niche is of concrete, and its walls in the vicinity of the embrasure are protected by thick iron plate. The rectangular armor plate of rolled iron rests against an elastic cushion of sand compactly rammed into an iron plate caisson. The conical embrasure traverses this cushion by means of a cast-steel piece firmly bolted to the caisson, and applied to the armor through the intermedium of a leaden ring. Externally, the cheeks of the embrasure and the merlons consist of blocks of concrete held in caissons of strong iron plate. The surrounding earthwork is of sand. For closing the embrasure, Commandant Mougin provides the armor with a disk, c, of heavy rolled iron, which contains two symmetrical apertures. This disk is movable around a horizontal axis, and its lower part and its trunnions are protected by the sloping mass of concrete that covers the head of the casemate. A windlass and chain give the disk the motion that brings one of its apertures opposite the embrasure or that closes the latter. When this portion of the disk has suffered too much from the enemy's fire, a simple maneuver gives it a half revolution, and the second aperture is then made use of.

The Schumann-Gruson Chilled Iron Cupola.--This cupola (Fig. 9) is dome-shaped, and thus offers but little surface to direct fire; but it can be struck by a vertical shot, and it may be inquired whether its top can withstand the shock of projectiles from a 10 inch rifled mortar. It is designed for two 6 inch guns placed parallel. Its internal diameter is 19½ feet, and the dome is 8 inches in thickness and has a radius of 16½ feet. It rests upon a pivot, p, around which it revolves through the intermedium of rollers placed in a circle, r. The dome is of relatively small bulk--a bad feature as regards resistance to shock. To obviate this difficulty, the inventor partitions it internally in such a way as to leave only sufficient space to maneuver the guns. The partitions consist of iron plate boxes filled with concrete. The form of the dome has one inconvenience, viz., the embrasure in it is necessarily very oblique, and offers quite an elongated ellipse to blows, and the edges of the bevel upon a portion of the circumference are not strong enough. In order to close the embrasure as tightly as possible, the gun is surrounded with a ring provided with trunnions that enter the sides of the embrasure. The motion of the piece necessary to aim it vertically is effected around this axis of rotation. The weight of the gun is balanced by a system of counterpoises and the chains, l, and the breech terminates in a hollow screw, f, and a nut, g, held between two directing sectors, h. The cupola is revolved by simply acting upon the rollers.

FIG. 9.--THE SCHUMANN-GRUSON CUPOLA.

Mougin's Rolled Iron Cupola.--The general form of this cupola (Fig. 1) is that of a cylindrical turret. It is 12¾ feet in diameter, and rises 3¼ feet above the top of the glacis. It has an advantage over the one just described in possessing more internal space, without having so large a diameter; and, as the embrasures are at right angles with the sides, the plates are less weakened. The turret consists of three plates assembled by slit and tongue joints, and rests upon a ring of strong iron plate strengthened by angle irons. Vertical partitions under the cheeks of the gun carriages serve as cross braces, and are connected with each other upon the table of the hydraulic pivot around which the entire affair revolves. This pivot terminates in a plunger that enters a strong steel press-cylinder embedded in the masonry of the lower concrete vault.

The iron plate ring carries wheels and rollers, through the intermedium of which the turret is revolved. The circular iron track over which these move is independent of the outer armor.

The whole is maneuvered through the action of one man upon the piston of a very small hydraulic press. The guns are mounted upon hydraulic carriages. The brake that limits the recoil consists of two bronze pump chambers, a and b (Fig. 10). The former of these is 4 inches in diameter, and its piston is connected with the gun, while the other is 8 inches in diameter, and its piston is connected with two rows of 26 couples of Belleville springs, d. The two cylinders communicate through a check valve.

When the gun is in battery, the liquid fills the chamber of the 4 inch pump, while the piston of the 8 inch one is at the end of its stroke. A recoil has the effect of driving in the 4 inch piston and forcing the liquid into the other chamber, whose piston compresses the springs. At the end of the recoil, the gunner has only to act upon the valve by means of a hand-wheel in order to bring the gun into battery as slowly as he desires, through the action of the springs.

FIG. 10.--MOUGIN'S HYDRAULIC GUN
CARRIAGE.

For high aiming, the gun and the movable part of its carriage are capable of revolving around a strong pin, c, so placed that the axis of the piece always passes very near the center of the embrasure, thus permitting of giving the latter minimum dimensions. The chamber of the 8 inch pump is provided with projections that slide between circular guides, and carries the strap of a small hydraulic piston, p, that suffices to move the entire affair in a vertical plane, the gun and movable carriage being balanced by a counterpoise, q.

The projectiles are hoisted to the breech of the gun by a crane.

Between the outer armor and turret sufficient space is left for a man to enter, in order to make repairs when necessary.

Each of the rolled iron plates of which the turret consists weighs 19 tons. The cupolas that we have examined in this article have been constructed on the hypothesis than an enemy will not be able to bring into the field guns of much greater caliber than 6 inches.--Le Genie Civil.

HIGH SPEED ON THE OCEAN.

To the Editor of the Scientific American:

Although not a naval engineer, I wish to reply to some arguments advanced by Capt. Giles, and published in the SCIENTIFIC AMERICAN of Jan. 2, 1886, in regard to high speed on the ocean.

Capt. Giles argues that because quadrupeds and birds do not in propelling themselves exert their force in a direct line with the plane of their motion, but at an angle to it, the same principle would, if applied to a steamship, increase its speed. But let us look at the subject from another standpoint. The quadruped has to support the weight of his body, and propel himself forward, with the same force. If the force be applied perpendicularly, the body is elevated, but not moved forward. If the force is applied horizontally, the body moves forward, but soon falls to the ground, because it is not supported. But when the force is applied at the proper angle, the body is moved forward and at the same time supported. Directly contrary to Capt. Giles' theory, the greater the speed of the quadruped, the nearer in a direct line with his motion does he apply the propulsive force, and vice versa. This may easily be seen by any one watching the motions of the horse, hound, deer, rabbit, etc., when in rapid motion. The water birds and animals, whose weight is supported by the water, do not exert the propulsive force in a downward direction, but in a direct line with the plane of their motion. The man who swims does not increase his motion by kicking out at an angle, but by drawing the feet together with the legs straight, thus using the water between them as a double inclined plane, on which his feet and legs slide and thus increase his motion. The weight of the steamship is already supported by the water, and all that is required of