. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 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
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 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
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