NO. 5 SOLAR VARIATION AND FORECASTING ABBOT 27 



4. The probable error of a fairly satisfactory mean, daily value for 

 one of these stations alone is 0.0065 calorie, and for a fairly satis- 

 factory daily mean value derived from results of both stations it is 

 0.0046 calorie. 



5. From a study of numbers of observations, and their departures 

 from the monthly means, in connection with these values of the 

 probable error, it is found that many more departures of magnitudes 

 of from I to 2 per cent are found than should arise from accidental 

 error. This investigation ignores the still larger departures of longer 

 periods which attend changes in solar activity. 



6. The theory of probability allows us to entertain a belief in 

 short-period solar variations as well as in long-period ones. 



7. Both short- and long-period solar variations are associated with 

 observable changes in the appearance of the sun. 



8. Two stations 4,000 miles apart agree in disclosing both short- 

 and long-period solar variations of several per cent amplitude. 



9. Both short- and long-period solar variations are attended by 

 alterations in the form of the solar energy-spectrum distribution. 

 These alterations are far greater for short-.wave rays than for long- 

 wave rays. 



10. There is a twofold cause for solar variation. Long-period 

 fluctuations are due to changes in solar activity. Short-period fluc- 

 tuations are due to obscurations in the solar atmosphere, which, 

 rotating with the sun, produce depressions whenever they point 

 towards the earth. For this cause, solar variation is not closely cor- 

 related with sun spots, because, though numerous sun spots betoken 

 great solar activity and high solar constants, yet each individual sun 

 spot, as it passes through the sun's center, carries its obscuring 

 tendency, and produces a temporary depression of solar radiation 

 as viewed from the earth. 



