170 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1913. 



A similar expianation is not so easy in the case of the spots. Tn 

 order that they may be more easily visible on the eastern than on 

 the western limb, we may suppose that they are followed, but not 

 preceded, in their general rotation by some kind of a cloud. Each 

 spot would then have its cloud, allowing the spot to be seen as it 

 approached but hid more and more as it departed. 



This explanation is not very convincing. In order that the cloud 

 have an appreciable effect upon a great spot it would have to be 

 at quite an elevation, and it is difficult to see how it would escape 

 observation at the border of the sun. Its influence would not be 

 felt except toward the ends of the spot's transit, and we have seen 

 that the inequalities are noted in the same sense in all pairs of sym- 

 metrical zones. 



(4) There remains one more test which we must not neglect. We 

 could not pretend that the earth alone has such an influence upon the 

 sun. If it is effective, then the other planets must be; and there are 

 apparently three, Jupiter, Mercury, and Venus, which should be 

 even more effective. How can we assure ourselves in this matter ? 



RESEARCHES OF THE KEW OBSERVERS. 



The problem had already been attacked long ago by De la Rue, 

 Balfour Stewart, Benjamin Loewy, astronomers at the Kew Observa- 

 tory. (Proc. Roy. Soc, p. 210, 1872.) As the observations never 

 related to but a half of the sun at a time, it was considered neces- 

 sary at the start to determine and try to eliminate the influence 

 which the position of the observer on the earth might have. 

 The two following conclusions resulted from the preliminary 

 examination : 



(a) Upon the hemisphere visible from the earth the mean area 

 occupied by the spots increases as the distance on either side of the 

 central meridian increases. 



(h) The spotted surface on the average is greater on the western 

 than on the eastern half of the visible disk. 



The second conclusion of the Kew observers is at variance with 

 that of the more recent investigators. However, the years examined 

 in the two cases have no part in common. The data used by Mrs. 

 Maunder was so much more homogeneous and abundant that her 

 conclusions should have greater weight. 



Having completed their first examination, the Kew observers con- 

 sidered how to correct their data for the position of the observer. 

 They could then, for any planet whatever, P, compare the hemisphere 

 turned toward the planet P with that turned away. Relative to the 

 circle limiting these two hemispheres, any other planet, P', could 

 have any possible position in its orbit. It seemed right to admit 

 that, if the interval considered be long enough, the effect of P' would 



