tested. The late
remarkable experiments in England--firing 130-and 150-pound Whitworth
steel shells, holding 3 to 5 pounds of powder, from a 7-inch Armstrong
gun, with 23 to 27 pounds of powder, through the Warrior target, and
bursting them in and beyond the backing--certainly show that large
calibres are not indispensable in fighting iron-clads. A destructive
blow requires a _heavy charge of powder_; which brings us to

_The Strain and Structure of Guns, and Cartridges_. The problem is, 1st,
to construct a gun which will stand the heaviest charge; 2d, to reduce
the strain on the gun without reducing the velocity of the shot. It
is probable that powder-gas, from the excessive suddenness of its
generation, exerts a percussive as well as a statical pressure, thus
requiring great elasticity and a certain degree of hardness in the
gun-metal, as well as high tensile strength. Cast-iron and bronze are
obviously inadequate. Solid wrought-iron forgings are not all that could
be desired in respect of elasticity and hardness, but their chief defect
is want of homogeneity, due to the crude process of puddling, and to
their numerous and indispensable welds. Low cast-steel, besides being
elastic, hard, tenacious, and homogeneous, has the crowning advantage of
being produced in large masses without flaw or weld. Krupp, of Prussia,
casts ingots of above 20 tons' weight, and has forged a cast-steel
cannon of 9 inches bore. One of these ingots, in the Great Exhibition,
measured 44 inches in diameter, and was uniform and fine-grained
throughout. His great success is chiefly due to the use of manganesian
iron, (which, however, is inferior to the Franklinite of New Jersey,
because it contains no zinc,) and to skill in heating the metal, and to
the use of heavy hammers. His heaviest hammer weighs 40 tons, falls 12
feet, and strikes a blow which does not draw the surface like a light
hammer, but compresses the whole mass to the core. Krupp is now
introducing the Bessemer process for producing ingots of any size at
about the cost of wrought-iron. These and other makes of low-steel
have endured extraordinary tests in the form of small guns and other
structures subject to concussion and strain; and both the theory and all
the evidence that we have promise its superiority for gun-metal. But
another element of resistance is required in guns with thick walls. The
explosion of the powder is so instantaneous that the exterior parts of
the metal do