405 | F. M. Hildebrandt 207 
of using the graph and a weather observer’s report for estimat- 
ing sunshine will be best shown by an example. 
Suppose it is desired to estimate the average daily sunshine 
intensity for some station in the general region of Mount 
Weather, for the first week of August. The average ordinate 
value for this week is first obtained from Kimball’s graph. 
For periods as short as a week or two this may be done by 
averaging the values for the first and last days of the period, 
since the curve may be taken as a straight line for such short 
intervals. From the report of the weather observer at the 
place in question, the number of clear, partly cloudy, and 
cloudy days is next determined for the days August 1 to 
August 7, inclusive, and some arbitrary weighting is given 
to each kind of day. We may, for instance, call clear days 
whole days of sunshine, partly cloudy days half days of sun- 
shine, and assume that cloudy days are days without any sun- 
shine. The scheme of weighting adopted must, of course, be 
adhered to in all the estimates made for different periods and 
stations. The system of weighting given above was used in the 
studies for which this method of approximating sunshine was 
developed. By summing these weighted daily values a num- 
ber is obtained which represents the equivalent number of 
clear days for the period considered. Suppose, in the example 
selected, that this equivalent number of clear days is 3.5, 
which is 0.5 of the total number of days in the period. The 
latter value may be termed “ the coefficient of clear weather.” 
By multiplying the average daily intensity value for clear 
days, as obtained from the curve, by this coefficient of clear 
weather a number is secured that is a rough approximation 
of the average daily sunshine intensity for the week. 
While it is certain that solar radiation affects plants in 
other ways than through its heating effect, it is no less cer- 
tain that by far the greater part of the energy of sunshine 
absorbed by plants is converted into heat (largely as latent 
heat of the vaporization of water), and it seems probable that 
the other effects produced upon the plant may be more or less 
proportional to the total energy equivalent of sunshine. The 
