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 Gen. Abbot.

Recent experiments in England have shown that the Whitehead torpedo, over which control ceases after it is fired, is not so formidable a weapon when fired at a ship under way as many supposed, for the simple reason that it can be dodged.  But an electrical torpedo, over which control is exercised while it is in motion through the water, cannot be dodged, provided it receives sufficient speed.  For effective work against ships capable of steaming fifteen knots per hour, the torpedo should have a speed of twenty knots.  There is no theoretical difficulty in the way of producing this, for a speed of eleven knots has already been recorded, though an electric torpedo, to get this speed, would have to be larger than a Whitehead having the same speed.  It may be conceived that a torpedo carrying 50 lb. of gun-cotton, capable of going 20 knots per hour, so that it would pass over a distance of 500 yards in about 45 sec., and yet be absolutely under control all the time, so that it can be constantly kept pointed at its target, would be a very unpleasant thing for an enemy to meet.

Military telegraphy is a second use of electricity in warfare.  Lieut.  Fiske traces its origin to our own civil war.  Foreign nations took the hint from us, and during the invasion of France the telegraph played a most important part.  In military telegraph trains, miles of wire are carried on reels in specially constructed wagons, which hold also batteries and instruments.  Some of the wire is insulated, so that it can rest on the ground, and thus be laid out with great speed, while other wire is bare, and is intended to be put on poles, trees, etc.  For mountain service the wires and implements are carried by pack animals.  Regularly trained men are employed, and are drilled in quickly running lines, setting up temporary stations, etc.  In the recent English operations in Egypt, the advance guard always kept in telegraphic communication with headquarters and with England, and after the battle of Tel-el-Kebir news of the victory was telegraphed to the Queen and her answer received in forty-five minutes.