152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



pieces of apparatus developed for use in the investigation. Early in 

 the work it was recognized that a standard instrument for precision 

 measurements of the sun's heat was required. By 1910 there was de- 

 veloped the standard water-flow pyrheliometer. This instrument is a 

 hollow chamber of a test-tube like form having hollow walls in which 

 circulates in a spiral channel a current of water. The sun's rays enter 

 this chamber through a vestibule of constant temperature. Just before 

 reaching the chamber they pass through a circular aperture of known 

 area and shine upon the blackened wall at the chamber's extreme rear. 

 Any remnant not fully absorbed by the blackened wall is reflected to 

 and thrown upon other parts of the chamber wall until fully absorbed. 

 In the flowing water current, just at the mouth of the chamber, are 

 found the arms of a platinum electrical thermometer, by means of 

 which the rise of temperature of the water due to the heat absorbed 

 from the sun-rays within the chamber is measured. The water which 

 has flowed through the apparatus is collected and weighed from time 

 to time so as to determine the rate of flow. Thus the intensity of the 

 sun's heat is measured in terms of the rise of temperature of a known 

 weight of water caused by the absorption of solar rays over a known 

 area in a given time. In order to get a check upon the accuracy of the 

 measurement, known quantities of electricity are caused to flow over 

 coils within the chamber and the heat thus developed is measured, as 

 if it were solar heat, by the flowing water. 



Comparisons of this instrument with other special devices for 

 measuring solar heat have been made from time to time for the past 

 10 years. No change in the scale of measurement has been detected 

 within this interval. Thus we may regard the whole body of Mount 

 Wilson data as an homogeneous system of measurements of that fun- 

 damental quantity, the intensity of the solar radiation available to 

 warm the earth. 



Over 30 copies of the secondary so-called " silver-disk pyrhelio- 

 meter," figure 1, also devised for the investigation, have been prepared 

 and standardized at the Smithsonian Institution and supplied at cost 

 to observers over all the earth. Thus the Smithsonian standard scale 

 of radiation measurements has become widely diffused. 



RESULTS ACCOMPLISHED UP TO 1910. 



The chief results accomplished in the research up to 1910 had been 

 as follows : 



1. The processes for determining the intensity of the sun's radia- 

 tion outside the earth's atmosphere had been perfected and the whole 

 investigation reduced to a well-organized routine. 



2. A standard scale of radiation measurements had been estab- 

 lished by the invention and construction of the standard water-flow 

 pyrheliometer. The silver-disk secondary pyrheliometer had been 

 perfected and had proved fully satisfactory for the daily observa- 

 tions. 



