PENETRATION OP IRON PLATES BY STEEL SHOT. 415 



breech-loading gun ; -with, what " work " shouUl the projectile strike in order 

 to accomplish this ? 



Here we have by equation (1) 



% 

 and substituting the values, we find that the necessary " work " or 



-jr— =905 foot-tons. 

 % 



Experiment gives 906 foot-tons as the '• Avork" necessary. 



It is unnecessary to give further examples of penetration through unbacked 

 plates : the subject of backed plates will bo further considered when the 

 results of experiments against various targets representing existing iron-clad 

 slips have been reviewed. 



On Oblique Fire.- — We have hitherto considered the fire as being direct ; 

 that is to say, the plate has been supposed to have been placed perpendicular 

 to the ground and the gun to have been so directed that the plane in which ' 

 the shot * moved was perpendicular to the face of the plate, or nearly so. 



Let lis suppose, however, that the plate has been set at an angle, or that the 

 gun fires obhquely at an upright plate. The shot has then a tendency to 

 glance oif and continue its motion in a new direction, and we shall have the 

 following proportion, viz. : — 



The force with which the shot, acting obhquely, will strike is to that with 

 which it would strike if acting directly as the sine of the angle of incidence is 

 to unity f. 



Equation (1) will therefore become 



and (2), 



/ W 



J = .sin0^^^^^ (5) 



It appears from this that the resistance of the plate increases as the value 

 of d diminishes. 



We have already shown that a 4-5-ineh unbacked plate, when fired at 

 direct, requires a force represented by 28 foot-tons per inch of shot's circum- 

 ference to ensure penetration. 



Let us siippose, however, that we place the plate in such a position that it 

 makes an angle of 38° with the ground. From equation (4) we find that the 

 force required to penetrate it in this position amounts to 1445 foot-tons for a 

 shot of 6-22 inches diameter, or 73-9 foot-tons per inch of the shot's circum- 

 ference. We may expect, therefore, that a less force wiU not penetrate a 

 4-5-inch unbacked plate placed at an angle of 38°. 



An experiment of this nature was actually tried by the Armstrong and 

 "^Tiitworth Committee ; they caused 4'5-inch plates to be set up at an angle 

 of 52° Avith the vertical, and fired at them from 200 yards distance with the 

 competitive Armstrong and Whitworth guns. 



It appears that the projectiles were solid steel shot of 70 lbs. weight and 

 6-34 inches diameter, that they struck with a "work" of 1049 foot-tons or 



* The shot is assumed to Imve no lateral deviation. 



t That is, the shot striking in a slanting direction may be supposed to have opposed 

 to it a plate of a thickness equal to the diagonal formed by the line of direction. 



