April 24, 1908] 



SCIENCE 



663 



cation work and cooperate in every possible 

 way to secure for the biological workers of 

 the country the form of publication and dis- 

 tribution which their researches deserve. 



SOCIETIES AND ACADEMIES 



THE WASHINGTON ACADEMY OF SCIENCES 



Mr. C. G. Abbot, the director of the Astro- 

 physical Observatory of the Smithsonian In- 

 stitution, delivered before the academy, March 

 24, an address on " Recent Studies of the 

 Sun." 



Mr. Abbot gave a summary of the re- 

 searches included in Volume II. of the Annals 

 of the Astrophysical Observatory of the 

 Smithsonian Institution, now about to be 

 issued. Besides this he gave a brief account 

 of the Smithsonian expedition to observe the 

 total solar eclipse of January 3, 1908. 



The mean value of the solar constant of 

 radiation in calories per square centimeter 

 per minute from 44 observations at Washing- 

 ton, D. C, 1902-6, is 2.061; from 59 observa- 

 tions on Mt. Wilson, California, in 1905, it 

 was 2.024, and from 62 observations at Mt. 

 Wilson in 1906 it was 2.020. Langley thought 

 it necessary to add about one third to his solar 

 constant value from Mt. Whitney observations 

 of 1881 because of a supposed failure of Bou- 

 guer's transmission formula. This correction 

 does not appear to be justified, and Langley's 

 values should be as follows : For Lone Pine 

 2.06, for Mountain Camp 2.22, and their dif- 

 ference is reasonably attributed to experi- 

 mental error, not difference of altitude. The 

 fact that so good agreement between the 

 Washington, Lone Pine, Mt. Wilson and Mt. 

 Whitney values is found makes it most prob- 

 able that the true solar constant value differs 

 very little from 2.1 calories. It was shown 

 from the temperature of the earth's radiating 

 surface that the solar constant can not exceed 

 2.33 calories unless the reflecting power of the 

 earth as a planet exceeds 37 per cent. The 

 latter value was derived by measuring the 

 reflecting power of clouds and other terrestrial 

 surfaces. 



Variations of the solar constant values were 

 noted both in Washington and on Mt. Wilson, 

 and these are so large and so well established 



by observation as to warrant the continuation 

 of solar constant work at two observatories in 

 cloudless regions of the earth well separated 

 from one another. A study of the surface 

 temperatures of the earth at 48 inland sta- 

 tions widely distributed over the globe indi- 

 cates that general variations of temperature 

 have occurred which may have been caused 

 by solar variations of short period. The sun- 

 spot cycle is clearly associated with a tem- 

 perature variation; for higher temperatures 

 occur at sun-spot minimum. 



The variation of brightness of the sun's 

 disk from center to limb has been observed 

 for various wave-lengths of light, and on nu- 

 merous days of observation. Changes of the 

 rate of this variation have been noted from 

 time to time, and these changes may prove to 

 be associated with variations of the solar con- 

 stant of radiation. Probably the cause of the 

 decreased brightness near the sun's limb is the 

 lower temperature of the sources of light near 

 the limb, due to the fact that the scattering 

 of light by the molecules of the gases of the 

 sun prevents us from seeing as deep near the 

 limb as at the center of the disk. The scat- 

 tering of rays is so great in the atmosphere 

 of the earth that, reasoning by analogy, scat- 

 tering probably prevents us from seeing at the 

 center of the sun's disk as much as 1 per cent, 

 of the solar radius below the outer photo- 

 spheric layers, and far less even than this at 

 the sun's limb, owin^ to the greater length 

 of path of the rays to a layer of given depth. 

 This explains the apparently sharp boundary 

 of the sun's disk, notwithstanding the neces- 

 sity of admitting the gaseous nature of the 

 sun on account of its extremely high tem- 

 perature. 



The Smithsonian Institution sent Messrs. 

 Abbot and Moore to Flint Island by invitation 

 of Director Campbell, of the Lick Observatory. 

 They observed there, on January 3, 1908, the 

 intensity of the rays of the solar corona at 

 five points, and found them at brightest only 

 1/1,000,000 as bright as sun rays. They em- 

 ployed a bolometer in focus of a twenty-inch 

 equatorial reflecting telescope. Glass was in 

 front of the bolometer to prevent exchanges 

 of long wave-length rays. By means of an 



