250 Messrs. W. E. Wilson and P. L. Gray. [Mar. 15, 



II. " Experimental Investigations on the Effective Temperature 

 of the Sun, made at Daramona, Streete, Co. Westmeath." 

 By W. E. WILSON, M.R.I.A., and P. L. GRAY, B.Sc., Assoc. 

 R.C.S., Demonstrator of Physics, Mason College, Birming- 

 ham. Communicated by G. JOHNSTONS STONEY, F.R.S. 

 Received January 4, 1#94. 



(Abstract.) 



The only tolerably complete series of investigations on this subject 

 np to the present time have been those of Rossetti and Le Chatelier. 

 The results given by other writers have depended more or less on 

 guesses relative to the law connecting radiation and temperature, 

 and differences on this point alone have given values varying 

 between 1500 and 3,000,000 to 5,000,000 C. 



Rossetti worked with a thermo-pile, exposed directly to the heat of 

 the sun ; the law connecting the deflections of the galvanometer with 

 the temperature of an artificial source of heat having been obtained 

 up to a temperature of about 2000 C., from the deflection produced 

 by the heat of the sun the solar temperature was calculated by extra- 

 polation. 



Le Cliatelier worked on an entirely different principle, measuring 

 the intensity of the light transmitted through a certain piece of red 

 glass, first from sources at known temperatures up to 1700 or 

 1800, and, secondly, from the sun, the temperature of which was 

 then obtained, as in Rossetti's case, by a process of extra-potation, 

 which is, of course, necessary in any method, until we can raise 

 substances to a temperature actually as high as that of the sun, an 

 experiment at present impossible. 



Rossetti obtained finally a temperature of 10,000 C., approximately, 

 while Le Chatelier gives 7600 ( + 1000) as the mean of his results. 

 In the paper the difference between Rossetti's result and our own 

 (6200 C.) is discussed, and a possible explanation given. 



The method adopted by the authors is a zero method, and the 

 essential point is the balancing of the heat from the sun with that 

 from a platinum strip heated to a high known temperature. 



The artificial source of heat was a modified form of Joly's " meldo- 

 meter," the calibration of which can be performed with a very high 

 degree of accuracy. The " radiation balance " is a form of Boys's 

 radio-micrometer containing a duplex circuit, so designed that the 

 heat from the sun can be made to exert a turning moment in the 

 opposite direction to that due to the artificial source of heat, and by 

 making the apparent area of the latter sufficiently great, the radia- 



