Mr Ritchie's Observations on Leslie's Photometer. 107 



in his interesting work on meteorology, has collected a great 

 number of observations on the heating power of the sun's rays, 

 from the polar to the equatorial regions. Most of these ob- 

 servations were made by comparing two thermometers, one of 

 which was kept in the shade, whilst the other, having its bulb 

 blackened, was exposed to the direct rays of the sun. These 

 experiments and observations are, however, quite vague and 

 unsatisfactory, as no correction seems to have been made for 

 the variable causes which abstract caloric from the blackened 

 ball of the exposed thermometer. ' 



A simple and ingenious method has lately been proposed by 

 J. F. W. Herschel, Esq. Sec. R.S. Lond., which promises com- 

 plete success. As I am not aware that his method has yet 

 been printed, I shall take the liberty of quoting his own de- 

 scription from one of his letters : — " My object in the instru- 

 ment alluded to above, was to ascertain, by direct experiment, 

 the relative heating power of the sun's rays. This I did, by 

 exposing in a glass vessel, or large thermometer, at different 

 times and places, a deep blue liquid, for a given time, to the 

 direct ray's of the sun — noting the increase of temperature, 

 which was purposely rendered very small, by properly adjust- 

 ing the capacity of the instrument, then shading the sun's di- 

 rect rays, and leaving it exposed for an equal time to the free 

 influence of all the other heating and cooling causes, radia- 

 tion, conduction, wind, &c. and again noting the effect of these. 

 The same difference of these, according to their signs, was the 

 effect of the mere solar radiation. Dividing this by the time 

 of exposure, I had the momentary effect, or differential co- 

 efficient, which is the true measure of the intensity of radia- 

 tion." 



Availing myself of the ingenious remarks of Mr Herschel, 

 I have been enabled to obviate the objections to which Mr 

 Leslie's photometer is obviously liable, and can now employ it 

 with considerable accuracy as a measurer of the sun's radia- 

 tion. This may perhaps form the subject of a future commu- 

 nication. I have the honour to be, 



Dear Sir, 



Yours truly, 



William Ritchie. 

 Tain Academy, Mai/ 20, 1825. 



