1912] on the Pressure of a Blow. 289 



wrought-iron or ordinary structural steel. Even sucli a special steel, 

 however, is handicapped as against the shell by the hard point of the 

 latter, which is able to force the softer material aside though itself 

 undamaged. This disability has been overcome by hardening the face 

 of the plate so that it now possesses the structure indicated in the 

 drawing, the back being tough and ductile, but the face as hard as it 

 is possible to make it. When such a plate is struck by the shell it is a 

 case of Greek meeting Greek, and this is the result (Fig. 9, rounds 

 583 and 588). Both the shell and the hardened face of the plate 

 are shattered by the pressure, sufficient of which is transmitted through 

 the substance of the plate to crack it right through, though, of course, 

 none of the shell has penetrated it. 



It w^ould seem that when it acquired the hard face the armour 

 plate more than over-took the shell in the race. Though the shell 

 might by sheer energy pierce a somewhat thinner plate, I am told 

 that it was apt to be smashed to pieces in the process. The balance 

 has of recent years been more than restored by the addition to the 

 shell of the soft steel cap. I have already shown you the effect of 

 firing an uncapped shell ; I will now draw your attention to that of 

 firing the same shell with cap at the same plate (Fig. 0, round 587). 

 The shell goes through minus its cap, but otherwise so completely 

 uninjured, iihat I am told it might in many cases be used again. It 

 punches a clean hole in the plate. The fate of the cap is interesting. 

 The shell punches a hole in it, as of course it must do before it reaches 

 the plate, and the cap forms a ring, which is held up by the plate and 

 through which the shell passes. The fragments of the cap are found 

 on the front side of the plate, and in some instances they have been 

 collected and put together, forming a ring. I have one such ring 

 here. Its largest diameter is that of the shell, its smallest about an 

 inch less, and it looks as" though the ring had got intact as far as the 

 shoulder of the projectile, but had then burst into several pieces. 



The usual explanation of this remarkable effect of a soft steel 

 cap is that it supports the point of the projectile. As I pointed 

 out in connection with billiard-balls, the destructive effect of pres- 

 sure depends on the difference of pressures in different directions, 

 and not on their absolute amounts, and it is obvious that by the 

 exercise of a sufficient lateral pressure the point might be com- 

 pletely protected. The difficulty is to see how the comparatively 

 weak material of which the cap is made can exert the very large 

 pressures which are necessary for effective support. It seems hardly 

 possible that such pressures could be generated by the mere act of 

 stretching or expanding the cap over the end of the shell. If this 

 be so, the inertia of the metal in the cap must play an important 

 part. At the critical moment when the hard point of the shell meets 

 the plate, there is a sudden distortion of the shell and plate near 

 the point of contact. This distortion is the cause of breakage. One 



