involved in the Construction of Artillery. 



287 



Fig. 2. 



279. Again let ^, d, Fig. 2, be the caliber of the gun, as before, and ^, a its 

 thickness, and let us assume this divided into a number of separate, closely 

 fitting concentric cylinders, 1, 2, 3, 4, 5, 6. Let 

 e or D be the middle point between the external 

 and internal surfaces of the gun, and let us sup- 

 pose that from the interior cylinder 6 to the 

 exterior 1, these have been in succession so su- _ 

 perimposed, that the three interior cylinders 4, 5, 

 and 6, are in a state of compression, while the 

 three external ones, 1, 2, and 3, are in a state of 

 extension, each set re-acting upon the other in 

 virtue of the elasticity of the material, just like so 

 many extended rings of Indian rubber, tightly grasping round the same number 

 of already compressed hollow cylinders, of the same material. In this state of 

 things, let us suppose pressure applied to the interior of the gun by discharge. 

 Its first effect is to act upon the internal compressed cylinders in succession, 

 which are in the state of so many compressed springs, and to relax their com- 

 pression by extending their circumferences until they have successively reached 

 their respective normal lengths, of uncompressed molecular equilibrium ; but 

 in doing this (as all the rings are absolutely in contact), a certain amount of 

 extension has been necessarily produced upon the three outer rings, which are 

 now in the condition of springs before slightly, and now still more, extended. 

 At this moment the elastic forces of the three internal rings begin to react 

 upon the pressure, as effective resistances to extension also, and the state of 

 things is such that the whole section of the metal of the gun, represented by 

 the six rings, has got into a state of equable extension, and offers effective resist- 

 ance to the pressure. But, although the interior rings are, as always must 

 happen, subjected to the greatest pressure per square inch of their circumfe- 

 rences, they are not in this case the most extended, or extended in proportion 

 to the intensity of the pressure, because we commenced applying the pressure 

 to them, while they were in a state of compression. 



280. We have thus been able in some degree to equalize the extension of the 

 three exterior and of the three interior cylinders ; but if we had been able in the 

 first instance to cause each successive cylinder to grasp and compress the pre- 



2p2 



