PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION 409 



Tiy20 after the outer shell had frozen, so as to prevent any supply 

 of metal to the center thereafter ; and this is related as the 

 cause which led to the hollow mode of casting. These cav- 

 ities do not occur of necessity in the center of a casting, but 

 of necessit}' in the center of the mass at equal distances 

 from tlie cooling surfaces if subjected to an equal rate of cooling ; and they 

 J j^l are to be found near the center of 



^^^^^^^^mrrr. ...„.,.. ^l^e mass in the Rodman gun as 



^^'■"•--^.1___^_,_^I*--.^^ the surfaces of the casting. Jf 



they icere generally didributed between the inner metal of the gun and the re-en- 

 force, a sufficient elasticity between these parts of the gun might be had, and a 

 similar result arrived at to that obtained by the drilling of the holes, as shown 

 by diagram. Their presence would account for the occasional endurance 

 of hollow cast guns, but the uncertainty of their uniformity would account 

 for the irregularity of this endurance. The Rodman 15-inch gun, fired at 

 Fortress Monroe, stood the indifferent test to which it was subjected, per- 

 haps, from the occurrence of this uncertain elasticity, and from the fact 

 that it was lired under a hot sun, with slow-burning powder, hollow shot, 

 and windage. Even this questionable success would rarely again be 

 obtained, as the requirements of service are unlikely to permit such favor- 

 able circumstances. 



The 15-inch guns have been shown to be inefficient, therefore, for they 

 only give a velocity of 750 feet per second; or they are unreliable (perhaps 

 both), for they will burst as often as the accidental porosity above spoken 

 of is not evenly distributed between the inner metal and the re-enforce. 

 And who can sa^' wlien these conditions are all fulfilled ? 



I here leave the subject to be considered by those who have followed me 

 thus far, with the remark, that I have shown that no large gun yet made 

 could be pronounced trustworthj', until it was destroyed. Yet I believe 

 that it is possible to so construct a gun, that not a single trial shot need 

 be fired for the purpose of demonstrating its valuable qualities. That 

 through the light I have thrown on the subject, trustworthy guns can be 

 constructed of any required size, and to give almost any required velocity 

 to Rhot, less than the unrestrained velocity of the gases along the bore. 



When I proposed to show that the heat evolved from the combustion of 

 powder in a gun was the principal agent in bursting, I found ordnance offi- 

 cers opposed the theory, on the ground that liut little heat was thus com- 

 municated to~guns. This was said to me by Major Wade, by Captain Rod- 

 man, and iudirectlj'^ by several others occupj-ing high positions. Major 

 Wade, however, afterwards told me he recollected having seen copper 

 melted by the heat of powder, at a time when a few kegs of powder ex- 

 ploded in a powder-mill. Captain Rodman, in a part of his book before 

 quoted, speaks of " higher temperature exhibited when larger masses of 

 powder are burned in an 11-inch gun, than in a 1-inch," as accounting for 

 three times the force in the larger gun than in th« smaller; and Captain 

 Dahlgren, in his report to the Secretary of the Navy for 1862, says " an 

 11-inch gun was so heated by firing that it was afterward eighteen hours 



