

] 4 s. Hirayama . 



upon only a single determination, and the second upon but two. 

 A comparison of Table X and Table XI shows a faster rate of 

 daily angular motion for Drift II than for Drift I. The mean 

 acceleration of the angular velocities is about 0°.35, within the 

 limits of observation, the corresponding acceleration in the rota- 

 tion period being O.'^Tl. For the lower latitudes, the rotation 

 periods of the spots belonging to Drift II agree approximately 

 with the recent spectroscopic results obtained by Messrs. Storey 

 and Wilson. "^'^ Perhaps certain groups of spots by a proper motion 

 of their own come to the same level as the reversing layer and 

 attain its angular velocity. 



As to the peculiar motions of sun-spots Prof. Sporer sa3^s: 

 "Die Beobachtungen haben ergeben, dass im östlichen Theile 

 einer Gruppe niemals übergrosse Rotationswinkel vorkommen. 

 Man findet daselbst X'erkleinerung der Rotationswinkel, aber selten 

 mit bedeutenden Betrage. Uebergrosse Rotationswinkel kommen 

 vor an der Westgrenze der Gruppen und bei neu entstandenen 

 Flecken." He then mentions 6G cases of great angular velocities 

 during the period 1880-84. This excess comes out to be about 

 0°.5 on the average. Perhaps such spots and those of short 

 duration niay greatly contribute to the existence of Drift II. 



8. Since the ratio of the amplitudes of the two drifts 

 depends on the choice of spots by each investigator, it is difficult 

 to find the true ratio from the investigations hitherto made. 

 By examining the tables in this article, it will be found that 

 it is actually different for different investigators. It also varies 

 irregularly in different zones. The mean ratio of the amplitudes 

 of the two drifts is about J, J, and 7 in Maunder' s, Carrington's 

 and Sporer's observations respectively. The researches of Mr. and 

 Mrs. Maunder were made on spots in general. They say: '' There 

 has been no selection of spots because they seemed to be steady in 

 motion or regular in shape, no rejection because of unsteadiness or 

 irregularity. The only criterion for the inclusion of a group in the 

 discussion has been that it lasted for six consecutive days " . This 



(I) storey and Wilson, Spectroscopic Observations of the Sun's Rotation, etc. MonfMy 

 Notices LXXI p. 674. 



