498 



Mr. W. Fairbairn on the Properties of Iron, [May 9, 



colleagues the laborious investigations in which we are now engaged, 

 and looking at the results of the recent experiment with the 300 -pounder 

 gun on the one hand, and the resisting targets on the other, there is 

 every prospect of an arduous and long-continued contest. 



From the Manchester experiments, to which I have alluded, we 

 find that with plates of different thicknesses, the resistance varies directly 

 as the thickness, that is, if the thickness be as the numbers 1, 2, 3, &c., 

 the resistance will be as 1, 2, 3, &c. ; but those obtained by impact at 

 Shoeburyness show, that up to a certain thickness of plate, the resist- 

 ance to projectiles increases nearly as the square of the thickness. That 

 is, if the thickness be as the numbers 1, 2, 3, 4, &c., the resistance will 

 be as the numbers 1, 4, 9, 16, &c. respectively. The measure there- 

 fore of the absolute destructive power of shot is its vis viva, not its 

 momentum as has been sometimes supposed, but the work accumulated 

 in it varies directly as the weight of the shot multiplied into the square 

 of the velocity. 



There is therefore a great difference between statical pressure and 

 dynamical effect ; and in order to ascertain the difference between flat- 

 ended and round-ended shot, a series of experiments were undertaken 

 with an instrument or punch exactly similar in size and diameter and 

 precisely corresponding with the steel shot of the wall piece '85 

 diameter employed in the experiments at Shoeburyness. The results 

 on tlie A, B, C, and D plates are as follows : — 



These figures show, that the statical resistance to punching is about 

 the same whether the punch be flat-ended or round-ended, the mean 

 being in the ratio of 1000 : 1085 or 8t per cent, greater in the round- 

 ended punch. It is, however, widely different, when we consider the 

 depth of indentation of the flat-ended punch and compare it with that 

 produced by the round-ended one, which is 3^ times greater. Hence, 

 we derive this remarkable deduction, that whilst the statical resistance 

 of plates to punching is nearly the same, whatever may be the form of 

 the punch, yet the dynamic resistance or work done in punching is 



