PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 465 



Mr. Roosevelt described a percussion shell invented by himself 

 in 1854. 



Mr. Babcock described several other explosive projectiles. 

 Sawyer's projectiles must strike with sufficient force to compress 

 a layer of lead before it can explode. The Shenkel shell has a 

 pin to prevent any motion of the tube containing the cap ; and 

 must strike with sufficient force to cut that pin off, in order to 

 produce an explosion. 



A percussion projectile may pass through three feet of timber 

 before it has time to explode. This is an advantage in most 

 cases ; but the percussion shell is of very little use against 

 infantry, because it will bury itself in the earth before explosion. 



Gov. Noble, of Wisconsin, stated that the rifled shell was 

 invented by Capt. Norton, as early as 1833. 



NEW METHOD OF RIFLING. 



Gov. Noble stated that it had been found from 500 experi- 

 ments that a rifled gun which would cause a penetration of eight 

 inches of ordinary pine board, could be made to cause a penetra- 

 tion of 24^ inches merely by making new grooves for a certain 

 distance. The theory proposed for the explanation of this fact, 

 but with which he was not satisfied, was : that there is a limit of 

 expansion to powder, which may be determined by experiment. 

 It is about thirty-two diameters from the breech of the gun. 

 Beyond that distance the motion of the ball is from its momen- 

 tum, and hence a vacuum is produced behind it, or the pressure of 

 the atmosphere before it is made available. From this point 

 therefore the extra rifling is introduced, which allows the atmos- 

 phere to get behind the ball. But if this theory were true, the 

 gun should be still more effective if cut entirely off at that point. 

 De Brame's experiments seem to involve the same principle ; and 

 it is desirable that some scientific explanation of the facts should 

 be given. 



Mr. Roosevelt said that experiments had been made to ascer- 

 tain the most effective lengths for the barrel ; and that the suc- 

 cess of the accelerating gun also, was against the theory. 



A gentleman suggested that the new arrangement might give 

 greater effect to the elasticity of the atmosphere, and might also 

 furnish oxygen for the more complete combustion of the powder. 



Air. Stetson said that instead of air passing into the gun, he 



[Am. Inst.] DD 



