MOUNT WILSON SOLAR OBSERVATORY. 259 



The hypothesis that the preceding members of a bipolar spot group 

 represent the opposite extremities of a semicircular vortex ring has 

 recently led to some laboratory experiments described on another 

 page (p. 280). On this hypothesis bipolar spots should exhibit 

 proper motions at right angles to the axis of the group, which usually 

 makes only a small angle with the solar equator. The direction of 

 motion, as determined from the above law of polarities, should there- 

 fore be toward the equator for low-latitude spots and toward the poles 

 for high-latitude spots. Carrington, and more recently Dyson and 

 Maunder, found proper motions (for all spots, single and multiple) 

 corresponding in sign with this requirement. But the inclusion of 

 single spots and the small magnitude of the motion in latitude renders 

 the agreement of doubtful significance, and further study of this ques- 

 tion will be necessary. 



The rule of polarities, in conjunction with the vortex experiments, 

 also provides the means of predicting the configuration of bipolar 

 flocculi in high and low latitudes, on the assumption that this is deter- 

 mined by hydrodynamic forces (see p. 256) . 



GENERAL MAGNETIC FIELD OF THE SUN. 



Our preliminary work on the general magnetic field of the sun served 

 to establish the existence of the field and to determine its intensity and 

 polarity. A second series of observations, besides giving improved 

 determinations of intensity, increased the number of lines showing the 

 field from 4 to 25, and indicated which fines should be chosen for further 

 work of the highest precision. Up to this time scattered observations, 

 made during the broken weather of the rainy season, served very satis- 

 factorily; but for the accurate determination of the inclination of 

 the sun's magnetic axis a continuous series of observations, well dis- 

 tributed in latitude and longitude, is required. Accordingly a large 

 number of photographs of spectra were taken with the 150-foot tower 

 telescope and 75-foot spectrograph during the summer of 1915. These 

 covered a period of 110 days, of which 3 days were lost on account of 

 clouds, and 2 days because of necessary repairs to the instruments. 

 Very few spots appeared on the sun during this period, and the occa- 

 sional spectra taken sufficiently near these to feel their magnetic influ- 

 ence in any degree were rejected. This large collection of photographs 

 is thus admirabl}^ suited for the difficult task of determining the in- 

 clination of the sun's magnetic axis and the period of its revolution 

 about the axis of rotation. 



During the present year Mr. van Maanen has measured the magnetic 

 displacements of the three lines X 5247.737, X 5300.929, and X 5329.329 

 on 819 of these spectra, representing 26 days distributed over 6 weeks. 

 The completion of the work will require some time, as the measure- 

 ment is very laborious and difficult. This will be appreciated when it 

 is remembered that the maximum displacement (at latitude -|-45°) 



