“They should be spherical; only instead of a hole for the fuse, as in a hollow shot, they should have a neck like a bottle, with a cap to screw over—not in—the neck. The case should be charged through the neck, and the wires let in through two holes, counter-sunk, diametrically opposite, the counter-sinking being for the purpose of receiving pitch or other resinous matter to keep the water out. The fuse, being adjusted to the wires, should be held in its place by a string through the neck, while the wires are drawn out taut and sealed within and without. Having proved the fuse, first fill and then drive in a wooden peg. Then fill the space between it and the screw-cap with red lead, and screw down tight so as to make it water-tight. Now secure the tails of the wires so that they will not be chafed or bruised, and the mine is ready to be packed for transportation. They are generally to be used in stone fougasses, the wire being buried at convenient depths, and all marks of fougasses and trenches removed as completely as possible. Any number, not exceeding twenty-five or thirty, may be arranged in a single circuit for the ebonite; but if the magnetic exploder of Wheatstone be preferred, and the ground be perfectly dry, hundreds may be planted in a ladder-circuit, which you have seen handled. “The operator may be at any distance from these mines when he explodes them, provided only he has established some mark or point which, on being reached by the enemy, should serve as his signal. The area of destruction of one fougasse, properly constructed, with a charge of twenty or twenty-five pounds of powder, may be assumed to be that of a circle seventy-five or eighty yards in diameter. Twenty mines would therefore serve for a mile. Several miles may be planted in a night, and the assailants may be enticed or invited out in the morning. Passes before an invading army may be mined in advance, and thus, if he cannot be destroyed, his progress may be so retarded by dummies or sham mines as almost literally to compel him to dig his way. “The power to telegraph through these torpedoes is of little consequence, inasmuch as there need be but one station and one operator. Using the testing-fuse manufactured by Abel, and a weak voltaic current, the operator can at any time satisfy himself as to continuity. Thus bridges and gulfs or breaks are not required for the land as they are in sea-mining. Ebonite has the further advantage on land that it takes but a single wire. “Forts may be protected against assault, and your own rifle-pits from occupation by an enemy, simply by a proper distribution of those new engines of war. They may be planted line within line, and one row above another, and so arranged that volcanoes may be sprung at will under the feet of assaulting columns.