MECHANICS AND USEFUL AKTS. 109 



at all times, as Sir William Ai-mstrong and Mr. Anderson testify, 

 — were far greater in 1842 than in 18(33. These difficulties were 

 mainly, if not wholly, in welding large masses of wrought-iron in 

 the shape of tubes or cylinders. It is for overcoming these diill- 

 culties that this medal is bestowed, and especially for the means 

 and appliances by which this difficult mechanical achievement was 

 effected in the furnace ' by the agency of fire.' 



" An incidental but noteworthy part of the improvement was the 

 welding by hydrostatic pressure, — an oijeration which is just now 

 coming into use in England, but has not yet attracted attention in 

 this country. 



" We come now to the second improvement in the construction 

 of artillery, the invention of the hooped gun. 



"This is not always clearly distinguished, even by those occupied 

 with the subject, from the gun formed of coiled rings. But a 

 simple statement will bring into view distinctly the new principle 

 of strength here introduced. 



' ' If an elastic hollow cylinder be subjected to internal fluid press- 

 ure, the successive cylindrical layers of the material composing 

 it, counting from within outwards, will be unequally distended, 

 and the resisting efficiency of the outer layer will be less than 

 that of any layer nearer the axis. And if the walls of the cylin- 

 der are thick, and the internal pressure surpasses the tensile 

 strength of the material, its inner layer will break before the outer 

 one has been notably strained. Hence the tensile strength of a 

 square inch-bar of the material is the measure of the maximum 

 pressure the cylinder can bear, when constructed as guns were 

 befoi-e the introduction of the improvement now under considera- 

 tion. The improvement does away with this limit, and enables 

 us to go indefinitely beyond it. 



" This is accomplished by so constructing the gun that the inner 

 laj'ers are compressed by the outer ; Avherel^y the internal press- 

 ure is first resisted by the outer layers, which must be distended 

 enough to allow the internal compressed portion to attain its 

 normal conditon before this internal portion (which is the first to 

 break in the common gun) is subject to any strain at all. It will 

 be perceived, that, if this principle could be rigorously applied, a 

 cannon could be made so perfect, that, when sul^jected to a burst- 

 ing pressure, every fibre, from the internal to the external sur- 

 face, would be at tliat instant equally extended, each contributing 

 its full share of resistance to fracture. The whole resistance 

 would 1)6 proportional to the area of fracture. 



" This was supposed to be the case in common cylinders before 

 the error was pointed out by Barlow, and also by Lamie and Cla- 

 peyron. Aud it was this erroneous supposition that led Count 

 Rumford to his exaggerated estimate of the force of gunpowdei-, 

 as tested by its power of bursting gun-barrels. If he had used 

 the theory which gave origin to the hooped gun, his results would 

 nearly have agreed with modern observations. 



"The demonstration of the superiority of the hooped gun, with 

 detailed directions for its construction, is contained in a pa2)er 

 read before this Academy in February, 1856, and published at the 

 10 



