1825.] Mr. Powell on Terrestrial Light and Heat. 



363 



In all these cases the increase of the ratio between the effects 

 of light and those of simple heat is very conspicuous ; and it 

 appears both from the results of Count Rumford, &c. as well as 

 these, that the increase of light is in a ratio greater than that of 

 the increase of temperature ; the effects of light being in these 

 experiments more than doubled when two flames were united, 

 whilst the heat radiated was less than doubled. 



(9.) I now extended the inquiry to the radiation from metal at 

 different stages of incandescence. For this purpose I employed 

 a mass of iron of a cylindrical form, about six inr.hes long and 

 1-5 diameter; heated to the brightest point which a common 

 fire could communicate, and suspended vertically. The photo- 

 meter was exposed to it at seven inches distance, placed oppo- 

 site to the middle point of its length. In the first sets of these 

 experiments, I observed the effect of light, using the glass case, 

 and therefore could make no compa.rison of the effect of the 

 light with that of the heat. I am well aware that these nume- 

 rical results can only be regarded as rough approximations ; yet 

 they will give some idea of the different law followed in the pro- 

 gression of the two parts of the effect. The value oC (I + h) 

 niay have been somewhat too small from a trifling heating eftect 

 of the small screen on the lower bulb. 



