SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. DO 



The water in the instrument is vigorously stirred ' by means of a 

 stirring device run by an electric motor. A platinum resistance 

 thermometer, fully bathed by the water of the pyrheliometer serves 

 to determine its rate of rise of temperature due to the absorption of 

 solar rays, and also its rates of change of temperature before and 

 after exposure to these rays, due to the influence of the surroundings. 

 In this manner we may determine the intensity of the radiation of 

 the sun in terms of the rise of temperature per minute of a calorimeter 

 of known water equivalent, and we may be assured that the solar 

 rays are completely absorbed to produce heat, because they are 

 absorbed by a blackened surface forming the inside of a deep chamber 

 closely approximating to the " absolutely black body." In this new 

 instrument, as in the water-flow pyrheliometers, means are pro- 

 vided for the introduction electrically of known quantities of heat 

 to test the accuracy of the apparatus. A full description of the 

 new water-stir pyrheliometer and of the water-flow pyrheliometers, 

 Nos. 2 and 3, will appear in Vol. 3 of the Annals of the Astrophysical 

 Observatory, now in preparation. 



The following table is a summary of the experiments made with 

 test quantities of electrical heating with standard pyrheliometers 

 Nos. 2,3 and 4: 



Pyrheliometer. 



Water-flow 2 .. 

 Water-flow 3 . . 

 Water-flow 3 . . 

 Water-stir 4. . . 



Dates. 



1910— May 12, 16, 25, 26, 31; June 7 



1910 — April 18, 22, 23 



191 1 — Oct. 10, u 



1912 — Oct. 24, 25 



Average 

 deviation. 



Per cent. 

 1.8 

 0.63 

 1.4 

 0-53 



From these experiments it appears that the test quantities of 

 electrical heating were recovered by each instrument to within one 

 per cent of the quantity introduced. 



The results of comparisons of the standard pyrheliometers with 

 secondary pyrheliometers are summarized in the following table: 



Dates. 



1910 — May 10, 17, 28; June 4 



1910 — Apr. 22 



1910 — Oct. 31 ; Nov. 1 



■June 27 



19 



1911 — Oct. 14, 15, 16, 22, 25, 31; 



No v. 2, 6, 7 



1912 — Nov. 16, 19, 21 



Probable 

 error. 



0.0022 

 0.0009 

 0.0013 



o.oniS 

 O.OOII 



0.0007 



0.0012 



1 Successful use requires rapid stirring. 



