fitted with wheels so that it forms a truck upon which the mine and the weight are placed, the whole running upon rails laid on the deck of the mine-layer. As this ship steams ahead the men push the mines along the rails, dropping them over the stern at regular intervals. When the thing reaches the water, the weight sinks the most rapidly, thereby tugging at the chain whereby it is connected to the anchor. The latter, being less compact, sinks more slowly so that the pull upon the rope is maintained until at last the weight rests upon the bottom. _Then and only then is the pull relaxed._ Now inside the anchor is a winch, upon which is wound a length of flexible wire rope, the other end of which is attached to the mine. The latter, it will be remembered, is light enough to float and so, since it lies upon the surface while the anchor sinks, the rope is drawn off the winch. But there is a spring catch which is able to hold the winch and to prevent it from paying out rope, and that catch is only held off by the pull of the weight. Consequently, as soon as the weight touches the bottom and its pull upon the anchor ceases, the winch is gripped by the catch, no more rope is paid out, and from that moment, as the anchor descends, it drags the mine down with it. The result, then, is that the mine becomes anchored at a depth below the surface roughly equal to the length of the rope connecting weight to anchor.