withstand both penetration and breakage; but these two conditions
unfortunately require opposite qualities. A metal of sufficient
ductility to withstand breakage is easily penetrated, and, conversely,
one that is hard and does not permit of penetration does not resist
shocks well. Up to the present, casehardened iron (Gruson) has appeared
to best satisfy the contradictory conditions of the problem. Upon the
tempered exterior of this, projectiles of chilled iron and cast steel
break upon striking, absorbing a part of their live force for their own
breakage.

In 1875 Commandant Mougin performed some experiments with a chilled iron
turret established after these plans. The thickness of the metal
normally to the blows was 23½ inches, and the projectiles were of cast
steel. The trial consisted in firing two solid 12 in. navy projectiles,
46 cylindrical 6 in. ones, weighing 100 lb., and 129 solid, pointed
ones, 12 in. in diameter. The 6 inch projectiles were fired from a
distance of 3,280 feet, with a remanent velocity of 1,300 feet. The
different phases of the experiment are shown in Figs. 4, 5, and 6. The
cupola was broken; but it is to be remarked that a movable and
well-covered one would not have been placed under so disadvantageous
circumstances as the one under consideration, upon which it was easy to
superpose the blows. An endeavor was next made to substitute a tougher
metal for casehardened iron, and steel was naturally thought of. But
hammered steel broke likewise, and a mixed or compound metal was still
less successful. It became necessary, therefore, to reject hard metals,
and to have recourse to malleable ones; and the one selected was rolled
iron. Armor plate composed of this latter has been submitted to several
tests, which appear to show that a thickness of 18 inches will serve as
a sufficient barrier to the shots of any gun that an enemy can
conveniently bring into the field.