THE SUN'S HEAT. 17 



according to the altitude of the sun and the condition of the air ; in- 

 deed, the most difficult part of the experimental problem lies in the 

 determination of the corrections to be applied on account of the ab- 

 sorption of the earth's atmosphere. It would take us too far to dis- 

 cuss the formulae and methods of calculation which have been proposed. 

 They are necessarily very complicated^those, at any, rate which are 

 tolerably accurate in their results because they have to take into ac- 

 count the meteorological conditions, especially the hygrometric state 

 of the air. Besides this, the absorption varies greatly for radiations 

 of different pitch ; so that the violet rays, which are photographically 

 the most active, suffer more than the green and yellow, which are 

 most effective in the growth of plants ; and these more than the red ; 

 and the red, in their turn, much more than the low-pitched, slowly 

 vibrating waves which, though invisible, are still the chief carriers 

 of energy, and do more to warm and vivify the earth than all the 

 others. 



Speaking loosely, it may be estimated that at the sea-level, in fair 

 weather, neither excessively moist nor dry, about thirty per cent, of the 

 solar radiation is absorbed when the sun is at the zenith, and at least 

 seventy-five per cent, at the horizon. Of the rays striking the upper 

 surface of the atmosphere, between forty-five and fifty per cent., there- 

 fore, are generally intercepted in the air, even when there are no 

 clouds. 



Of course, it does not follow that the heat absorbed in our atmos- 

 phere is lost to the earth. Far from it : the air itself becomes warmed 

 and communicates its heat to the earth ; and, since the atmosphere in- 

 tercepts a large proportion of the heat which the earth would radiate 

 into space, if not thus blanketed, the temperature of the earth is kept 

 much higher than it would be if there were no air. 



It is now generally customary to express the intensity of the solar 

 radiation in a somewhat different way from that which has been in- 

 dicated. Instead of stating how much ice would be melted in a min- 

 ute by a given sunbeam, we give the number of calories * received 

 per minute by one square metre exposed perpendicularly to the sun's 

 rays at the upper surface of the atmosphere. This number is called 

 the solar constant, and according to different experimenters ranges 

 from Pouillet's estimate, 17'6, to that of Forbes, who found 28"2. The 

 most reliable recent determinations by Crova and Violle set it at 23 '2 

 and 25'4 respectively. Probably 24 is very near the truth, though 

 there remains a considerable amount of uncertainty, since the results 

 obtained by the same observer on different days, after all possible 

 pains is taken with the corrections, are even more discordant than the 

 numbers given above. A continued series of observations at some 

 very elevated station would improve the data. 



* The calory is that quantity of heat which will raise the temperature of one kilo- 

 gramme of water from to 1 centigrade. 

 VOL. XTIII. 2 



