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AREQUIPA PYRHELIOMETRY 

 By C. G. ABBOT 1 



In 1910 the Committee on Solar Radiation of the International 

 Union for Cooperation in Solar Research recommended that regular 

 observations of the intensity of solar radiation should be under- 

 taken at additional stations in relatively cloudless regions far re- 

 moved from existing stations. Prof. E. C. Pickering thereupon 

 offered to undertake such observations at the Arequipa, Peru, 

 station of the Harvard College Observatory if suitable apparatus 

 should be furnished. In conversation between Messrs. Pickering 

 and Abbot it appeared inexpedient to undertake a complete spectro- 

 bolometric program for the determination of the solar constant of 

 radiation, but pyrheliometric observations were proposed whenever 

 weather should permit. 



By authority of the Secretary of the Smithsonian Institution, a 

 silver disk pyrheliometer was lent for the purpose. This unfortu- 

 nately was broken in transportation, and much time was lost owing 

 to the delays of communication, so that it was not until the summer 

 of 1912 that silver disk pyrheliometer S. I. 17 arrived at Arequipa. 

 This instrument also was damaged in transportation, by loss of 

 mercury from the cavity in the silver disk. But this defect was 

 skillfully repaired by Senor J. E. Muniz. 



It is probable that this alteration involved some slight change in 

 the constant of the instrument, but probably not more than 1 per 

 cent. Until we obtain further knowledge we may therefore retain 

 the value of the constant as stated in " Smithsonian Pyrheliometry 

 Revised," namely, 0.3635. 



Individual measurements were made at Arequipa in the manner 

 described in the publication just cited. The general plan of the 

 work, as proposed by Mr. Abbot, was to secure measurements of the 

 pyrheliometer and psychrometer at highest sun, and also at a solar 

 zenith distance of about 70 , corresponding to three times the path 

 in air which obtains at zenith sun. Some delay occurred in making 

 these requirements fully understood at Arequipa, and it is to be 



1 Published by the Smithsonian Institution by request of Director E. C. 

 Pickering of the Harvard College Observatory. 



Smithsonian Miscellaneous Collections, Vol. 65, No. 9. 



