346 



SCIENCE 



[N. S. Vol. XXXIX. No. 1001 



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 observ- 

 ing station on days when the sky condi- 

 tions 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 hap-hazard character. I 

 mean by this that the values would pro- 

 gress 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 direc- 

 tion through other successive days, and do 

 not fluctuate widely from high values to 

 low as would be the case if the irreg- 

 ularities were due merely to instrumental 

 error. Since, then, it appeared that the 

 fluctuations were neither of an accidental 

 instrumental character nor of a character 

 associated with the altitude of the observ- 

 ing station, it appeared most reasonable to 

 conclude that these fluctuations were due 

 to changes in the sun's emission. 



To test this important conclusion it ap- 

 peared necessary to establish a second sta- 

 tion, 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 

 alter the results at both stations in the 

 same direction on the same day. Such a 

 station was established at Bassour, Algeria, 

 in the years 1911 and 1912. Seventy-five 

 flays of simultaneous measurement at Mount 

 Wilson and at Bassour were obtained, and 

 of these days about 50 were so far free 

 from the occurrence of clouds or other dis- 

 turbing influences at both stations as to be 

 retained for purposes of comparison. The 

 result of the comparison shows that when 



high values are obtained at Bassour, high 

 values are obtained also at Mount Wilson 

 and vice versa. Thus the fluctuations 

 which have been found appear to be truly 

 existing in the solar radiation outside the 

 earth's atmosphere, for the solar constant 

 values obtained at two stations separated 

 by about one third the circumference of 

 the earth unite in showing them. 



VALUE OF THE SOLAE CONSTANT 



During the whole solar constant cam- 

 paign from 1902 to 1913, about 700 meas- 

 urements of the solar constant of radiation 

 have been obtained, all but three of the 

 values ranging between 1.80 calories and 

 2.10 calories. The range of these numbers 

 is mainly attributable to the actual fluctua- 

 tion of the sun itself, though part, espe- 

 cially in Washington work, is due to acci- 

 dental errors of measurement. The mean 

 value from 690 measurements is 1.933 cal- 

 ories per square centimeter per minute. It 

 is believed that this number represents the 

 average value of the solar constant of 

 radiation for the epoch 1902 to 1913 

 within 1 per cent. There is still the pos- 

 sibility, however, that an appreciable 

 quantity of solar radiation beyond the 

 wave-length of 0.3 ju in the ultra-violet has 

 been absorbed by ozone in the higher 

 atmosphere of the earth, and has been im- 

 possible of determination at the stations 

 employed. However, from the considera- 

 tion of the form of energy curve of the 

 sun's spectrum it is improbable that this 

 lost ultra-violet radiation can exceed 1 or 

 2 per cent. 



SOLAE VAEIABILITT ASSOCIATED WITH 

 SUN-SPOTS 



Besides the short irregular fluctuation 

 of solar radiation above mentioned as hav- 

 ing been shown by the simultaneous meas- 

 urements at Mount Wilson and Bassour, 



