wc 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 phenological 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+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 (1) 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, while 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 
Marié-Davy, that the bright and black bulb thermometers in vacuo, or 
the so-called “ conjugate thermometers,” give us the total radiation 
(C-+1) as for the horizontal surface, and that this is the quantity in 
which vegetation is interested. 
2667—05 M——6 (81) 
