There are various methods of firing these mines, all of which work perforce by the concussion of the ship itself. In some cases the sudden tilting over causes an electric contact to be made, and permits a battery in the mine to cause the explosion. Another way is to furnish the mine with projecting horns of soft metal, inside which are glass vessels containing chemicals. The ship, striking a horn, bends it, breaks the glass, and liberates the chemicals which cause the explosion. In the type of mine largely used by the British Navy there is a projecting arm pivoted on the top of the mine and projecting from it horizontally. The mine itself rolls along the side of the passing ship, but the arm simply trails or scrapes along. Thus the mine turns in relation to the arm, and a trigger is thereby released, which fires the mine. In this, be it noted, the ship only pulls the trigger, so to speak, and releases a hammer which does the work, just as the trigger of a gun releases the hammer. The motive force which makes the hammer do its work when the trigger is "pulled" is the pull on the anchor rope. That arrangement has a virtue which is not apparent at first sight. Since it is the pull on the anchor rope which actually fires the mine, it follows that if such a mine break away from its moorings it instantly becomes harmless. Safety for the men who lay the mines is secured in several ways. One is by the use of a hydrostatic valve. The firing mechanism is locked until the pressure of water releases it, and that pressure does not exist until the mine is several feet under water. Another way is to seal up the firing mechanism with a soluble seal made of some substance such as sal-ammoniac. The mine cannot then explode until it has been under water long enough for the seal to be melted. It now remains to relate how these mines are swept up and removed, yet there is very little really to tell, for the process is so exceedingly simple. So far as is generally known, no method has been found that is superior to the primitive plan of dragging a rope along between two ships so as to catch the anchor ropes. The vessels employed are usually of very light draft, so that they stand a good chance of passing over the mines themselves, and the rope used is as long as possible, so that a mine, if exploded by being caught in the loop of the rope, explodes so far away as to do no harm. When dragged to the surface the mines are exploded from a distance by shots from a small gun, or even from a rifle. In the case of those mines which have horns, a blow from a bullet is enough to break the glass and cause explosion, and in all cases mines seem sooner or later to succumb to a sharp blow. Thus they are destroyed, by their own action, at a safe distance from the sweepers. Accidents happen, however, and mine-sweeping is no job for anyone but the bravest.