Nitro-glycerine............................ 1.6
Blasting gelatine.......................... 1.45
Forcite      "   .......................... 1.51
Dynamite No. 1............................. 1.6
Wet gun-cotton............................. 1.32
Dry     "     ............................. 1.06
Gunpowder.................................. 0.9

Their relative efficiency under water per cubic foot, according to
Bucknill, is as follows: 

Blasting gelatine.......................... 1.38
Forcite     "    .......................... 1.27
Dynamite No. 1............................. 1.00
Dry gun-cotton............................. 0.66
Wet      "    ............................. 0.66
Gunpowder.................................. 0.14

The wet gun-cotton has twenty-five per cent. of added water.

Mines for harbor defense are of two kinds—­buoyant and ground.  The buoyant are usually spherical, and contain from 400 to 500 pounds of explosive.  They bring the charge near to the ship’s bottom, but are difficult to manage in a tideway, and can be easily found by dragging.  The ground mines can be made of any size and are not easily found by dragging, but are of little value in very deep water.  They are either cylindrical or hemispherical in shape, and contain from 500 to 1,500 pounds of explosive in from thirty to eighty feet of water.  Mines of any kind are exceedingly difficult to render efficient when the water is over 100 feet deep.  On account of the tendency of all high explosives to detonate by influence or sympathy, and the liability of the cases to collapse by great exterior pressure, harbor mines are separated a certain distance, according as they are buoyant or ground, and according to the nature of the explosive.

Five hundred pounds buoyant gun-cotton mines require 320 feet spacing.

Five hundred pounds buoyant blasting gelatine mines require 450 feet spacing.

Six hundred pounds ground gun-cotton mines require 180 feet spacing.

Six hundred pounds ground blasting gelatine mines require 230 feet spacing.

Of torpedoes, other than those described, we have several modern varieties; submarine projectiles, submarine rockets, automobile and controllable locomotive torpedoes.  The first two varieties, though feasible, are not developed and have not yet advanced beyond the experimental stage.  Of the automobile, we have the Whitehead, Swartzkopf and Howell.  The first two are propelled by means of compressed air and an engine; the last by the stored-up energy of a heavy fly-wheel.  Generally speaking, they are cigar-shaped crafts, from 10 to 18 feet long and 15 to 17 inches in diameter, capable of carrying from 75 to 250 pounds of explosive at a rate of 25 to 30 knots for 400 yards, at any depth at which they may be set.  Of the controllable locomotive torpedoes, the three representative types are the Patrick, Sims and Brennan.  They are in general terms cigar boats, about 40 feet long and