RADIATION OF THE SUN^ — ABBOT. 165 



and compared, with solar-constant values from Mount Wilson, seem 

 to show that there is a slight but distinct change of distribution of 

 the radiation along the diameter of the sun's disk depending on the 

 value of the solar constant. This change is such that when the 

 intensity of solar radiation outside the atmosphere is high as observed 

 at Mount Wilson, the contrast in brightness between the center and 

 the edge of the sun's disk is great, and vice versa. This change in 

 contrast holds true for all wave lengths observed at Washmgton. 



Thus wo have two mdependent proofs that the sun is an iiTcgularly 

 variable star of a short irregular periodicity of 7 to 10 days and mag- 

 nitude of fluctuation reaching often 3 per cent but rarely 8 per cent. 

 Besides this newly discovered irregularity there is of course also the 

 long period variability of 11 years mdicated by sun spots. Curiously 

 enough the solar radiation proves to be above normal when the sun 

 spots are at a maximum ; although, as shown in figure 7, the tempera- 

 ture of the earth is then below the normal. It is not unlikely that 

 other solar changes of great interest may be brought to light in 

 future years by the methods above described. Nothing has yet been 

 done to see how far the newly discovered short period solar changes 

 affect the earth's climate, but evidently a most interesting field is 

 opening there. 



S5360°— SM 1912 12 



