180 ANNUAL EEPORT SMITHSONIAN INSTITUIION, 1913. 



he was able to remove or alter the individual components of the 

 spectrum lines in a manner adapted to show the magnetic field exist- 

 ing in the sun spots where the light was produced. This most inter- 

 esting discovery he has now pushed still further, and has examined 

 the magnetic field of the whole surface of the sun. He finds that 

 there exists upon the sun a magnetic field similar in many of its char- 

 acteristics to that which exists in the earth, although the intensity of 

 the field is so extremely slight that the shifts or alterations of spec- 

 trum lines caused by it are almost beyond the possibility of disclosure. 



Kecently it was shown by Evershed that in the penumbras or dark- 

 ened edges of sun spots, there are found shiftings of the spectrum 

 lines which show that the vapors are moving outward from the 

 center of the spot, or umbra, toward the outlying parts of the penum- 

 bra. Later investigation shows that this outflow of the gases from 

 the umbra toward the outer part of the penumbra is accompanied 

 by a motion of rotation also around the umbra, so that the motion 

 resolves itself into a whirling of these vapors or gases similar to 

 that which is found in a waterspout. This has a very important 

 bearing on the explanation of the magnetic field in sun spots dis- 

 covered by Hale, for it was shown by Rowland many years ago that 

 an electric charge in motion has the property of an electric current of 

 producing a magnetic field. Thus if there are in sun spots materials 

 under dissimilar electric conditions, and these materials be whirled 

 as in a waterspout, they must necessarily produce a magnetic field. 

 St. John, of the Mount Wilson Solar Observatory, has made a thor- 

 ough investigation of the motions of the vapors in the neighborhood 

 of sun spots, using the spectrum lines of many of the chemical ele- 

 ments. He finds that the displacements of the spectrum lines of iron 

 and some other well-known metals indicates a motion away from the 

 umbra. The motion, on the other hand, of magnesium and hydrogen 

 and some other of the lighter chemical elements is toward the 

 umbra. Is was also shown some years ago in a photograph by St. 

 John that hydrogen gas is sometimes sucked into the center of a sun 

 spot. 



All these various lines of evidence indicate that a sun spot is a 

 whirl in the gases of the outer part of the sun, analagous to a water- 

 spout, and that this whirl comes from within outward. Associated 

 with the whirl there is produced a magnetic field, and associated 

 with the outward motion of the materials a decrease of pressure. 

 The decreased pressure of the gases causes their expansion and conse- 

 quent cooling, so that the coolness of the sun spot is thereby ex- 

 plained. As the gases spread out at the surface of the sun, the 

 lighter gases — hydrogen and others — which are found in the outer- 

 most solar layers, are sucked into the partial vacuum at the center 

 of the whirl. 



