73 



puting oil {g) these data we find, that the temperature at which 

 the soft cast iron nieUs, is about SjSSS'^ : now we know, that 

 this fuses at 130^ of Wedgewood, each of his degrees is, tliere- 

 fore IQ'' or 20° of Fahrenheit nearly. 



Robins found, that air by a white heat, or about 35° or 40" of 

 Wedgewood was quadrupled in volume, therefore, supposing it 

 to expand uniformly, it must b^ the known expansion of air 

 have been heated 3x480 or 1440'', which would give l'^ of the 

 pyrometer=:26'^. Lastly, the experiments of Beaunier and Gal- 

 lois, related in the 70th Number of the Journal des Mines, give us 

 an approximation. They exposed in a refining furnace mas- 

 ses of iron of known weight and capacity till they had ac- 

 quired its temperature, these were rapidly withdra^vn, and 

 plunged into a quantity of cold water ; the augmentation of its 

 temperature gave the heat of the iron. They found that at 

 the instant when the last pellicle of litharge disappears from 

 the melted silver, its heat was 700 of Reaumur or 1573° F* 

 and as the silver was perfectly fused though sunk deep in the hol- 

 low of the cupell beneath the current of flame, while the iron 

 was in the hottest part, we may fairly suppose it to have been 

 10° W. hotter than the silver and therefore at 35? W. This 

 gives l** W=28* F. (Ji) I offer these estimations not as certain 



VOL. XIII. M 



(g) Let W and w be the weights of the gases and iron, C and c their capacities, and the 



temperature, then 8x(WC+wc) =10000", therefore 9 =, = — taking 



WC-|-wc S . 5 



the capacity of iron to be ,lt; it is probably something greater. 



(h) In these computations I have subducted from the numbers expressing the heats, 

 600 ' or the number of degrees before ignition commence?, as Wedgewood's scale begins at that 

 point. 



