EADIATION OF THE SUN ABBOT. 



163 



years, corresponding with the fluctuations of the number of sun spots, 

 the changes in form of the solar corona, and other solar phenomena, 

 and inducing changes in the prevalence of the aurora borealis, the 

 magnetic force and declination, and other terrestrial phenomena. 

 Fisure 7 shows in its first curve the variation from normal of the tern- 

 l^eraturc of the United States, second the variation of sun-spot num- 

 bers, third and fourth the fluctuations in terrestrial magnetic elements 

 for the period 1878 to 1900. It would be of great interest if we had 

 accurate solar-constant measurements to compare with these changes 

 for a long period of time. Unfortunately we have none such earlier 

 than 1903 and only scattering measurements until 1905. 



In figure 8 the results of solar-constant measurements on Mount 

 Wilson, 1905 to 1909, are given. On their face the}^ indicate a fluctua_ 



Fig. 8.— Intensity of solae radiation outside the earth's atmosphere as measured at Mount 

 Wilson in the years 1905 to 1909. 



Values given in calorics per square centimeter per minute. 



tion of the intensity of solar radiation. The fluctuation extends over 

 a total range of nearly 10 per cent. In the later j^ears the measure- 

 ments were made almost daily, and seem to bring out with certainty 

 that the changes are not haphazard in their character. Successive 

 days of observation indicate a gradual march of the solar constant 

 values from a high to a low, and back. Had the variations been due 

 merely to accidental errors we should not have expected this regu- 

 larity. Hence we conclude that the changes observed are either 

 really solar or are due to some obscure source of error in estimating 

 the losses in our atmosphere. The latter supposition is not very 

 reasonable, because as already shown, we get practically identical solar 

 constant values whether we observe at sea level (Washington), 1 mile 

 (Mount Wilson), or 3 miles elevation (Mount Whitney). Hence it 



