PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 551 



familiar example that would show that they understood the effects of heat 

 upon metal as I have been so long endeavoring to illustrate them. 



58. The pressure of powder is undoubtedly uniform, and it is a folly to 

 attribute greater pressure to it at one time than another, unless such acci- 

 dents occur as not ramming the shot home, when the straining effect on 

 the gun is said by some to be greater, or by using heavier projectiles, to 

 overcome the vis inertia of which requires more nearly the entire effort of 

 the puwder. 



69. It is undoubtedly true that the pressure upon a square inch of sur- 

 face of a chamber filled with powder of one size will be the same as upon a 

 like size or area of surface of a larger chamber also filled, and cast-iron 

 chambers have endured charges of powder from which there was no escape 

 of the gaso^, as well as where the gases escaped slowly, as through a small 

 vent, which is positive evidence that the full pressure of the gases of pow- 

 der can be restrained by cast-iron walls, whether the chamber be filled or 

 portly full. 



A PRACTICAL EXAMPLE. 



30. When the steamship Great Eastern was launched, Braham or hydrau- 

 lic presses were used of ten-inch calibre, and with ten inches thickness of 

 wa'ils of cast-iron, consequently these presses had the usual proportion of 

 thickness of walls in a ten-inch gun. Ten-inch guns have been fired with 

 double-shotted charges without bursting. These press cylinders burst as 

 often as five thousand pounds pressure to the inch was applied on the 

 occasion alluded to. 



ANOTHER EXAMPLE. 



Gl. Major Wade and Major Hagner conducted experiments at Springfield 

 armory, in which water pressure was applied at different points in the 

 length of musket barrels. Twenty-five hundred pounds to the inch would 

 permanently enlarge a musket barrel in the "thinnest part, near the muzzle, 

 and five thousand pounds to the inch would permanently enlarge it in the 

 strongest part," a result, says the report, never attained by the proof 

 charges of powder and bullet. In these examples, five thousand pounds 

 to the inch had a greater effect upon the barrels or cylinder, both of large 

 and small sizes, and of wrought and cast iron, than the pressure of powder. 



62. Having seen that initial strains from unequal cooling, are sufficient 

 to rupture all kinds of guns, and that the pressure of powder is uniform 

 under like circumstances, we must conclude that some other force besides 

 the pressure of the powder, is the cause of bursting. It has been estab- 

 lished in previous papers, and is undoubted-, that guns are heated unequally 

 by firing rapidly the very circumstances that attend the bursting. Then 

 to what else shall we attribute the accidents. 



POSITIVE EVIDENCE. 



63. One of Blakely's first guns, as exhibited in diagram number 8, 

 failed by the longitudinal extension of the inner tube, and the bauds between 

 the trunnion ring and the cascabel. The gun was strengthened longitudi- 

 nally by four bolts, reaching from the trunnion ring to a cascabel piece 

 against the breech of the "jrun. ' 



