326 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



selves, and though we found by these intercomparisons that the scale 

 remained unchanged from year to year, we yet felt sure that it was 

 not the standard scale of calories. In 1903, not knowing of Michel- 

 son's idea of a decade earlier, I conceived the idea of employing the 

 hollow chamber, or " absolutely black-body " principle. Instead of 

 combining with it the Bunsen ice-calorimeter method I proposed to 

 make the walls of the chamber hollow and to circulate a measured 

 current of water through them, which should carry off the heat as 

 fast as formed. The rise of temperature of this water I proposed to 

 measure by a platinum-resistance thermometer immersed half in the 

 incoming, half in the outflowing water. To test the accuracy of the 

 results I proposed to insert a coil of resistance wire within the cham- 

 ber and to measure a known quantity of electrical heating which 

 could be introduced thereby in terms of the rise of temperature it 

 produced in the water. This program, after seven j^ears, and the 

 successive building of three, all supposedly final, water-flow pyr- 

 heliometers is now satisfactorily completed. I can not praise too 

 highly Mr. Kramer's admirable skill in the construction of these 

 instruments. A careful comparison, completed by Mr. Aldrich this, 

 spring in Washington, of standard pyrheliometers No. 2 and No. 3 

 with secondary pyrheliometer No. 8, and through this with secondary 

 No. 4, used since 1906, on Mount Wilson, has resulted as follows : 



Constant of secondary pyrlieliometer No. J/. 



By standard No. 2 0. 8553 



By standard No. 3 0.8550 



In these comparisons electrical heating was frequently introduced 

 as a check, and the heat found seldom deviated more than 1 per cent 

 from 100 per cent of that introduced. On the average about 99.5 

 per cent was found. 



I consider that now the obstacle to solar-constant work called 

 " formidable " by Langley is overcome, and that we may know the 

 amount of solar heat received at the earth's surface within a quarter 

 of 1 per cent. 



It is not probable that I should have been here this evening if it 

 had not happened that our " solar constant " values of 1903 indicated 

 a fall of solar radiation of about 10 per cent at a time just before 

 there occurred a general fall of several degrees centigrade from the 

 normal temperature of the United States and Europe. This led to 

 the suspicion that the " solar constant " was a mii?nomer, and that 

 the sun's emission is reallj^ variable. After further studies in Wash- 

 ington, hindered by long periods of cloudiness, I was sent by Mr. 

 Langley in 1905, at Prof. Hale's invitation, to occupy for the summer 

 a temporary station here on Mount Wilson. As 1 was about to start 

 Mr. Langley directed me to remember that I was going not to fix the 



