Modern mines are of various shapes and sizes but are as a rule either spherical or shaped like a pear with the stem down. The anchor is a hollow, flat-bottomed cylinder, containing its own anchor cable wound on a windlass, and making a convenient base or stand for the explosive chamber or mine proper, so that the whole apparatus can be stood or trundled about the deck of a mine-layer like a barrel. Once placed in the water either by being dropped through the overhanging stern-ports of a large sea-going mine-planter like the U.S.S. _San Francisco_, or lowered over the side of a smaller craft by a derrick boom, the weight of its anchor causes the mine to assume an upright position. This releases a small weight or plummet at the end of a short line attached to a spring that keeps the windlass inside the anchor from revolving. When the plummet has sunk to the end of its cord, its weight pulls down the spring, and the windlass begins to revolve and unreel the cable, the end of which is, of course, made fast to the bottom of the mine. This causes the anchor, which has been held up by the buoyancy of the mine, to sink, and follows the plummet till the latter touches the bottom. Freed of the plummet’s weight, the spring now flies up and stops the windlass. But the hollow anchor is now filled with water, whose additional weight drags the mine under. When the anchor rests on the bottom, the mine will be at the same distance beneath the surface of the water as the anchor had to sink after the windlass stopped, or the length of the plummet’s line. By regulating that, a mine can be made automatically to set itself at any desired depth. [Illustration: (Redrawn from the London Sphere.) Modern Contact-Mine. A, Mine-Planter; B, Mine being dropped overboard; C, Plummet-line extended; D, Anchor sinking; E, Plummet touching bottom; F, Mine submerged and anchored; G, Battleship striking mine; 1, The “Striker”; 2, Charge of Explosives; 3, Air-space, for Buoyancy; 4, Mine-case; 5, Anchor; 6, Plummet. ] Mines are almost never laid singly but in groups, the area of water so planted being called a “mine field.” A secret, zigzag channel is often left clear for the benefit of friendly craft. The rows of mines are usually “staggered” or placed like the men on a checker-board, so that if a hostile vessel passes through an opening in the first row she will strike a mine in the second. Another device is to couple together the mooring cables of two or more mines so that a ship passing between them will draw them in against her sides.