Chapter V. 



THE METHODS OF MEASURING DIRECT OR DIFFUSE SUNSHINE 

 AS TO INTENSITY OR DURATION. 



Sunshine may be measured as to its quality or wave length, its 

 intensity, or its duration. The methods used in measuring either of ' 

 these must be understood in order to intelligently compare the pub- 

 lished observations with phsenological phenomena. The following 

 section considers some of the methods of measuring or registering the 

 duration or intensity of sunshine, or the intensity of the skylight, 

 at least in so far as these have been used in agricultural studies. 



THEORETICAL RELATION OF DIRECT AND DIFFUSED SUNSHINE. 



The relative intensity of any radiation may be measured by its heat 

 or light or chemical effect. The insolation received by a horizontal 

 surface, whether directly from the sun or diffusely from the sky, is 

 subject in a general way to calculation, but the irregularities intro- 

 duced by haze and clouds can not be so calculated and must be ob- 

 served daily. The following table gives, for a clear blue sky, the 

 values obtained by Clausius for the radiation (S) that falls upon a 

 horizontal surface directly from the sun, and in the third column 

 the diffuse radiation (C) that falls from the whole sky upon that 

 same surface; the total radiation (S-j-C) is the sum of these two. 

 If, however, the surface is normal to the sunlight, instead of hori- 

 zontal, it receives the quantity in the fifth column (I) directly from 

 the sun, and (c) which is less than the quantity (C) from the sky, 

 depending upon the altitude of the sun, the total being, as before, the 

 sum of these (I-|-c). The study of these columns shows us the 

 maximum and minimum amounts of sunshine that may fall upon a 

 given leaf surface, since a leaf will in general be in some position to 

 receive the full sunshine normally to its surface, Avhile others will be 

 horizontal, or vertical, or in the shade, and receive only a part of the 

 diffuse light from the sky. 



It is assumed by Radau, in his actinometry (1877), as also by 

 Marie-Davy, that the bright and black bulb thermometers in vacuo, or 

 the so-called *' conjugate thermometers." give us the total radiation 

 (C-f I) as for the horizontal surface, and that this is the quantity in 

 which vegetation is interested. 



2607—05 M 6 (81) 



