Inventions of the Great War

by A. Russell Bond

The mines may be arranged to be fired by electricity from shore stations, in which case they are anchored at such a depth that ships can sail over them without touching them. If a hostile vessel tried to dash into the harbor, the touch of a button on shore would sink it when it passed over one of the mines. But the success of electrically fired mines would depend upon the "seeing." In a heavy fog they would prove no protection.

Another way of using electric mines is to have telltale devices which a ship would strike and which would indicate to the operator on shore that a vessel was riding over the mines and would also let him know over which particular mines it was at the moment passing. No friendly vessel would undertake to enter the harbor in a fog or after dark and the operator would not hesitate to blow up the invader even if he could not see him.

However, the ordinary method of mining a harbor is to lay fields of anchored mines across the channels and entrances to the harbor—sensitive mines that will blow up at the slightest touch of a ship's hull—and leave tortuous passages through the fields for friendly shipping. Of course pilots have to guide the ships through the passages and lest enemy spies learn just where the openings are the mine-fields must be shifted now and then.

The mines are, therefore, made so that they can be taken up by friendly mine-sweepers who know just how to handle them, and planted elsewhere. These are defensive mines, but there are other mines that are not intended to be moved. They are planted in front of enemy harbors to block enemy shipping and they are made so sensitive or of such design that they will surely explode if tampered with.

THE MINE THAT DOES ITS OWN SOUNDING

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.

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.