The electric light will be found very useful for signal purposes by fitting a plane mirror in front of the catadioptric lens; so arranged that it be turned to any desired angle to the axis of the beam of light. By altering the angle of the mirror, the reflected beam of light can be swept from the horizon on one side, through the zenith, to the horizon on the other side. The time of passing the zenith being equivalent to the long and short flashes of the usual night signal code. In addition to using the electric light to detect the approach of torpedo boats, it may be used by the boats themselves to prevent the attacked vessel from discerning them. In turret ships, electric lights may be so arranged that the instant an object is brought into the field of the beam of light, the turret guns will be bearing on it. One great disadvantage of electric lights is the impossibility of protecting them from the enemy's fire, and this is a defect that cannot be eradicated, though it may be lessened, by manipulating them from the tops of a ship. _Torpedo Guns._--Hitherto by torpedo guns has been meant small guns mounted on carriages so constructed that a shot may be fired into the water only a few feet from the ship's side, or mitrailleuses, Gatlings, &c. Here the term is applied only to machine guns, which are constructed to fire either volleys, or, extremely rapidly, single shot, each shot of which would be capable of _penetrating_ and _sinking_ torpedo boats, such as Messrs. Yarrow and Thornycroft are daily launching from their yards. Of such weapons there are at present only two, viz., the "Nordenfelt" and "Hotchkiss" gun. The former has, after very exhaustive experiments, been adopted by the English, Austrian, Swedish, and other naval authorities, while the latter has been adopted by the French government. _Nordenfelt Torpedo Gun._--This gun, as it at present is constructed, consists of four barrels of 1 inch calibre. The barrels are fixed in a horizontal plane, and are not moved during the firing; and the movement of the lever, the loading, the firing, and the extracting are all performed in the same plane, so that the _elevation_ of the gun is not disturbed by the firing. The gun is fed by means of hoppers, each of which contains ten rounds per barrel, _i. e._, forty shots. The continuous supply of cartridges, as well as the firing and extracting, are all performed by one motion of the lever, thus enabling the gunner to use his left hand to lay the gun. A volley of four shots can be fired at the same moment, or one shot can be fired separately. Eight shots can be fired in 1-1/4 seconds; twenty, thirty, or forty shots can be fired at a rapidity of two hundred shots per minute without difficulty. The recoil being taken up by the whole framework of the gun does not in the least disturb the aim. The entire mechanism of the gun can be opened up without undoing a single screw, in less than 20 seconds.