1909] on Solar Vortices and Magnetic Fields. 621. 



to indicate rotation in a clockwise direction in the southern hemi- 

 sphere, and in a counter-clockwise direction in the northern hemi- 

 sphere (assuming the direction of motion to be inward towards the 

 spot). However, this cannot be taken as a general law, correspond- 

 ing to the law of terrestrial cyclones. Indeed, many instances have 

 been found of closely adjoining spots, in the same hemisphere, and 

 frequently in the same spot-group, having magnetic fields of opposite 

 polarity, produced by vortices rotating in opposite directions. 



In some cases, at least, these vortices seem to exercise a powerful 

 attraction on the surrounding gases, as a series of pliotographs taken 

 on June 3, 1908, illustrates. A long dark hydrogen prominence, first 

 photographed in elevation at the sun's limb on May 28, had advanced 

 half-way across the solar disc. It lay at the outer boundary of a well- 

 defined vortex, centred on a sun-spot. This spot had been gradually 

 separating into two parts, and on June 3 the separation was complete. 

 The first photogi'aph of a series of nine was made on this day at 4 h. 

 58 m. Several successive photographs indicated no appreciable change, 

 but one taken at 5 h. 7 m. showed that the prominence was developing 

 an extension toward the spot. At 5 h. 14 m. this had assumed the 

 appearance illustrated in the next photograph, and 8 m. later, when 

 the last photograph of this series was taken, the extension had 

 almost reached the spot. It will be seen that it divided into two 

 parts, which indicates that each umbra was a centre of attraction. 

 The average velocity of the motion toward the spot was over 100 km. 

 per second. Later photographs, made on the following days, show a 

 ring of bright hydrogen surrounding the spots, suggesting that the 

 comparatively cool hydrogen carried down into the spots was reheated 

 and returned to the surface, after escaping from the lower end of the 

 vortex. We thus seem to be observing some of the phenomena of an 

 actual vortex in the sun. But it must not be supposed that cases of 

 this kind are common. In many instances the hydrogen floccuH do 

 not appear to move toward or away from spots, but undergo changes 

 of intensity, as though the physical condition of the gas was con- 

 stantly changing. But before proceeding further with a discussion of 

 these sun-spot vortices, let us turn to another phase of the subject, 

 which will afford much new information indispensable for this purpose. 



We are all familiar with the effect produced by passing an electric 

 current through a wire helix. The lines of force of the resulting 

 magnetic field are parallel to the axis of the helix, and its intensity 

 is determined by the diameter of the helix, the number of turns of 

 wire and the strength of the current. We also know, from Rowland's 

 experiment, that the rapid revolution of an electrically charged body 

 will produce a magnetic field. Thus if a sufficient number of electri- 

 cally charged particles were set into rapid revolution by the solar 

 vortices a magnetic field should result. What warrant have we for 

 assuming the existence of charged particles in the sun, and how could 

 such a field be detected ? 



