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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



[VOL. 47 



get a value of the solar radiation at least partly corrected for the 

 absorption in the earth's atmosphere, the observations are plotted 

 with air-masses as abscissae, and logarithms of the radiation meas- 

 ures as ordinates. 



Now the range of air-masses is such that the points thus plotted 

 are very nearly linear, although a slight curvature convex towards 



Fig. 62. — Bonguer's Formula and Actinometry of Solar Radiation. Abscissae are 

 air-masses. Ordinates are logarithms of actinometer measures. 



the axis of abscissae might be expected in accordance with what 

 .Air. Langley has shown, and is indeed at times noticeable where the 

 range of air-masses is great. The straight line best representing 

 these points is then drawn and its intersection with the axis of or- 

 dinates (w = o) gives a preliminary too low value of the extra- 

 polated solar radiation. 



