But that is by the way. The point is how to make the torpedo very clearly visible while it is still under water. A short mast might be used, but that would be liable to be shot away. The inventor had a happy inspiration when he made it blow up a jet of water, like a whale does. This jet is quite easy to see, yet no shot can destroy it. Compressed air blows up this tell-tale jet which the observer can see, and by its means he can guide the torpedo at will. A submarine mine may be regarded as a stationary torpedo. It consists of a metal case filled with a powerful charge of explosive which floats harmlessly in the water until some unfortunate vessel strikes against it, when it blows up with sufficient force to make a hole in the stoutest ship. 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.