For use in the open sea a different type of mine is required. This must be quite self-contained and automatic in action, exploding when struck by a passing vessel. The exploding mechanism may take different forms. The blow given by a ship may be made to withdraw a pin, thus releasing a sort of plunger, which, actuated by a powerful spring, detonates the charge. A similar result is obtained by the use of a suspended weight, in place of plunger and spring. Still another form of mine is fired electrically by means of a battery, the circuit of which is closed automatically by the percussion. Deep-sea mines may be anchored or floating free. Free mines are particularly dangerous on account of the impossibility of knowing where they may be at any given moment. They are liable to drift for considerable distances, and to pass into neutral seas; and to safeguard neutral shipping international rules require them to have some sort of clockwork mechanism which renders them harmless after a period of one hour. It is quite certain that some, at least, of the German free mines have no such mechanism, so that neutral shipping is greatly endangered. Submarine mines are known as _ground_ mines, or _buoyant_ mines, according to whether they rest on the sea bottom or float below the surface. Ground mines are generally made in the form of a cylinder, buoyant mines being usually spherical. The cases are made of steel, and buoyancy is given when required by enclosing air spaces. Open-sea mines are laid by special vessels, mostly old cruisers. The stern of these ships is partly cut away, and the mines are run along rails to the stern, and so overboard. The explosive employed is generally gun-cotton, fired by a detonator, charges up to 500 lb. or more being used, according to the depth of submersion and the horizontal distance at which the mine is desired to be effective. Ground mines can be used only in shallow water, and even then they require a heavier charge than mines floating near the surface. Mines must not be laid too close together, as the explosion of one might damage others. The distance apart at which they are placed depends upon the amount of charge, 500-lb. mines requiring to be about 300 feet apart for safety. CHAPTER XXXI WHAT IS ELECTRICITY? The question which heads this, our final chapter, is one which must occur to every one who takes even the most casual interest in matters scientific, and it would be very satisfactory if we could bring this volume to a conclusion by providing a full and complete answer. Unfortunately this is impossible. In years to come the tireless labours of scientific investigators may lead to a solution of the problem; but, as Professor Fleming puts it: “The question--What is electricity?--no more admits of a complete and final answer to-day than does the question--What is life?”