282 CARNEGIE INSTITUTION OF WASHINGTON. 



RESEARCH WORK IN WASHINGTON. 

 TERRESTRIAL MAGNETISM. 



The receipt of the requisite data for reducing to a common standard 

 the magnetic results accumulated since the publication of Volumes II 

 and III of the ''Researches of the Department of Terrestrial Mag- 

 netism" will make it possible to complete early in 1921 the 

 manuscript for Volume IV. That volume will contain the final re- 

 sults of the land-magnetic observations 1914-1919 and the ocean work 

 1917-1919, as also some preliminary results for 1920. About ten 

 reports on special researches, covering the subjects of terrestrial 

 magnetism, terrestrial electricity, atmospheric refraction, and related 

 topics, will accompany the volume. 



The discussion of the results obtained from the geophysical obser- 

 vations by the Department and cooperating magnetic observatories 

 in connection with the famous solar eclipse of May 29, 1919, has proved 

 of extreme interest from various standpoints. The magnetic results 

 bear out the conclusions drawn from those of previous eclipses that 

 appreciable changes in the magnetic elements occur during an eclipse 

 of the Sun. These changes, while of subordinate magnitude, are 

 generally of the same character as the oscillatory changes which the 

 Earth's magnetic condition daily undergoes. In the case of the solar- 

 diurnal magnetic variation, during the Earth's rotation, an eclipse of 

 the Sun occurs daily, sunlight being cut off from one side of the Earth 

 by the opposite side. In the case of the eclipse magnetic variation, 

 it is the interposition of the Moon between the Sun and the Earth 

 that momentarily withdraws from the Earth the Sun's radL:itions, in 

 varying amount, over the region of visibility. Approximately it may 

 be said that the ranges of the solar-diurnal variation and of the eclipse 

 magnetic variation vary in accordance with the cross-section of the 

 Sun and Moon, respectively. It is believed that the investigation 

 of the eclipse magnetic phenomena will have an important bearing 

 upon the complete determination of the causes of the magnetic changes 

 which occur during both the solar day and the lunar day. (For 

 further information, see p. 298 and abstract, pp. 317 and 318. See 

 also pp. 307 and 319 regarding the eclipse electric results, as well as 

 abstract, p. 315, concerning possible bearing of the eclipse meteoro- 

 logical observations upon the deflections of light observed by the 

 British expeditions.) 



The discussion of the results of the intercomparisons of magnetic 

 standards, obtained by the observers of the Department in all regions 

 of the Earth, forms the subject of a special report for Volume IV, 

 referred to above. For assistance in the control of the magnetic 

 standards at Washington, it is hoped that before long the sine gal- 

 vanometer, referred to on page 281 of the 1919 annual report, will be 

 available . 



