involved in the Construction of Artillery. 189 



discovered by Leslie) to cool faster than a smooth gun-metal gun, 

 with a semi-bronzed and polished surface, so far as radiation 

 alone is concerned, which, being equal all round the gun, tends to 

 make it, so far, cool equably, but the heat is carried off by evec- 

 tion faster from the surface of the gun-metal, in consequence of its 

 higher conducting power, and it is the evected heat (lost chiefly 

 from the lower side of the gun, as has been already shown) that 

 produces the main inequality of temperature in the piece ; the 

 disadvantage, therefore, is all on the side of the gun-metal. 



103. Pursuing the method of Dulong and Petit, and ofRegnault, for deter- 

 mining specific heats, equal masses in cooling, under like conditions, lose 

 quantities of heat proportional to 



^(T-t)xcP. ' (9) 



2 being the specific heat ; <r, the specific gravity ; T and t, the temperatures of 

 the body and the medium ; and P the volume in each case. 



If T—t he the same for each of two different bodies, with the same volume 

 and form, &c., equal cooling shall, under similar conditions, occur in times 

 proportionate to 



•2xaP, 



or in equal times the heat lost will be in the same proportion. 

 This for gun-metal and cast-iron will be. 



Gun-metal, . . 0110x8400= 924, 

 Cast-iron, . . 0-134 x 7500 = 1005. 



The gun-metal cooling, so far as its specific heat and density alone are concerned, 

 rather the more slowly. 



104. But from the recent experiments of Prevostaye and Desaines, the 

 relative radiating power of rough cast-iron, painted black, to imperfectly polished 

 and weather bronzed gun-metal, may be assumed as 90 : 15. 



Combining, then, the three principal elements in each case, for equal and 

 similar volumes at equal temperatures, and in the same media of equal tempe- 

 ratures, we have the relative rates of cooling for gun-metal and for cast-iron 

 in the following ratio : — 



VOL. XXIII. 2 c 



