48 PROCEEDINGS OF THE AMERICAN ACADEMY 



stratcd before 1845 as they have been since, not only was a fall ac- 

 count of the process and of the results published here in that year, but 

 a French translation of his pamphlet was published in Paris, in 1848, 

 by a professor in the school of artillery at Vincennes ; and Mr. Ti-ead- 

 well's patent, with full specifications, was published in England before 

 Sir William Armstrong began his experiments. 



The difficulties to be overcome in making such a gun, — great at all 

 times, as Sir William Armstrong and Mr. Anderson testify, — were 

 far greater in 1842 than in 1863. 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 difficulties 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 operation 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 

 th^ subject, from the gun formed of coiled rings. But a simple state- 

 ment will bring into view distinctly the new principle of strength here 

 introduced. 



If an elastic hollow cylinder be subjected to internal fluid pressure, 

 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 cylinder 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 before the introduction of the improvement now under con- 

 sideration. 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 layers 

 are compressed by the outer ; whereby the internal pressure is first 

 resisted by the outer layers, which must be distended enough to allow 



