194 I'ir. Mallet on the Physical Conditions 



from which for a length of time the most reliable guns (of cast-iron) in the 

 British service have been produced — viz., the Carron, the Low Moor, and the 

 Gospel Oak Works, are those whose "makes" of iron present precisely the 

 character here insisted upon — namely, an extremely high coefficient of exten- 

 sion ( Tc) as compared with their absolute ultimate resistance to rupture. 



Having in the foregoing pages pointed out some of the circumstances, almost 

 all of a character purely physical, i. e., molecular, which afiect both the construc- 

 tion, and the destruction of ordnance, I propose to discuss briefly the relative 

 advantages and disadvantages of the same class, which belong to each of the 

 four principal materials that have been in use, or are now proposed for their 

 fabrication. These are, gun-metal, cast-iron, wrought-iron, and steel. 



18. — The General Relations of Elasticity to the Construction of Guns. 



114. The elasticity of solids is of two sorts — cubic elasticity^ which is the 

 resistance that the body presents to change oi volume by the application of pres- 

 sure ; and linear elasticity, which is that which opposes change oiform. These are 

 very different in different bodies, and different from each other in the same body. 



Glass or steel, for example, powerfully resists either change of volume or of 

 form ; caoutchouc admits of a large alteration of either. Cork readily changes 

 its form ; its lineal elasticity is great, while its cubic elasticity is small : while 

 cold carpenter's glue or whalebone probably present to it, relations exactly the 

 reverse. 



When change of form takes place in a solid, whether by extension or com- 

 pression, it is always accompanied by some change of volume ; otherwise the 

 heat evolved on rapid and considerable change of form (as in tearing asunder 

 a bar of iron) is hard to be accounted for. The specific heat diminishing as 

 the density increases, by decrease of volume, at or near the point of rupture ; 

 or possibly the heat evolved being the mechanical equivalent of the force 

 employed in producing the changes in volume. It cannot be that, of the force 

 employed in producing change of form only — because mere fracture, as when a 

 hole is struck out from a thick plate of iron by a cannon shot (under whose rapid 

 stroke the toughest iron breaks as a brittle body), produces a remarkable rise 

 of temperature in the adjacent parts of the plate. 



