Bigelow — Magnetic Theory of the Solar Corona. 257 



magnetization and high temperature in combination in the 

 central portions. 



Instructive as the preceding line of investigation has already 

 become it will doubtless be much more profitable in further 

 studies, such as it is possible to bestow whenever the minimum 

 of the sun-spot period prevails. I hope to continue this 

 research on the coronal photographs, as soon as the results of 

 the eclipse of 1901 are in hand. The pictures of 1898, 1900 

 and 1901 may then be combined with the preceding computa- 

 tions, not only for a closer location of the coronal poles, but 

 also for the period of revolution of the solar nucleus, as dis- 

 tinguished from that of the sun-spot belts, which are evidently 

 an atmospheric drift over a more stable central body. We are 

 not, however, limited to this single line of attack on the prob- 

 lem of the solar magnetic field, but possess at another place 

 on the solar magnetic lines, namely, in the earth's atmosphere, 

 an opening of equal validity, and apparently of a practical 

 nature. This has been gradually built up, as in the preceding 

 case, out of several different researches, at first independent of 

 each other. 



An example of the method employed for obtaining the 

 deflecting vectors in the earth's magnetic field, which are 

 observed to be impressed upon its normal field, is contained in 

 " Solar and Terrestrial Magnetism," Chapter 3, Bulletin No. 

 21, W. B., 1898. The earth's field is disturbed from its 

 normal form by the addition of forces whose direction in 

 space, whatever may be their cause, can be computed mechan- 

 ically by combining the residuals of the given rectangular 

 coordinate vectors into a single resultant vector located by 

 polar' coordinates. Taking the available magnetic stations in 

 different latitudes, 26 in number, and using the mean diurnal 

 elements, the horizontal force, the declination, and the vertical 

 force, so that no question of geographical longitude is involved, 

 the conclusion is reached that the disturbing vectors are 

 closely confined to north and south meridian planes ; the 

 lengths and angles that the vectors make with the surface at 

 different latitudes are shown on Chart 10 of that chapter, 

 which represents one of the meridian sections described. This 

 is reproduced in the wave line of the adjacent diagram accom- 

 panying this paper, and it is indicated by the smooth inflexed 

 full line. The vector directions are here omitted, as they can 

 be readily explained by the theory of a permeable shell 

 immersed in an external magnetic field. The peculiar curva- 

 ture of the line that was found to bound the average length of 

 these vectors is, however, one that has been difficult to account 

 for, and lias caused some students to doubt the reliability of 

 my result. But, in fact, it is now believed to be a substantial 



