APPENDIX III 1 



SOME PYRHELIOMETRIC OBSERVATIONS ON MOUNT 

 WHITNEY 



By A. K. Angstrom and E. H. Kennard 



In the summer of 1913 an expedition supported by a grant from the 

 Smithsonian Institution proceeded to California in order to study the noc- 

 turnal radiation under different atmospheric conditions. In connection with 

 these investigations we had an opportunity to measure the intensity of the 

 solar radiation during seven clear days on the summit of Mount Whitney 

 (4,420 m.). These measurements were made for different air masses and 

 include observations of the total radiation and of the radiation in a special 

 part of the spectrum, selected by means of an absorbing screen, as had been 

 proposed by K. Angstrom. 2 Our paper will present the results of the observa- 

 tions and a computation from them of the solar constant. 



INSTRUMENTS 



The observations were made with Angstrom's pyrheliometer No. 158. With 

 this instrument the energy of the radiation falling" upon the exposed strip is 

 given in calories per square centimeter per minute by the relation I = kC*, 

 where C is the compensating current sent through the shadowed strip, and k is 

 a constant which was determined for this instrument at the solar observatory 

 of the Physical Institute in Upsala and found to be 13. s8. 3 The compensating 

 current was furnished by four dry cells, which proved entirely suited to the 

 purpose. It was measured by a Siemens and Halske milliammeter. For 

 further details of the instrument and the method of using it, we refer to the 

 original paper. 4 



The absorbing screen, used in order to study a limited part of the spectrum, 

 was composed of a water cell, in which the water layer had a thickness of 

 1 cm., and a colored glass plate, Schott and Genossen, 436 111 , the thickness of 

 which was 2.53 mm. The transmission of the combination for different wave 

 lengths as previously determined at Upsala by Mr. A. K. Angstrom is given 

 in figure 16. The maximum of transmission occurs at wave length 0.526 \i, 

 and 85 per cent of the transmitted light is included between 0.484 \i and 0.570 \i. 



1 Reprinted by permission from the Astrophysical Journal, Vol. 39, No. 4, 

 PP. 350-360. 



2 Nova Acta Reg. Soc, Sc. Upsal., Ser. IV, 1, No. 7. 



3 A comparison made at the Smithsonian Institution in Washington showed 

 that the readings of this instrument are 4.57 per cent lower than the Smith- 

 sonian scale. 



4 Astrophysical Journal, 9, 332, 1899. 



ISO 



