82 Mr. Powell on Solar Light and Heat. [Aug. 



Let the portion of the surface of the bulb exposed to 



radiant matter = a 



Diameter of the bulb = d 



Its surface = s 



The observed rise in a given time = r 



The power of the coating for absorbing heat (of what- 

 ever kind) = p 



And for radiating it = k 



The intensity of heating power = It 



The general formula easily deduced on the above considera- 

 tions will be, 



r =h. >bfc*Ai 



cP . (» — n) ft 



= h a ' p 



a (s — a) ft 



,.h p - = r d - (s -"\ 

 k a 



When the whole bulb is exposed, 



• 



~* 2 



> 



.'.a n - and - — - = 1 , 



* a 



And .-.^ = ,•</. 



ft 



Comparing two different cases, 



h P*i _ r.d.a,U - a) , .. 



/', Pi A "" r, . rf,. a (s, — a,) ** ' 



If h = A, we thus obtain the value ot —, '-. 



If the bulbs are equal, this = - L - L . (B) 



* ' r, . a (s — a,) v ' 



And if the coating be the same, it = -, andif/* = //, it as 1, 



When the whole bulb is exposed, we have 



I' p ftt r . d .~ 



h,p,k ~ r,.d t 



Tf in this last case the thermometers be exposed to simple 

 radiant heat, assuming the universality of the law, that the 

 absorptive is proportional to the radiating power of a surface, 

 we shall have 



p — k, and p x = fe, 



And if h = h. then — T = 1, or - = y. 



r,d, r, d 



Or hence we might derive a neat and simple method of veri- 

 fying that law. 



The relative values ofp and k as compared with a surface of 

 glass in particular cases, may be obtained by coating only half 



