[Illustration:  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 123/4 feet in diameter, and rises 31/4 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.

[Illustration:  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.