Mines of this kind can, of course, be fired electrically by the tilting of a cup of mercury or similar device as already described. Another arrangement is to fit projecting horns upon the surface of the mine made of soft metal so that they will be bent or crushed by a strong blow such as a passing ship would give. This breaks a glass vessel inside, liberating chemicals which cause detonation. The method adopted in the Elia mines is to have a projecting arm pivoted upon the top of the mine. The mine is spherical (they are nearly all either spherical or cylindrical), with the rope attached to the South Pole, so to speak, and the arm pivoted to the North Pole. As the mine floats in the water the arm projects out horizontally. The effect of this arrangement is that when a ship strikes the mine the latter rolls along its side, but the arm being too long, simply trails along. Thus the spherical case of the mine turns while the arm remains still and that is made to unscrew and eventually release a hammer which, striking the detonator, fires the mine. In other words, this type of mine is exploded not by the ship giving it a blow, but by its rubbing itself along in contact with the mine. The great advantage of this is that it is only a ship that can do this. No chance commotion in the water can do it: no chance blow from floating wreckage can do it: only the rubbing action of a ship can accomplish it. Such a mine, too, is less likely to be affected by counter-mining, of which more presently. Apparently the laying of these mines must be very dangerous work, for since a blow will explode most of them, what is to prevent their receiving that blow while on the deck of the mine-layer, or at all events as they are dropped into the water. In all cases, precautions are taken against such an event. Sometimes a hydrostatic valve is employed, the arrangement being that the firing mechanism is locked until released by the valve, until, that is, the mine is immersed to a predetermined depth in the water. Another device for the same purpose is a lump of sugar. The mine is so made that it cannot be fired until this lump has been melted by the action of the water: sal ammoniac is another substance employed for the same purpose. The technical term for this is a "soluble seal." The firing arrangement, whatever it may be, is sealed up so that it cannot come into operation until the seal has been dissolved away by the water, or until the mine has been in the water long enough for the mine-layer to get out of harm's way. Another interesting feature of the Elia mine is connected with the source of the power which drives the hammer which causes the explosion. The anchor, it will be remembered, pulls the mine down under water, the latter being of itself buoyant. There is a continual pull, therefore, upon the rope by which the mine is held under. It is that pull which works the hammer.