A favorite type of mine used during the war was one which automatically adjusted itself to sink to the desired depth. Submerged mines are more dangerous to the enemy because they cannot be seen and avoided. They should float far enough under the surface to remain hidden and yet not so deep that a shallow-draft ship can pass over them without hitting them. As the sea bottom may be very irregular, it is impossible to tell how long the anchor cable should be without sounding the depth of the water at every point at which a mine is planted. But the automatic anchor takes care of this. Very ingeniously it does its own sounding and holds the mine down to the depth for which it is set. The mine cable is wound up on a reel in the anchor and the mine is held fast to the anchor by a latch. The anchor is of box-shape or cylindrical form, with perforations in it. At first it sinks comparatively slowly, but as it fills with water it goes down faster. Attached to the anchor is a plummet or weight, connected by a cord to the latch. The length of this cord determines the depth at which the mine will float. [Illustration: Courtesy of the "Scientific American" FIG. 22. How the mine automatically adjusts itself to various depths of water] The operation of the mine is shown in Fig. 22. When it is thrown overboard (1) it immediately turns over so that the buoyant mine _A_ floats on the surface (2). While the anchor is slowly filling and sinking, the plummet _B_ runs out (3). If the mines are to float at a depth of, say, ten feet, this cord must be ten feet long. As soon as it runs out to its full length (4) it springs a latch, _C_, releasing the mine _A_. Then the mine cable _D_ pays out, as the anchor _E_ sinks, until the plummet _B_ strikes bottom (5). As soon as the plummet cord slackens a spring-pressed pawl is released and locks the mine-cable reel, so that as the anchor continues to sink it draws the mine down with it, until it touches bottom (6), and as the anchor was ten feet from the bottom when the plummet touched bottom and locked the reel, the mine must necessarily be dragged down to a depth of ten feet below the surface. The mine itself, or the "devil's egg" as it is called, is usually a big buoyant sphere of metal filled with TNT or some other powerful explosive; and projecting from it are a number of very fragile prongs which if broken or even cracked will set off the mine. There is a safety-lever or pin that makes the mine harmless when it is being handled, and this must be withdrawn just before the mine is to be launched. In some mines the prongs are little plungers that are withdrawn into the mine-shell and held by a cement which softens after the mine is submerged and lets the plungers spring out. When the plungers are broken, water enters and, coming in contact with certain chemicals, produces enough heat to set off a cartridge which fires the mine. PICKING INFERNAL MACHINES OUT OF THE SEA