involved in the Construction of Artillery. 



179 



latter member of the equation only would apply. In the absence, however, of 

 any knowledge of the constants that would admit of an exact expression for the 

 phenomena before us, Biot's may serve as an illustration of the enormous dis- 

 proportion in temperature between the interior and exterior of the gun. 



86. But a more precise expression for the facts may be had. M. Weidemann, 

 in a recent Memoir on the Transmission of Heat in Metals (Poggen. Annal., 

 St. 95, p. 337), has found experimentally that a bar of zinc, having thermo- 

 meters placed at distances of two inches apart along its length, and heated 

 constantly at one end to 100°, had the following temperatures at each of the 

 thermometric points, viz.: — 



Now these results, obtained experimentally for zinc, are to a near approxi- 

 mation true for iron (and probably for cas<-iron), as the relative conducting 

 powers for heat of zinc and iron are as the numbers 363 and 374 (Daniel), 

 Chem. Phil. p. 121). We may, therefore, conclude that a cast-iron gun, whose 

 thickness is 10 inches, and whose interior is heated to 100', will lose heat from 

 its exterior, at the rate due about to the temperature of 7'2°. 



87. These conclusions, though not strictly correct, are sufficient to indicate 

 the enormous disproportion in temperature that must subsist between tlie interior 

 and the exterior, and that is all we are at present concerned with. 



Experimental data are as yet wanting, to enable this question to be pursued 

 with exactitude. It is necessary first to learn experimentally what is the ex- 

 treme of temperature acquired by the interior of each class of gun, over that of 

 its exterior, in firing hot sliot ; and the actual rate in time at which the wave 

 of heat travels outwards by conduction only through the substance of the gun, 

 a constant as yet unknown for any substance. 



