10 



AMPLITUDE OF THE SOLAR-DIURNAL VARIATION 



The distinctive features of the above table are next to be considered analytically 

 as well as graphically. The inequality in the diurnal variation is most conspicuous 

 when the tabular numbers in the horizontal lines for the months of February and 

 August are compared. The annual variation appears plainest by carrying the eye 

 over the vertical column at the hours 6 or 7 A. M. The annual variation depends 

 on the earth's position in its orbit; the diurnal variation being subject to an in- 

 equality depending on the sun's declination. The diurnal range is greater when 

 the sun has north declination, and smaller when south declination ; the pheno- 

 menon passing from one state to the other about the time of the equinoxes. To 

 show the diurnal variation at these periods, the summer and winter means, as well 

 as t lie annual means, were tabulated. The months from April to September (in- 

 clusive) comprise the summer period, and from October to March (inclusive) the 

 winter period. The first diagram (A) shows this variation, and contains the type 

 curves for these half yearly periods. We find for the summer months a diurnal 

 range of nearly LOi minutes, and for the winter months of but •">:■ minutes. These 

 and other curves will be further analyzed hereafter. 



