86 
of his above-quoted work of 1884, on the Temperature of the Atmos- 
phere and the Earth’s Surface). 
Until such a method has been perfected (see an article by 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 Marié-Davy and others; but the reader must bear in 
mind that these results from the hypothesis assumed by Marié-Davy 
that the observed difference 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 Marié-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 degrees=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, 6, 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 6 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 with it should be associated a 
simultaneous record of the cloud or haze as given by the sunshine 
recorder. 
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