48 NATIONAL ACADEMY OF SCIENCES 



onstrated the existence of solar magnetism, it is confined to 

 limited areas. We must look further if we would throw new 

 light on the theory of the magnetic properties of rotating bodies. 



This leads us to the question with which we started : is the sun 

 a magnet like the earth? The structure of the corona, as re- 

 vealed at total eclipses, points strongly in this direction. Re- 

 membering the lines of force of our magnetized steel sphere, we 

 can not fail to be struck by their close resemblance to the polar 

 streamers in these beautiful photographs of the corona (Fig. 12) 

 taken by Lick Observatory eclipse parties, for which I am 

 indebted to Professor Campbell. Bigelow, in 1889, investigated 

 this coronal structure, and showed that it is very similar to the 

 lines of force of a spherical magnet. Stormer, guided by his 

 own researches on the aurora, has calculated the trajectories of 

 electrons moving out from the sun under the influence of a gen- 

 eral magnetic field, and compared these trajectories with the 

 coronal streamers.* The resemblance is apparently too close to 

 be the result of chance. Finally, Deslandres has investigated 

 the forms and motions of solar prominences, which he finds to 

 behave as they would in a magnetic field of intensity about one 

 millionth that of the earth. We may thus infer the existence of 

 a general solar magnetic field. But since the sign of the charge 

 of the outflowing electrons is not certainly known, we can not 

 determine the polarity of the sun in this way. Furthermore, our 

 present uncertainty as to the proportion at different levels of 

 positive and negative electrons, and of the perturbations due to 

 currents in the solar atmosphere, must delay the most effective 

 application of these methods, though they promise much future 

 knowledge of the magnetic field at high levels in the solar at- 

 mosphere. 



Of the field at low levels, however, they may tell us little or 

 nothing. To detect this low-level field we must resort to the 

 method employed in the case of sun-spots the study of the Zee- 

 man effect. If this is successful, it will not only show beyond 

 doubt whether the sun is a magnet: it will also permit the polarity 



* See also the important investigations of Birkeland. 



