A submarine mine of to-day consists of a mine case, shaped like a ball or egg, about one yard in diameter, mounted on an anchor in the form of an iron box about 30 inches square, connected by a wire rope mooring cable, about ⅜ inch in diameter. The mine case contains the charge of high explosive—300 pounds of TNT in our mines—and the firing mechanism. The combination stands about 5 feet high and weighs 1400 pounds. Four small car wheels on the anchor run on steel tracks, allowing the mines to be easily moved along the decks to the launching point. [Illustration: THE SEQUENCE OF OPERATIONS AFTER A MINE IS LAUNCHED.] When the mine dives overboard, the mine and anchor come to the surface and float for a time, still held together, part of the mine case above water. Outside the anchor is a 90-pound plummet, containing a reel of ⅛-inch diameter steel wire “plummet cord,” made the same length that the mine is to be below the surface. Thus, if the mine is to be 160 feet beneath the surface, the cord is made 160 feet long. The plummet drops off when the mine goes overboard, unreels its cord, coming to the end with a jerk that trips the slip hook which holds the mine and anchor together. The pull on the cord also lifts the latch on the reel inside the anchor, allowing the mooring wire to unwind. The nearly solid plummet tends to sink faster than the more bulky anchor, thus keeping the cord taut until the plummet strikes bottom. The cord then at once slackens, releasing the latch, locking the reel, and preventing any more mooring wire unwinding. The anchor, continuing to sink, pulls the mine under until the anchor strikes bottom. The mine is thus finally moored always at the desired depth beneath the surface, no matter how irregular the ocean bed may be. The mine cases are buoyant enough to pull straight up from their anchors ordinarily, but in a current they are swayed away from the vertical, which dips them down somewhat deeper than intended. For this reason, any locality where the currents are strong is unfavorable for a minefield—one of the difficulties the British Navy had to contend with in closing the Dover Strait. The new mine having, by October, been carried past the experimental stage as to its principal features, by the Naval Torpedo Station at Newport, R.I., some important mechanical details of the mine yet remaining were now worked out by the _Baltimore_, Captain A. W. Marshall, working directly under the Bureau of Ordnance. By the time complete units were ready, the _Baltimore_ had been sent abroad, so the proof testing devolved upon the _San Francisco_, Captain H. V. Butler. This came in March and April.