MOUNT WILSON SOLAR OBSERVATORY. 



149 



results for corresponding latitudes in the northern and southern hemispheres, 

 reduced from synodic to sidereal values. 



Miss Ware has also measured the motions of 828 points in the hydrogen 

 fiocculi on 35 H8 plates (table i). The difficulties of identification are much 

 greater than in the case of the calcium fiocculi, since the hydrogen flocculi 

 change more rapidly in fonn. It is thus necessary to use plates separated 

 by an interval of only about 12 hours, whereas the same calcium flocculus 

 may frequently be measured on plates separated by an interval of two or 

 three days. 



Table i. 



It will be seen that while the motions of the calcium flocculi clearly show a 

 decrease in angular velocity toward the poles of about the same magnitude as 

 in the case of sun-spots, the hydrogen flocculi do not appear to follow the 

 same law. 



This work was near completion when the first Ha photographs of the disk 

 were obtained. The extensive whirls shown on these photographs indicate 

 that the flocculi lying in the neighborhood of sun-spots, or within any region 

 occupied by whirls, are unsuitable for the determination of the solar rotation. 

 The HS plates apparently represent the hydrogen in a lower region of the 

 solar atmosphere, where the whirls are not clearly shown. Nevertheless, the 

 motions of the H8 flocculi are probably affected by the whirls, which may ac- 

 count for many peculiarities encountered in the measurement of the plates. 

 It accordingly becomes necessary to study the rotation at the Ha level, by 

 means of flocculi which are well outside of the whirls. 



A further extension of the spectroheliograph work will soon be possible, as 

 the 30-foot spectroheliograph, constructed in our instrument-shop during the 

 year, is now nearly ready for trial. 



