106 abbot: solar constant of radiation 



The results from these two stations reduced to outside the atmos- 

 phere at mean solar distance, like those which had formerly been 

 obtained simultaneously at Washington and Mount Wilson, were 

 identical within the limit of the accuracy of the determinations. 

 The accuracy of the work at Mount Wilson and Mount Whitney 

 was so great that the average divergence between the observa- 

 tions of the same days was only 1 per cent. At Washington, the 

 sky conditions being less perfect, the average divergence from 

 simultaneous solar-constant results of Mount Wilson was about 

 3 per cent. 



Evidences of solar variahility of short irregular periods. Numer- 

 ous observations of several years at Mount Wilson indicated a 

 fluctuation in the solar-constant values having a range of about 

 10 per cent. The fluctuations seemed to occur irregularly, some- 

 times running their course of 10 per cent or less within the period 

 of a week or ten days, and at other times keeping nearly constant. 

 It had been shown by the observations made simultaneously at 

 Mount Wilson and at Mount Whitney that the results, as re- 

 duced outside the atmosphere, appear to be independent of the 

 altitude of the observing station, on days when the sky conditions 

 appeared to the eye to be excellent. The march of the apparent 

 fluctuation of the solar constant values at Mount Wilson has not 

 been of a haphazard character. I mean by this that the values 

 progress in a definite direction, as for instance from a low value 

 to a high value by steps through several successive days, and 

 then as definitely progress in the opposite direction through other 

 successive days, and do not fluctuate widely from high values 

 to low, as would be the case if the irregularities were due merely 

 to instrumental error. Since, then, it appeared that the fluctua- 

 tions were neither of an accidental instrumental character, nor 

 of a character associated with the altitude of the observing sta- 

 tion, it appeared most reasonable to conclude that these fluctua- 

 tions were due to changes in the sun's emission. 



To test this important conclusion it appeared necessary to 

 establish a second station, equally favorably situated with re- 

 gard to sky conditions as Mount Wilson, but so far remote from 

 Mount Wilson that local influences could not be expected to 



