86 



of his above-quoted work of 1884, on the Temperature of the Atmos- 

 phere and the Earth's Surface). 



Until such a method lias been i^erfected (see an article l^y Ferrel 

 in Am. Jour. Sci., May, 1891, 3, Vol. XLI, p. 378) we will for the 

 present quote the actinometric degrees and other figures as ordinarily 

 published by Marie-Davy and others; but the reader must bear in 

 mind that these results from the hypothesis assumed by Marie-Davy 

 that the observed ditference between the bright and black bulb is pro- 

 portional to and therefore a proper measure of the intensity of the 

 radiant heat that falls upon these thermometers; a hypothesis which, 

 as Ferrel has shown, is far from being true. The error of this hypoth- 

 esis is of such a nature that for a given difference or a given 

 actinometric degree the true intensity of radiation is greater at high 

 temperatures than at low temperatures. Probably the recorded acti- 

 nometric degrees therefore give a rather low value for the solar and 

 sky radiation during the hottest portions of summer days. 



The accompanying table, as published by Marie-Davy, shows the 

 actinometric degrees calculated for the clearest of skies at Paris at 

 noon of each day. They are computed according to the preceding 

 formula, viz, A= actinometric degTees= 100X0.875*"; in which, as 

 before said, the coefficient, 0.875, represents the penetration or the 

 total heat which penetrates to the observer, both from the sun 

 and the surrounding sky, and includes even that small part that is 

 directly reflected from the surrounding grassy lawn or other surface 

 when the sun is in the zenith; if there were no atmosphere pres- 

 ent the total amount received would be 100. It will be less confusing 

 if the reader will consider these so-called " actinometric degrees "' 

 as "■ percentages of what would be received in the absence of the 

 atmosphere." 



Columns 5, (>, and 7 of our table give the mean value of the five 

 actinometric percentages observed on the clearest days at 6 a. m., 

 9 a. m., noon, 3 p. m., and G p. m. ; in the absence of actual observa- 

 tions these means may be employed in our study, provided we make 

 a proper allowance for the influence of hazy and cloudy skies. It is, 

 however, always desirable that the actual observation of the acti- 

 nometer should be available, and witli it should l)e associated a 

 simultaneous record of the cloud or haze as given by the sunshine 

 recorder. 



