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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



rate of 1.93 calories per square centimeter per minute. What a prize 

 would be given to one who could bring us similar measurements 

 dating, let us say, from the time of Archimedes ! 



Before considering the possible variability of the sun we may pause 

 to note some of the by-products of the solar-constant measurements. 

 Referring again to figure 1, we see before us the distribution of 



Fig. 5.— Solar spectrum energy curves outside the earth's atmosphere. 



"All." — Radiation from the whole solar disk. 



" Center."— Kadlatioa from cenler of sun's disk. 



65, 82.5, and 95.— Radiation from points at these percentages of radius from center of sun's disk. 



energy in the sun's spectrum observed at a particular time on Mount 

 Wilson with certain apparatus. Knowing from other observations 

 of the same day the losses suffered by the rays in the air, and being 

 able to measure also the losses suffered in their passage of the optical 

 apparatus, it is possible to compute the form which the solar energy 

 spectrum would assume if observed outside our atmosphere, with 



