involved in the Construction of Artillery. 289 



of extension : — we have a state of things in which the total section of resisting 

 material, a, 6, shall be operative against the internal pressure, to the highest 

 possible advantage, for — 



1°. We have reduced the moment of strain per square inch of the metal 



oi all the cylinders, 1, 2, 3, 4, by removing them further out from 



c, the axis. 

 T. We have equalized the extension, in relation to the maximum pres- 



siure, upon all parts of the metal from b to a, so that, it all re-acts 



alike, with an equable strain, and as though subjected to an ordinary 



straight pull. 



282. We have now arrived, therefore, at a built-up gun of wrought-iron, in 

 which the ex-ternal cylinders, ^ , i, as they cannot practically be superimposed 

 and closely fitted, in the required conditions, if consisting each of a single piece 

 equal to the whole length of the gun, must consist of a number of compara- 

 tively short rings, applied end to end. The longitudinal strains due to the 

 projection of the ball or to the recoil, as well as to the coherence of the whole 

 together, must, however, be provided for, and for this the internal longitudinal 

 voussoir prisms, g, b, are efficient; or a cylinder in one piece may (if not too 

 thick) be substituted for these voussoirs. 



A gun or mortar thus formed will consist of one or more plies of longitu- 

 dinal bars extending the whole length of the gun, and fitting closely at the 

 edges, forming by their internal surfaces (when together), the cylindric cavity 

 of the chase, upon which a number of circular rings will be closely fitted by 

 shrinking-on, at a suitable heat; and upon these again, other rings (breaking 

 joint of the abutting edges), until the necessary strength be reached. Various 

 modes of closing the breech end are practicable; but into these or other 

 questions of merely operative detail, it is not my purpose here to enter, as we 

 are dealing with principles, though not losing sight of practice. The theory of 

 the strength of guns thus built up, and the methods of calculating their propor- 

 tions, will be found in Note W. Some subsidiary questions remain to be here 

 considered. 



283. In guns thus constructed, the whole of the normal bursting strains are 

 sustained by the external rings, — their total strength having been determined for 

 the maximum pressure per square inch, regard being had to the proper limited 



