abbot: solar constant of radiation 105 



hollow-chamber instrument in which the chamber was bathed 

 with stirred water was employed to check the results of the 

 standard water-flow instruments. In each of these types of 

 standard instruments it is possible to introduce electrically known 

 quantities of heat for testing purposes, and in many experiments 

 it has been proved that the test quantities of heat thus intro- 

 duced may be recovered to within 1 per cent. Accordingly it is 

 believed that the standard scale of radiation has been thoroughly 

 established. The silver disk instruments are standardized by com- 

 paring them with such standard instruments, and the standard 

 scale of radiation so produced, which is believed to be accurate 

 to at least one-half of 1 per cent, has been diffused generally over 

 the world by the Smithsonian Institution. About twenty-five 

 copies of the silver disk pyrheliometer have been standardized 

 and sent out to Europe, North America, and South America for 

 this, purpose. The Smithsonian instruments read about 3.5 per 



o 



cent above those of Angstrom. 



jNIeasurements of the solar constant of radiation were begun 

 in Washington in 1902 and have been continued at Washing- 

 ton or elsewhere in every succeeding year until the present 

 time. In 1903 it was noticed that the values of the solar radia- 

 tion outside the atmosphere obtained in Washington were vari- 

 able within the limits of about 10 per cent, and as some of the 

 changes appeared to occur between days which were of the high- 

 est order of excellence, it was thought possible that these changes 

 might occur in the sun, and not be caused by alterations of the' 

 transparency of the earth's atmosphere. To test this possibil- 

 ity, a station was established on ]\lount Wilson, California, in 

 1905 by invitation of Director Hale of the Mount Wilson Solar 

 Observatory. The station proved to be very favorable for the 

 work, and in 1908 a permanent structure of cement was built 

 there for the use of the Smithsonian Astrophysical Observatory. 

 In the years 1909 and 1910 spectro-bolometric observations for 

 the determination of the solar constant of radiation were also 

 made on the extreme summit of Mount Whitney in California 

 at an altitude of 4420 meters. At the same time observations 

 were being made at Mount Wilson at .an altitude of 1730 meters. 



