This photograph was taken looking down upon the deck of the ship. The mines run upon rails, and are pushed by the men towards the stern, whence they are dropped one at a time into the water. The splash indicates that one has just fallen.] Contact mines are often fired electrically, sometimes by batteries of their own inside their own cases, or else by current from the shore through wires, the circuit being completed by an automatic device of some sort actuated unwittingly by the unfortunate victim. One of these contact devices will illustrate the general character of them all. Imagine a little vessel with mercury in it: it is, generally speaking, of some insulating material, but right at the bottom is a metal stud with which the mercury makes contact. The rim may likewise be of metal or a metal rod may project downwards into it: it matters not which, for we can see at once that it is quite easy so to arrange things that whereas, while upright, the mercury shall be well clear of the upper contact, it shall when the vessel is tilted flow on to it, thereby bridging from lower contact to upper contact and completing the circuit. Of course, a mine must only go off when actually struck by a ship and not when it is gently swung to and fro by the action of tide or current in the water. That is easily arranged, for the vessel and contacts can be so shaped that contact is not made until an angle of tilt is reached which no tide or ordinary commotion of the water could bring about. It is clearly possible, too, to combine the contact and observation arrangements in such a way that contact mines can be made safe for friendly ships during the daytime. It is only necessary to adopt the shore battery arrangement already mentioned and disconnect the batteries during the day or when no enemy is in sight, restoring the connection during the darkness or in the event of hostile ships trying to rush the passage. Another interesting scheme for keeping mines safe until required is to anchor them in what is termed a "dormant" condition. This means that a loop is taken in the wire rope by which they are anchored, the loop being fastened by means of a link. This link, however, contains a small quantity of explosive which can be fired from the shore. This has the effect of breaking the link, releasing the loop and allowing the mine to float upwards to the full length of the rope. Thus the mine is down deep, well below the bottom of the biggest ship until released for action. It is doubtful whether much use is made nowadays of permanent mines of the types just described, for they have, no doubt, been largely displaced by the temporary mine which can be laid in a moment by simply being dropping overboard from a ship, but it is quite possible that some of the defences of, say, the Dardanelles, were of the permanent nature.