wo fine steel wires which, in unwinding, drive the twin screw
propellers. This is the Brennan torpedo. The Sims-Edison torpedo is
both propelled and steered by electricity from the shore, transmitted
to a motor and steering relay in the torpedo by an insulated cable.
This cable has two cores and is paid out by the torpedo as it travels
through the water just as a spider pays out its web. The cable is
about half an inch in diameter and two miles long, and the torpedo can
be driven at about eighteen miles per hour with a current of thirty
amperes and 1,800 volts pressure.

Still another auxiliary weapon of defense is the dynamite gun, or
rather, a pneumatic gun, that throws long projectiles carrying from
250 to 450 pounds of dynamite, to a distance of about two miles. The
shells are arranged to explode soon after striking the water, by an
ingenious battery that ignites the fuse as soon as the salt water
enters it. The gun, which is known as the Zalinski gun, is some sixty
feet long and fifteen inches in caliber, the compressed air being
suddenly admitted to it from the reservoirs at any desired pressure by
a special form of valve that regulates the range. These guns are to be
mounted in deep pits and fired at somewhat higher elevations than
ordinary guns, but it has great accuracy within reasonable limits of
range.

FIELD FORTIFICATIONS.

In field fortification an enormous quantity of work was done during
our last war. Washington, Richmond, Nashville, Petersburg, Norfolk,
New Berne, Plymouth, Vicksburg, and many other cities were elaborately
fortified by field works which involved the handling of vast
quantities of earth, and, where the opposing lines were near together,
ditches, abbatis, ground torpedoes, and wire entanglements were freely
used. In some cases the same ground was fortified in succession by
both armies, so that the total amount of work expended, in this way,
would have built several hundred miles of railway. Around Richmond and
Petersburg alone the development of field works was far greater than
Wellington's celebrated lines at Torres Vedras. In all future wars,
when large armies are opposed to each other, it is probable that field
works will play even a more important part than in the past. The great
advantage of such works, since the introduction of the deadly breech
loading rifles and machine guns, was shown at Plevna, where the
Russians were almost annihilated in attempting to capture the Turkish