191 1.] ABBOT— SOLAR CONSTANT OF RADIATION. 243 



Solar-Constant Mean Values. 



Our observations indicate as the mean value of the solar con- 

 stant of radiation : 



1.922 calories (15° C.) per square centimeter per minute. 



The observations having been obtained mainly near the time of 

 sun-spot maximum we think it probable that their mean is hardly 

 high enough to represent the average condition of the sun. We 

 incline to think this because it has been shown by Koppen, Nord- 

 mann. Xewcomb, Abbot and Fowle, Bigelow, Arctowski and others 

 that the earth's temperature is a little lower at sun-spot maximum 

 than at sun-spot minimum. This probable correction cannot exceed 

 one or two per cent. 



There is another reason why our value of the solar constant may 

 be too low. \\''e have not been able to observe, even on ]^Iount 

 Whitney, any radiation beyond the wave-length 0.29/i, in the ultra- 

 violet spectrum. Whether the rays of less wave-length are oblit- 

 erated in the earth's atmosphere or in that of the sun we cannot 

 know, but we do know that ozone, which is perhaps formed in the 

 upper atmosphere, exercises powerful selective absorption beyond 

 wave-length 0.29/x. Hence it may be that we are forced to neglect 

 some radiation not quite negligible. It is very improbable that the 

 amount thus neglected exceeds i or 2 per cent. 



As for the supposed variability of the sun, our determinations 

 strongly indicate that the so-called solar constant is not really a 

 constant, but fluctuates over a range of about 8 per cent. This 

 result is apparently the direct outcome of our observations, but the 

 question may well be asked if the apparent fluctuation is not due 

 either to the inaccuracy of the observations or to incorrect estimates 

 of the transmission of the atmosphere. If it were due merely to 

 accidental errors of observations, a gradual march, step by step, day 

 by day, from a low value to a high one and return woukl be the ex- 



PROC. AMER. PHIL. SOC. L. I99 P, PRINTED JUNE 29, I9II. 



