Makch 6, 1914] 



SCIENCE 



845 



standard primary instrument to compare it 

 with. As early as the year 1904 experi- 

 ments were begun to produce a pyrheliom- 

 eter based upon the hollow chamber 

 "black body" type, with a flowing liquid 

 to carry off the heat produced by the ab- 

 sorption of the solar rays within such a 

 chamber. After numerous experiments and 

 long trial the waterflow standard pyrhe- 

 liometer was fully developed in the year 

 1910. Later still, another hollow chamber 

 instrument in which the chamber is bathed 

 with stirred water was employed to check 

 the results of the standard water-flow in- 

 struments. In each of these types of stand- 

 ard instruments it is possible to introduce 

 electrically known quantities of heat for 

 testing pui-poses, and in many experiments 

 it has been proved that the test quantities 

 of heat thus introduced may be recovered 

 to within 1 per cent. Accordingly it is be- 

 lieved that the standard scale of radiation 

 has been thoroughly established. The silver 

 disk instruments are standardized by com- 

 paring them with such standard instru- 

 ments, and the standard scale of radiation 

 so produced, which is believed to be accu- 

 rate to at least % of 1 per cent., has been 

 diffused generally over the world by the 

 Smithsonian Institution. About 25 copies 

 of the silver-disk pyrheliometer have been 

 standardized and sent out to Europe, North 

 America and South America for this pur- 

 pose. The Smithsonian instruments read 

 about 3.5 per cent, above those of Angstrom. 

 Measurements of the solar constant of 

 radiation were begun in Washington in 

 the year 1902 and have been continued at 

 Washington and elsewhere in every suc- 

 ceeding year until the present time. In 

 1903 it was noticed that the values of the 

 solar radiation outside the atmosphere ob- 

 tained in Washington were distinctly 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 pos- 

 sible 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 possibility, a station was estab- 

 lished on Mount Wilson, California, in 

 the year 1905 by invitation of Director 

 Hale of the Mount Wilson Solar Observa- 

 tory. The station proved to be very favor- 

 able for the work, and in 1908 a perma- 

 nent structure of cement was built there 

 for the use of the Smithsonian Astrophys- 

 ical 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 4,420 meters. At the same 

 time observations were being made at Mount 

 Wilson at an altitude of 1,730 meters. The 

 results from these two stations reduced to 

 outside the atmosphere at mean solar dis- 

 tance, like those which had formerly been 

 obtained simultaneously at Washington 

 and Mount Wilson, were identical within 

 the limit of the accuracy of the determina- 

 tions. The accuracy of the work at Mount 

 Wilson and Mount Whitney was so great 

 that the average divergence between the 

 observations of the same days was only 1 

 per cent. At Washington the sky condi- 

 tions being less perfect, the average diver- 

 gence from simultaneous solar-constant 

 results of Mount Wilson was about 3 per 

 cent. 



EVIDENCES OP SOLAR VABIABILITY OP SHORT 

 IRREGULAR PERIODS 



Numerous observations of several years 

 at Mount Wilson indicated a fluctuation in 

 the solar constant values having a range 

 of about 10 per cent. The fluctuations 

 seemed to occur irregularly, sometimes 

 running their coiirse of 10 per cent, or less 



