The production of steel by this method furnishes employment for eight or nine hundred men daily in the Krupp works. The Bessemer process for converting iron into steel is also largely used there for making steel for certain purposes. All material used in the different classes of manufactures is subjected at every stage to extreme and exact tests; the standards being fixed with reference to the purpose to which the metal is to be applied, and any material that proves faulty when suitably tested is rigorously rejected. The guns originally manufactured by the Krupp firm were formed from solid ingots of steel, which were bored, turned, and fashioned as in the case of cast-iron smooth-bore cannon. With the development of the power of artillery, the greater strain caused by the increased powder-charges and by the adoption of rifling--involving enhanced friction between the projectile and the bore--had the result of demonstrating the weakness inherent in the construction of a gun thus made entirely from one solid forging, and that plan was eventually discarded. Artillerists have learnt that the strain produced by an explosive force operating in the interior of a cannon is not felt equally throughout the thickness of the metal from the bore to the exterior, but varies inversely as the square of the distance of each portion of the metal from the seat of effort. For example, in a gun cast solid, if two points be taken, one at the distance of one inch from the bore, and the other four inches from the bore, the metal at the former point will during the explosion be strained sixteen times as much as that at the distance of four inches. The greater the thickness of the material, the greater will be the inequality between the strains acting at the points respectively nearest to and farthest from the interior. The metal nearest the seat of explosion may thus be strained beyond its tensile strength, while that more remote is in imperfect accord with it. In such a case, disruption of the metal at the inner surface ensues, and extends successively through the whole thickness to the exterior, thus entailing the destruction of the gun.