2'3 



clear, partly cloudy, and cloudy days is next determined for 



the days August 1 to August 7, inclusive, and some arbitrary 



v;eighting is given to eahh kind of day. This was done in 



the oresent instance by recarding days reported "clear" as 

 i;7hole days 



of sunshine, those reported "partly cloudy" as half days of 

 sunshine, and those reported "cloudy" as without any sunshine. 

 The same scheme of weighting must of course be adhered to in 

 all the estimates used for comparative purposes in any dis- 

 cussion. By summing these weighted daily values a number is 

 obtained that represents the equivalent number of clear days 

 for the period considered. Suppose, in the example selected, 

 that this equivalent number of clear days 's 3.5 v;hich is 0.5 

 of the total number of days in the week period. The latter 

 value Tiay be termed "the coefficient of clear weather". By 

 multiplying the average daily intensity value for clear days, 

 as already obtained, by this^oeff icient of clear weather a 

 value is secured tha. t may be taken as a rough approximation 

 of the average daily sunshine index for the v/eek. 



While it is certain that solar radiation affects plants 

 in other ways than through its heating effect, it is no less 

 certain that by far the greater part of the energy of susishine 

 absorbed by plants is converted into heat (largely as latent 

 heat of vaporization), and it seems probable that the other 

 effects produced upon the plant may be more or less proportion- 

 al to the total energy equivalent of su shine. This method 

 of deriving sunshine indices, although it is to be taken as only 

 a rough approximation, has been shown, as a matter of -^'act, to 

 give quantities rather definitely correlated with the plant 

 groivth values in this study. It has been found, for instance, 

 that the amount of dry substance produced 



