There are two classes of mine: one which is laid in peace time, to protect harbours and channels; and the other, which is laid during actual warfare.
The former are anchored in a more or less permanent way. The services of divers are used to place them in position. In some cases they float well down in the water, out of the way of passing ships, but come up nearer the surface when needed. This result is achieved by having an anchor chain of such a length that when fully extended the mine floats a little way under the surface, just high enough to be struck by a passing ship, together with what is called an "explosive link." The link is used to loop together two parts of the chain, and so, in effect, to reduce its length. Wires pass from the link to the shore, and when an electric current is sent along these wires the link bursts asunder, liberates the chain, and the mine floats up to the full length of its chain.
Another plan is to let the mines float high up always, but to fire them, not by the touch of the ship but by electricity from the shore. In this way a safe channel is kept for friendly vessels, while an enemy can be destroyed.
Necessarily, those mines which are hurriedly laid in war time are very different from these. To be of much use, a mine must be concealed below the surface. If it floats upon the water it will be visible, and can be avoided, or, at all events, easily picked up. It is practically impossible to set a floating object at a certain depth in the water, except by anchoring it to another, heavier, object, which will lie at the bottom. Therefore mines have to be anchored in some way.
But the sea varies in depth, so that the length of the anchor chain must be varied, or else some of the mines will be on the surface, thereby advertising the presence of the mine-field, while others will be below the depth of even the biggest ship. In warfare, however, mines need to be laid quickly. There is no time to sound for the depth and then to adjust the length of cable accordingly. Hence the mine must be so made as to set itself correctly at a pre-determined depth.
Possibly some readers may think that such things might be made to float, of themselves, at the right depth. It is a fact, however, that a thing either floats upon the surface of water or falls to the bottom. Water is practically incompressible, so that the water at the bottom of the sea is no heavier than that near the surface. The conditions which prevail in air and allow a balloon to float at any desired height do not apply. The only thing, in this case, is to have an anchor chain or rope of the right length.
So let us picture a mine-laying ship steaming along, probably in the dead of night, surreptitiously laying mines in the hope that the enemy will run into them on the morrow.
Along the deck of the ship are small railway lines, and on these lines stand what appear to be trains of small trucks, each truck having small wheels to run on, and each bearing a large round metal ball. As the ship travels along, the crew, handling these deadly things quite freely, as if they were innocent of any danger, propel them along to the stern, and at regular intervals push one overboard. That is all.
The freedom with which the men handle them is not folly, for they are then quite harmless. Nor need they trouble about the length of rope, for that adjusts itself. Just tumble the things overboard, and in due time they anchor themselves at the right depth and set themselves in the right condition for blowing up any ship which may get amongst them.
The truck-like object upon wheels is not the mine itself: it is the sinker which lies at the bottom of the sea. The round ball which it bears is the mine, and the two are connected together by a wire rope. To commence with, this rope is coiled upon a drum in the sinker, which drum is either held tightly or is free to revolve according to the position of a catch. That catch is held open, so that the drum is free, by a weight at the end of a short rope. Let us assume that that rope is ten feet long.