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 the torpedo is thus simple, the defense of a
harbor becomes a complex problem, on account of the time and expense
required to perfect it, and the training of a corps of men to operate the
torpedoes.

In order to detect the presence of torpedoes in an enemy's harbor, an
instrument has been invented by Capt. McEvoy, called the "torpedo
detecter," in which the action is somewhat similar to that of the
induction balance, the iron of a torpedo case having the effect of
increasing the number of lines of force embraced by one of two opposing
coils, so that the current induced in it overpowers that induced in the
other, and a distinct sound is heard in a telephone receiver in circuit
with them. As yet, this instrument has met with little practical success,
but, its principle being correct, we can say with considerable confidence
that the reason of its non-success probably is that the coils and current
used are both too small.

Lieut. Fiske described the spar torpedo and the various classes of
movable torpedoes, including the Lay. His conclusion is that the most
successful of the movable torpedoes is the Simms, with which very
promising experiments have been conducted under the superintendence of