272 Mr. Mallet on the Physical Conditions 



is reduced ; and that the maximum tension is attained when the shot has been 

 displaced 0'075 metres. It is, then, for the gun-cotton, = 493-4 atmospheres, 

 while at the corresponding length for gunpowder it is only = 227-7 atmo- 

 spheres. 



It follows, then, that the strain upon the gun for equal ranges, and equal 

 weight of shot, is with gun-cotton about double that with gunpowder. 



249. Had we learned, experimentally, the actual time required for the igni- 

 tion and complete combustion of given weights and volumes of gunpowder 

 and of gun-cotton, more precise conclusions could be arrived at as to the best 

 material for cannon intended to be fired with the latter. With the exception, 

 however, of a single set of experiments made by the author, as to the time of 

 explosion of some rather large charges of gunpowder, by means of the chrono- 

 graph, incidental to his experiments on earthquake-wave transit (Trans. Brit. 

 Assoc), no experiments seem as yet to have been made on the subject. 



250. With guns whose resistance to ultimate rupture shall be so propor- 

 tioned by excessive scantling as to be far within the limits of safety, there can be 

 no doubt that the metal whose period of force transmission is highest will suffer 

 least from the rapid blow of gun-cotton, while the ductility of gun-metal must 

 render it susceptible of rapid injury of local form by it ; and should the extreme 

 lightness of gun-cotton as ammunition for field artillery ever induce its general 

 adoption for that arm (as the experiments in Bavaria and Austria seem to 

 render somewhat probable), there can be no doubt that wrought-iron field- 

 guns would be found the best fitted to resist its action, if properly made ; their 

 length being reduced and thickness varied, in accordance with the above 

 experiments, and the external contour of the gun wholly altered from the estab- 

 lished models of brass field-guns. 



251. While, however, a metal comparatively rigid and highly elastic will 

 suffer least distortion under the stroke of gun-cotton, or the like rapidly 

 exploding agents, there may be ground to apprehend that internal molecular 

 injury, and final dislocation, may more or less slowly result from the shattering 

 jar of such explosions, and most of all so, in a loosely coherent and crystalline 

 mass, such as cast-iron, although such might not occur with gunpowder, or be 

 much more slowly produced. 



252. That particular form of destruction upon long-continued firing which 



