48 ON THE CONSTRUCTION OF HOOPED CANNON. 



by me, twenty years ago, in the trial, to extremity, of two 32-poiuuler wrought 

 iron guns. In both of these, the fracture began upon the outside and worked 

 slowly inwards. 



The preceding statement cannot fail, I think, to convince any one that Mr. 

 Barlow's theory is wholly inapplicable to guns made of wrought-iron, or any like 

 malleable material, and, indeed, is to be applied, in its complete and unlimited 

 extent, only to such materials as highly hardened steel, glass, and those crys- 

 talline or wholly unyielding bodies, in which the ultimate particles or molecules are 

 incapable of being made to change place permanently in relation to each other, 

 but in which the limit of elasticity ends in complete separation or fracture. When 

 applied to hollow cylinders made of substances of this latter kind, it is probably 

 true to the letter. But what is cast-iron ? And are we to be guided by Barlow's 

 theory in computing the strength of cannon made of this material ? Believing, 



as I do, that most kinds of cast-iron are, to some though a very limited 



extent, malleable, or at least that they admit of some small permanent change 

 of form without fracture, we ought not, in my judgment, to apply Barlow's 

 theory, without some modification to express the strength of guns made of such 

 material, as they really possess greater strength than the formula given by that 



theory assigns them; though for many of the harder and completely crystalline 

 kinds of iron we must consider it applicable, as a safe, if not an entirely accu- 

 rate, guide for practical purposes. 



Following this property of malleability, from the cast-iron, or body, of the gun 

 to the wrought-iron or steel hoops with which the body is encircled and com- 

 pressed, let us next see what method of constructing the hoops should be adopted 

 to obtain the greatest strength to the gun. 



It is a fact well known to all smiths, or actual workers in the metals, and to 

 many engineers, whose knowledge is often derived principally from books, that all 

 the metals, by being subjected for a considerable time to hammering, rolling, or 

 wire-drawing, acquire a great increase of elasticity and hardness. Indeed, if any 

 of these processes be carried beyond a certain extent, the metal loses its mallea- 

 bility and ruptures or cracks under the continued operation. The hardness and 

 elasticity thus induced are, however, easily destroyed, and the original malleability 

 is restored by simply subjecting the hardened metal to heat, which should be 

 considerably below its melting point. For tin, it is said that the heat of boiling 

 water is sufficient for the purpose. But for gold, silver, copper, and iron, the heat 





