PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 



401 



HOW GUXS BURST. 



When gunpowder is fired from a gun, two prominent phenomena are to 

 be observed; namely, the wonderful expansive force which ejects the shot, 

 aud the heat which results from the combustion of the powder. 



Let us exhibit the eifect of heat on metals by a familiar experiment. 



T^^l 



Pour boiling water into a glass tumbler; the heat, com- 

 municating more quickly to the thin sides than to the thick 

 bottom, breaks the glass from unequal expansion. If v. e 

 wish the tumbler to withstand the sudden communication 

 of heat, we must make it everywhere thin alike, so that 

 the heat may pass through it uniformly and quickly. Hot 

 water may then be poured into it with impunity. But it 

 we wish it to withstand a pressure of cold fluid, it will be -^y^- 



necessary to make the walls equally thick; it will then 

 withstand a considerable pressure on its interior surface, 

 even if communicated suddenly. But if, after having pre- 

 T!<j-3. pared it to withstand the pressure, we wish 



to communicate a pressure accompanied by U^ j) 



•heat, as of a considerable height of col- ^— =^ 



umn of melted metal, although the thickness of the walls 

 would be sufficient to withstand the pressure, the heat 

 communicated to the inner surface of the wall would 



\ y expand it within the outer metal, before the heat reaches 



the outside, and it would be broken by this unequal communication of the 

 heat. Now, this unequal communication of heat has a similar effect upon 



large guns. This may, also, be 

 illustrated by a glass model of a 

 gun, which, although strong enough 

 to withstand a pressure on the 

 inner surface of 400 lbs. to the inch, 

 would be broken by the insertion 

 of a heated rod of iron of smaller diameter than the bore, even though 

 so inserted as not to come in contact with its sides, and not accompanied 

 by any pressure against the surface. Three models might thus be brokon 

 quickly, in succession, by the insertion of an iron rod heated to a high 

 temperature, while the fourth would break slowly, or not at all, the rod 

 being reduced in temperature, from the heat lost by communication to the 

 broken models. If, however, after waiting a time /or Die model to he slowhi 

 heated throughout its xohole mass, the outer surface of the gun be touched 

 by the wetted finger, the evaporation of the moisture will make the 

 heat sufficiently unequal, and the model will break. This example may 

 exhibit the direct cause of the bursting of the 100-pounder Parrott gun, on 

 the steamer Naugatuck, on the James river, before Fort Darling, when other 

 guns of the same kind on the steamer Galena, though fired with groat 

 rapidity, and oftener, did not burst; all of which may be accounted for by 

 the fact that it was raining at the time, and that the gun of the Naugatuck 

 being on the upper deck and exposed to the rain, was subjected to a more 

 u.ipqual heating than the guns of the Galena, which were between decks. 

 [Am. Inst.] 26 



