t from
ship or shore, makes no such mistake, and becomes harmless when detached
from the battery. The condition of the mine at any time can also be told
by sending a very minute current through it, though miles away and buried
deep beneath the sea.

When a current of electricity goes through a wire, it heats it; and if
the current be made strong enough, and a white hot wire thus comes in
contact with powder or fulminate of mercury in a torpedo, an explosion
will result. But it is important to know exactly when to explode the
torpedo, especially during the night or in a fog; and hence torpedoes are
often made automatic by what is called a circuit closer. This is a device
which automatically bridges over the distance between two points which
were separated, thus allowing the current to pass between them. In
submarine torpedoes it is usual to employ a small weight, which, when the
torpedo is struck, is thrown by the force of the blow across two contact
points, one of which points is in connection with the fuse and the other
in connection with the battery, so that the current immediately runs over
the bridge thus offered, and through the fuse. In practice, these two
contact points are connected by a wire, even when the torpedo is not in
the state of being struck; but the wire is of such great resistance that
the current is too weak to heat the wire in the fuse. Yet when the weight
above mentioned is thrown across the two contact points, the current runs
across the bridge, instead of through the resistance wire, and is then
strong enough to heat the wire in the fuse and explode the torpedo. The
advantage of having a wire of high resistance between the contact points,
instead of having no wire between them, is that the current which then
passes through the fuse, though too weak to fire it, shows by its very
existence to the men on shore that the circuit through the torpedo is all
right.

But instead of having the increased current caused by striking the
torpedo to fire the torpedo directly, a better way is to have it simply
make a signal on shore. Then, when friendly vessels are to pass, the
firing battery can be disconnected; and when the friendly ship bumps the
torpedo, the working of the signal shows not only that the circuit
through the fuse is all right, but also that the circuit closer is all
right, so that, had the friendly ship been a hostile ship, she would
certainly have been destroyed.

While the management of t