the Surface- Temperature of the Planets. 165 



To get that for sea-level we shall take Crova's self-regis- 

 tering actinometric measures at Montpellier (height 40 m.) 

 made on August 13, 1888, at 12 h 30 m under a barometer of 

 761 mm., and with them those made simultaneously on 

 Mt. Ventoux (height 2000 m.). The respective calories he 

 obtained were : — 



Montpellier. Mt. Ventoux. 



Calories. Bar. Calories. Bar. 



Aug. 13, 12 h 30 m , 1888... 0*975 761-1 mm. 1*360 613-5 mm. 



We shall reduce these to the same scale as the Lone Pine 

 results, made with the pyrheliometer and used by Very, 

 which were : — 



Lone Pine. 

 Aug.ll,12 ; 14: 12 h -12 h 30 m ,1881. 1*533 calories, bar. 663 mm. 

 giving for 



Montpellier. Mt. Ventoux. 



1*180 calories. 1*643 calories. 



This value of 1*180 is one which is probably about the 

 average of clear days in our latitude, the day in question 

 being registered by Crova as " very clear " 



From these several data we find the following values for 

 the solar radiation received at the respective posts, in calories 

 in one column, in percentage of that entering the atmosphere 

 in another : — 



Solar Radiation. 



Bar. Calories. Percentage. 



Outside the atmosphere .. . 3*127 1*000 



Camp Whitney 500 mm. 1*896 0*606 



Lone Pine 663 „ 1*533 0*490 



Montpellier 761 „ 1*180 0*377 



The loss in the visible spectrum as shown by the bolometer 

 is almost wholly from selective or general reflexion and from 

 diffraction, that in the invisible one from selective absorption. 

 The absorptive loss by bands in the former is only about 

 1 per cent, of the whole, and the loss by reflexion in the latter 

 probably not over 7 per cent, of its depletion. 



Depletion in Visible Rays. — We may now find the depletion 

 in the visible part of the spectrum which is not in general 

 the same as that for the invisible part, decreasing relatively 

 with the altitude and reversely increasing as the air envelope 

 becomes thicker. It does this at a greater rate than the 

 increase of the air-mass because the particles suspended in 



