DEPARTMENT OF TERRESTRIAL MAGNETISM. 317 



with the same accuracy as in land work. Special emphasis has been placed 

 upon reducing the observations promptly, and, in fact, the chief results are 

 made known to the leading hydrographic establishments within a few months 

 after the observations have actually been made. 



Errors of importance to mariners in the magnetic charts used have been 

 disclosed from time to time, as also matters of general geographical interest. 

 For fuller information reference must be made to the annual reports of the 

 Department, and especially to Volume III of Researches of the Department 

 of Terrestrial Magnetism, "Ocean Magnetic Observations 1905-1916 and 

 Reports on Special Researches." This latter volume contains a chart showing 

 the tracks of the chief vessels engaged in exploratory work in the Pacific 

 Ocean during the past 75 years. Volume IV of the Researches, giving an ac- 

 count of the cruises and work between 1917 and 1920, is in preparation. 



Results and analysis of magnetic observations during the solar eclipse of May 29, 1919. 

 By Louis A. Bauer.^ 



With the aid of magnetic data from 9 stations within the region of visibility 

 of the solar eclipse of May 29, 1919, 5 of these stations being those at which 

 observations were made by the expeditions of the Department of Terrestrial 

 Magnetism of the Carnegie Institution of Washington, and from about 18 

 cooperative stations distributed outside the region, the following main con- 

 clusions were drawn : 



(a) Magnetic effects of appreciable and determinable magnitude were 

 observed during the solar eclipse of May 29, 1919, at stations inside the region 

 of totahty as well as at certain stations in the sunlit region, the magnitude 

 and character of the effects being siinilar to those observed during previous 

 solar eclipses and showing a distinct connection with the eclipse circum- 

 stances. The magnetic data for stations in the night region of the globe did 

 not exhibit similar effects. 



(6) There were two principal variations (with some subordinate ones), as 

 shown especially at stations near the totality-belt, having periods approxi- 

 mating that of the entire ecHpse (5''10"'), and that of the local ecHpse (on 

 the average about 2 hours from first to last contact). There are evidences 

 that the effects continued for some time after the end of the eclipse at sunset 

 on the southeast coast of Africa. The amplitude (semi-range) of the short 

 wave was, on the average, about one-half of that for the long wave. In the 

 case of the m.agnetic declination, for example, the amplitude of the long wave 

 for stations inside or near the totality-belt approximated, on the average, 

 1 minute of arc, which was equivalent to a horizontal deflecting force of 

 about 0.01 per cent that of the average west-east component of the Earth's 

 magnetism. 



(c) A preliminary analysis of the magnetic effects at stations within the 

 region of visibility, or in close proximity, showed that the effects in declina- 

 tion and horizontal intensity were similar to those produced by a north-end 

 attracting focus located in the vicinity of the shadow-cone. With the aid 

 of the vertical-intensity effects it was found that the eclipse magnetic system 

 was composed of an external and an internal system of forces. At 12''30™ 

 G. M. T., May 29, 1919, just before the maximum development of the ecKpse 

 system, the north-end attracting focus of the external system was located 

 east-southeast of the shadow-cone, and that of the internal system was to 

 the northward of the cone and approximately northward of the point where 



^Presented before the Philosophical Society of Washington, October 9, 1920. See Terr. Mag., 

 Sept. 1920, pp. 81-98. 



