85 



Thichncss of the layer of air trarrrscd hii the ilireci solar rai/ff as coiiipiitnl lijf 

 Laphiec'ft formula, and the vorrvsjHtndhin raliir of I. the absolute inhasit n of 

 direet sunshine in calories per minute /ler stjuarc ci-iiliiuctcr irhieli fall nor- 

 mally on any surface through the purest air. as i/inii Ini 1 o///r. 



Observation shows that no two such Arago-Davy actinometers 

 placed side by side will give exactly the same results; therefore the 

 rule has been adopted of comparing all instruments with the stand- 

 ard kept at INIont.souris, and a standardizing factor is thereby obtained 

 by Avhich the observed difference between the bright and black bulb 

 of any pair is to be multiplied in order to reduce it to a common 

 standard. 



In addition to the standardizing factor of the preceding paragraph, 

 Marie-Davy has also introduced the conception of an ideal standard 

 actinometer, graduated in such a way that the first factor, 17° in 

 the above-given formula as expressed in centigrade degrees, shall 

 be represented by 100 " actinometric degrees '* in his ideal in.stru- 

 ment ; that is to say, all the differences (t-t^) observed with any 

 actinometer, after being multiplied by the standardizing factor, have 

 still to be multiplied by the factor 5. 88 in order to convert them into 

 ideal actinometric degrees. For convenience both these factors may 

 be replaced by one, and in this way the instrument and Mario-Davy's 

 methods have been extensively employed in studying the relation 

 between sunshine and crops. 



In such study Marie-Davy and his pupils take the " sum of the 

 total number of actinometric degrees " as the datum for comparison 

 with crop reports, instead of the sum of the temperatures of the air 

 observed in the shade, or the sum of the soil temperatures as used 

 by other investigators. If we divide the actinometric degrees given 

 in any case by the factor 5.88 we shall obtain the excess of the black 

 bulb over the bright bulb as originally observed in centigrade 

 degrees. P^rom this we can obtain the true rehitive (|uantities of solar 

 radiation by a modification of the nielhod given by Ferrel (pp. -H-HO 



