MECHANICS AXD USEFUL ARTS. 



75 



exhibits a fibrous fracture when broken by bending, presents a widely- 

 difierent aspect when suddenly snapped asunder by vibration, or by 

 a sharp blow from a shot. In the former case the fibre is elongated 

 by bending, and becomes developed in the shape of threads as fine 

 as silk, whilst in the latter the fibres are broken short, and exhibit a 

 decidedly crystalline fracture. But, in fact, every description of 

 iron is crystalline in the first instance ; and these crystals, by every 

 succeeding process of hammering, rolling, etc., become elongated, and 

 resolve themselves into fibres. There is, therefore, a wide differ- 

 ence in the appearance of the fracture of iron when broken by tear- 

 ing and bending, and when broken by impact, where time is not an 

 element in the force producing rupture. 



From a series of experiments made at Manchester it was found 

 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, 

 etc., the resistance will be as 1, 2, 3, etc.; but those obtained by 

 impact at Shoeburyness show that up to a certain thickness of plate 

 the resistance of projectiles increases nearly as the square of the thick- 

 ness. That is, if the thickness be as the numbers 1, 2, 3, 4, etc., the 

 resistance will be as the numbers 1, 4, 9, 16, etc., respectively. The 

 measure, therefore, 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 multi- 

 plied 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 under- 

 taken with an instrument or punch exactly similar in size and diame- 

 ter and precisely corresponding with the steel shot employed in the 

 experiments at Shoeburyness. The results on the A, B, C, and D 

 plates are shown in the following table : 



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 : 1,085, or 8 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 



