THE USE OF IRON IN FORTIFICATION.

Roumania is thinking of protecting a portion of the artillery of the forts surrounding her capital by metallic cupolas.  But, before deciding upon the mode of constructing these formidable and costly affairs, and before ordering them, she has desired to ascertain their efficacy and the respective merits of the chilled iron armor which was recently in fashion and of rolled iron, which looks as if it were to be the fashion hereafter.

[Illustration:  Fig. 1.—­Mougin’s rolled iron turret.]

The Krupp works have recommended and constructed a cupola of casehardened iron, while the Saint Chamond works have offered a turret of rolled iron.  Both of these recommend themselves by various merits, and by remarkably ingenious arrangements, and it only remains to be seen how they will behave under the fire of the largest pieces of artillery.

[Illustration:  Fig. 2.]

We are far in advance of the time when cannons with smooth bore were obliged to approach to within a very short range of a scarp in order to open a breach, and we are far beyond that first rifled artillery which effected so great a revolution in tactics.

[Illustration:  Fig. 3.]

To-day we station the batteries that are to tear open a rampart at distances therefrom of from 1,000 to 2,000 yards, and the long, 6 inch cannon that arms them has for probable deviations, under a charge of 20 pounds of powder, and at a distance of 1,000 yards, 28 feet in range, 16 inches in direct fire and 8 inches in curved.

The weight of the projectile is 88 pounds, and its remanent velocity at the moment of impact is 1,295 feet.  Under this enormous live force, the masonry gradually crumbles, and carries along the earth of the parapet, and opens a breach for the assaulting columns.

[Illustration:  Fig. 4—­State of A cupola after the action of thirty-seven 6 in.  Projectiles.]

In order to protect the masonry of the scarp, engineers first lowered the cordon to the level of the covert-way.  Under these circumstances, the enemy, although he could no longer see it, reached it by a curved or “plunging” shot.  When, in fact, for a given distance we load a gun with the heaviest charge that it will stand, the trajectory, AMB (Fig. 2), is as depressed as possible, and the angles, a and a’, at the start and arrival are small, and we have a direct shot.  If we raise the chase of the piece, the projectile will describe a curve in space which would be a perfect parabola were it not for the resistance of the air, and the summit of such curve will rise in proportion as the angle so increases.  So long as the falling angle, a, remains less than 45 deg., we shall have a curved shot.  When the angle exceeds this, the shot is called “vertical.”  If we preserve the same charge, the parabolic curve in rising will meet the horizontal plane at a greater distance off.  This is, as well known, the process employed for reaching more and more distant objects.