282 REPORT — 1861. 



different aspect when suddenly snapped asunder by vibration or a sharp 

 blow from a shot. In the former case the fibre is elongated by bending, 

 arid 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, &c., 

 become elongated, and resolve themselves into fibres. There is, therefore, 

 a wide difference in the appearance of the fracture of iron when broken by 

 tearing and bending, and when broken by impact, where time is not an ele- 

 ment in the force producing rupture. 



The mechanical properties of iron best calculated to resist the penetration 

 of shot at high velocities are enumerated as follows. 



The plates were subjected to statical tensile strain, to compression, and to 

 punching, with the following results. 



1. Specific Gravity. 



The mean specific gravity of the 1|, 2, 2|, and 3-inch plates of each series 

 were as follows : — 



A plates 7-8083 



B plates 7-7035 



C plates 7'904-2 



D plates 7-6322 



The order of merit is therefore C, A, B, D. These results coincide with 

 the following tests. 



2. Tensile Strength. 



The statical resistance to tensile strain was as follows : — 



Tensile strain per square inch in tons. 

 Thinner plates. Thicker plates. 



A plates 25 047 24.-644 



B plates 25 606 23-354 



C plates 31-770 27'032 



D plates 18-333 24-171 



The general order of merit in this case is C, A, B, D. The homogeneous 

 metal plates have the highest tenacity, but decrease in strength progressively 

 as the plates increase in thickness. 



3. Ductility of the Plates. 



A measure of the ductility of the plates ia afforded by the ultimate 

 elongation under tensile strain. 



Ultimate elongation per unit of length* 

 Thinner plates. Thicker plates. 



A plates -0690 -2723 



B plates -0566 '2459 



C plates -1880 -2725 



D plates -0197 '1913 



Here the order of merit is nearly the same as that for density and tenacity. 

 On the whole the elongations increase progressively with the thickness for 

 iron plates, and decrease for homogeneous metal plates. But with iron the 

 ductility is nearly the same for 2, 2g, and 3-inch plates. 



4. Resistance to Impact. 



Mr. Mallet has pointed out that the product of the tensile breaking weight 



