On Solar Badiation. 116 



intensity.* The intercepted heat goes, of course, to warm 

 the air. 



The object of this note is, however, to record a different 

 set of observations, performed with an instrument of inferior 

 delicacy to the actinometer, but still capable of yielding very 

 remarkable results, — I mean Leslie's photometer. Its prin- 

 ciple may be briefly described as measuring the difference of 

 the heat absorbed by a dark and clear thermometer-ball. It 

 is well known that this instrument gives, on some occasions, 

 results which appear highly paradoxical, but which, if consis- 

 tent, require to be explained, and ought, therefore, to be dis- 

 tinctly established. My observations with it were directed to 

 two points. 



1. To ascertain the effect of the presence of a coating of 

 snow on the ground, in magnifying the apparent Solar Ra- 

 diation. To this effect has been ascribed the extraordinary 

 force of the sun's rays observed in arctic climates, and also 

 some singular variations from one season to another, supposed 

 to depend on the presence of snow on the ground. t Now, 

 the few experiments which I obtained before breaking my in- 

 strument last summer (1842), gave me the following most 

 striking results : — 



Surrounded by grass, and exposed to direct sunshine, the photometer in- 

 dicated, 78° 



Exposed upon snow instead of grass, it rose to ... 121* 



The whiteness of the snow is all important in this respect ; 

 dirty snow produces comparatively little effect, and so does 

 ice; thus, 



The photometer exposed on a dirty part of the Mer de Glace, stood 



at 70- 



Placed upon a neighbouring patch of snow, .... 140** 



This action fully explains the intensity of the effect of fresh 

 snow upon the eyesight. I have myself found that exposure 

 for several weeks to the moderate glare of sun-light reflected 

 from a glacier surface, produced little effect upon me, whilst 



• Phil. Trans, as above. 



t Edin. New Phil. Journal 1841. A paper bj Dr Richardson, with remarks. 



