SMITHSONIAN PYRHELIOMETRY REVISED 

 By C. G. ABBOT, Director, and L. B. ALDRICH, Bolometric Assistant, 



ASTROPHYSICAL OBSERVATORY OF THE SMITHSONIAN INSTITUTION. 



In a paper entitled " The Silver Disk Pyrheliometer " 1 it was 

 stated that in order to promote pyrheliometric measurements of the 

 solar radiation in other parts of the world with instruments whose 

 indications are quite comparable, several copies of the Silver Disk 

 Pyrheliometer have been sent out by the Smithsonian Institution. 

 The number of these instruments which have now been sent out has 

 reached about twenty. The present paper gives a revision of the 

 constants of these instruments and a statement of their dependence 

 on experiments to determine the standard scale of radiation. 



Three copies of the Standard Water-flow Pyrheliometer 2 have 

 been prepared at the shop of the Astrophysical Observatory. The 

 principle of these instruments consists in receiving the solar radia- 

 tion in a blackened chamber composing a perfect absorber or " ab- 

 solutely black body " and in carrying away the heat developed as 

 fast as formed by a current of water circulating around in the walls 

 of the receiving chamber. The rate of flow of the water, the rise of 

 temperature due to the solar heating and the aperture through 

 which the solar rays enter being known, the heating due to the solar 

 rays is determined in calories per square centimeter per minute. In 

 test experiments heat may be introduced electrically within coils in 

 the absorption chamber, and this may be measured as if it were solar 

 heat. The complete recovery of such test quantities of heat serves to 

 prove the accuracy of the instrument. 



Quite recently a new standard pyrheliometer which we have 

 called " Standard Water-stir Pyrheliometer No. 4 " has been devised 

 and tested by us. This instrument employs the ordinary method 

 of calorimetry. A blackened tubular chamber for the absorption of 

 the solar heat is provided as for the water-flow pyrheliometer. In 

 the new instrument the absorption chamber is enclosed by a known 

 quantity of water in a copper vessel, so that the whole apparatus 

 comprises what is in effect a calorimeter for the method of mixtures. 



1 Smithsonian Miscellaneous Collections, Vol. 56, No. 19. 

 " See Annals, Astrophysical Observatory, Vol. 2, pp. 39 to 47, 1908. Also 

 The Astrophysical Journal, Vol. 33, pp. 125 to 129, 191 1. 



Smithsonian Miscellaneous Collections, Vol. 60, No. 18 



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