MECHANICS AND USEFUL ARTS. 57 



ship, or to the timbers in a wooden one. If iron plates of the corru- 

 gated form were backed with an inch plate of steel, hardened and tem- 

 pered, they would, I think, prove impenetrable ; and even smooth iron 

 plates of 4 inches thus laid upon steel would be more effective than iron 

 plates even of 7 inches thick, backed by timber. In explaining the 

 mode in which I conceive this kind of compound armor-plating to act, 

 it is necessary to consider to what the great force of flat-headed projec- 

 tiles is due. It will be admitted that this is not to be attributed to the 

 velocity of the shot, but to its flat form and great cohesive strength 5 

 because spherical and conical projectiles sent with higher velocity do 

 not pierce iron plates. The rationale of the punching action may not 

 yet be quite clearly understood, but this much seems to be established, 

 that a flat-headed shot pierces because its whole force is applied equally 

 in one direction, and free from the lateral resistance which the conical 

 and round shot meet with. I venture also to state that the suddenness 

 of the impact of such a body has much to do with the effect; it would 

 seem that time is an important element in the consideration, and there- 

 fore if this perfect impact can be delayed, and still more, if it can be 

 prevented altogether, the piercing of the plate will not be effected. As 

 an illustration of the difference between force applied with time and 

 force without it, may be pointed out the simple experiment of striking 

 an anvil with a small hammer, and placing the same force in weight 

 gradually, or with time. In the first case, the force is not conducted 

 away into the mass of the anvil, but is spent in repelling the hammer, 

 and possibly in breaking it into fragments. This is also exemplified in 

 the smashing of round shot against an iron target of proper thickness. 

 In the case of the hammer's being pressed upon the anvil, no effect is 

 produced upon either the one or the other. Another familiar illustra- 

 tion is in the effect of soft materials, such as gun wads or tallow candle, 

 being sent at high velocity through wooden planks, and, as frequently 

 occurred, killing persons struck by them. Velocity, then, may be de- 

 scribed as force with the least possible element of time ; the best exam- 

 ple, perhaps, of which is electric force in the form of lightning. The 

 analogue of this force is seen in the flash of flame which occurs when a 

 shot strikes at high velocity upon an iron plate. Now, the object of 

 the steel plates, being hardened and tempered, is to delay the shot by 

 its cohesive strength, and to prevent, by the corrugated surface, the 

 whole area of the flat-head projectile bearing upon the plate at the 

 same moment of impact. The shot is delayed partly by this means, 

 and partly by meeting with a metal as hard and tough as itself, and 

 thus time is allowed for the conduction of the force away into the sur- 

 rounding metal without fracture and penetration ; the objects which I 

 consider most important being, first, to prevent penetration of the outer 

 plate, and to oppose a shot which did pierce it, when it has expended 

 its force, by covering the skin of the ship with a thin steel plate, in 

 preference to increasing the thickness of the outer plating. The me- 

 chanical strength of this arrangement of plates consists in the double 

 arch form combined with a certain amount of " play " and elasticity, ob- 

 tained by fixing the corrugated plate upon a flat one. The flat action 

 of the projectile is also prevented by, as it were, converting it into a 

 round head or a hollow head before it reaches the inner plate. The 

 space between the corrugations being about four and a half inches, a 



