PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 535 



sure the force applied to punch a hole of the same area as that made by 

 the cannon ball, we can nearly determine the pressure it takes to shear off 

 metal for a given area and thickness. Mr. Wiard, by this process, approx- 

 imates to the force of powder wliicli he assumes the maximum force to be 

 11,000 and some odd pounds. How this pressure is distributed through the 

 length of the gun, we have as yet no knowledge. Captain Rodman has made an 

 apparatus to find this out, but the principle on which it is constructed 

 would lead us to doubt his conclusions. 



Mr. Norman Wiard. — The experiments made at the Springfield armory 

 showed that gun barrels were not permanently enlarged by gunpowder, but 

 that they were by water at a pressure of 5,000 on the square inch. From 

 this fact it appears to me that the pressure of gunpowder must be even 

 less than this. 



The great problem to be solved is to find out the actual pressure of gun- 

 powder. The great diversity of opinion is really astonishing. It is calcu- 

 lated by some to be 100,000 — by others 200,000, while some authors calcu- 

 late it at 150,000 pounds on the square inch. Now, if a pressure of 10,000 

 pounds on the square inch will punch a hole through iron, would not five 

 times that pressure blow a gun all to pieces ? And I will not ask what 

 would be the efiect of 150,000 pounds pressure. Now, what bursts a gun ? 

 The theory by which I explain it is, that it is due to the unequal expan- 

 sion of tlie metal from the heat of the powder, and I have not as yet found 

 any one to controvert it. 



In making guns for the Ordnance Department, they insist that the ten- 

 sile strain of the iron shall not be above 28,000 pounds, and they would be 

 rejected if above that. I make metal of 38,000 pounds tensile strain, but 

 am not allowed to use it. The conclusion is, that guns of a lower strength, 

 of even 16,000 pounds to the square inch, have stood the best. A lOinch 

 gun, one of the best I made, which had a tensile strain of 38,000 pounds 

 burst on the fourth round. It is a remarkable fact that cast-steel guns will 

 burst when cast-iron ones will not so readily, and that guns made of metal 

 of a tensile strain of 16,000 pounds on the square inch, will stand the best. 

 The 500-pounders at Charleston which threw a shot five miles, burst after 

 a few rounds, as did also the 200 and lOO-pounders, and Gen. Gilmore is 

 now using 30 and 50-pounders. 



Mr. Benjamin Garvey made several diagrams on the black-board illus- 

 trating the mathematical principles involved in using projectiles with high 

 and low velocities. He said there was one point which he had never seen 

 taken into account by those measuring the force of gunpowder, and 

 that was, that at the moment the powder is ignited, the force is so great 

 that the back section of the ball, particularly in rifle shots, is bulged or 

 swelled up at that end, and causes increased friction, wliicli diminishes 

 much force of the powder. 



" The Use of Lead Pipe," and " Magnetism and Electricity," were select- 

 ed as the subjects for the next discussion. 



Adjourned. 



