394 Solar Radiation 



favourable circumstances from the vertical sun is i'2gr.C. 

 per square centimetre per minute, and if we ascribe to the 

 atmosphere a coefficient of transmission no greater than 

 two-thirds, the value of the solar constant^ or the heating 

 power which the sun's rays would exert on a surface of one 

 square centimetre exposed to them for one minute at a point 

 on the earth's orbit, is r8 gr. C. As the transmission coef- 

 ficient is probably greater than two-thirds, the value of the 

 solar constant is probably less than r8. Vallot, by giving 

 effect to the rate of absorption actually observed in the air 

 separating his two stations, arrives at 17 gr. C. as the most 

 probable value. These values are in substantial agreement 

 with the older ones, such as those of Herschel and Pouillet ; 

 but there is a feeling at present (1901) that not much weight is 

 to be attached to these results, and much higher figures seem 

 to be more readily accepted. In a recent work, Strahlung 

 und Temperatur der Sonne, p. 38, J. Scheiner sums up the 

 discussion of this subject by giving 4 as the most probable 

 value of the solar constant. 



As we have seen, the heat which arrives at the sea-level 

 has to support the temperature of the land and that of the 

 sea ; it has also to supply the energy for all the movements 

 of the ocean ; it has to warm and expand the air, and to 

 furnish the latent heat represented by the aqueous vapour 

 in the atmosphere, and it is mainly accountable for winds 

 and storms. All this is maintained on certainly less than 

 1 '5 g r - C. per square centimetre per minute. But when the 

 above catalogue of functions has been repeated, there is 

 nothing left to be accounted for. If the sun's rays enter at 

 the top of the atmosphere with an intensity of 4 and come 

 out at the bottom of it with an intensity of only 1-5, how is 

 the loss to be accounted for? It represents nearly double 

 the energy which reaches the sea-level and produces such 

 far-reaching effects. If it really entered the atmosphere it 

 must be still there, either as heat or as its equivalent. But 

 we know that the air is not made appreciably warmer by 

 it, and we see no mechanical manifestations which can in 

 any way be put forward as an equivalent. We conclude 



