46 ON THE CONSTRUCTION OF HOOPED CANNON. 



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" There may, at the first view, seem to be a great practical difficulty in making 

 the hoops of the exact size required to produce the necessary compression. This 

 would be true if the hoops were made of cast-iron, or any body which fractures 

 when extended in the least degree beyond the limit of its elasticity. But wrou 

 iron and all malleable bodies are capable of being extended without fracture much 

 beyond their power of elasticity. They may, therefore, be greatly elongated with- 

 out being weakened. Hence we have only to form the hoops small in excess, and 

 they will accommodate themselves under the strain without the least injury." 



And again, in a note, I said : " Mr. Barlow does not limit the application of his 

 investigation to any kind of material, but it is evident that his conclusions are 

 not applicable to any malleable metal like bronze ; for in a cylinder constructed of 

 hoops of this material the inner hoops may be elongated by the pressure acting 

 as a crushing force, and by this means be enlarged without any diminution of 

 tenacity. Perhaps some kinds of soft cast-iron may accommodate themselves to 

 an enlargement in the same way. But with hard crystalline cast-iron, no actual 



displacement of the constituent particles can take place without fracture; and 



although the effect of the fluid as a crushing force may act as an auxiliary to 

 the strain, as any estimate of its amount would be a mere guess, I shall not 

 attempt any modification of Mr. Barlow's conclusion, when applied, as in this 

 case, to hard cast-iron gun-metal." 



However important I might have considered the effects of the crushing force, 

 and the partial or imperfect malleability of cast-iron, by which the gun may be 

 permanently distended, a further examination of the subject has convinced me, 

 that Mr. Barlow's theory must be in all cases modified and limited by the elongation 

 or yielding from this malleability under the crushing pressure of the fluid ; and, 

 in many cases, as where the material is bronze or wrought-iron, the whole theory 

 must be discarded as inapplicable. To show this, I will state the following exper- 

 iment. I took a ring or hoop of wrought-iron, made up of four concentric rings, 

 one placed over another, after one of the methods practised by me in making 

 my wrought-iron guns in 1840-1843. These rimaL when welded together, formed 



a hollow cylinder 1 inch long, having an internal diameter of 1| inches, and an 

 external diameter of 3 inches;' consequently its walls were f of an inch thick. 

 This cylinder, after being smartly hammered or sledged when cold, was subjected 

 to distension by driving into it a conical plug or pin, by blows with a heavy 

 sledge. By this means the inside diameter was increased to 2J inches. This dis- 

 tension, from y to V, was far from rupturing the ring, although it produced a 



