1872.] 



on Planetary Influence upon Solar Activity. 



213 



a sort of cylindrical wall round the spot. . Now the effect of such a wall 

 would be to allow the whole spot to be seen when at or near the centre of 

 the disk, but to hide part of the spot as it approached the border on either 

 Inside. A spot thus affected would therefore appear to be more diminished 

 by foreshortening than the usual formula would indicate ; and we should 

 therefore expect, if this were the case, that, on the whole, and after making 

 the usual allowance for foreshortening, spots would nevertheless be found 

 deficient in area near the borders as compared with their area at the centre 

 of the disk. As a matter of fact we have something of this kind, as will 

 be seen from the following Table, in which we have used the whole body 

 of spots forming the catalogue to which we have made allusion. In this 

 Table the first column denotes the heliocentric longitude from the centre 

 of the disk reckoned as zero ; the second denotes the united areas at the 

 various longitudes of all those groups from both series, the behaviour of 

 which we have been able to obtain with accuracy ; while the third column 

 exhibits the residual factor for foreshortening, which will bring the areas of 

 the second column into equality with each other. 



Table I. 



Longitude 

 observed. 



United areas of all 

 groups at longitude 

 of column 1 . 



Residual factor for 

 foreshortening 

 necessary to 

 equalize the areas of 

 column 2. 







-63 



147,508 



1-229 



-49 



156,758 



1-156 



— 35 



168,697 



1-075 



-21 



176,41/ 



1-028 



- 7 



178,990 



1-013 



+ 7 



181,336 



1-000 



+ 21 



178,638 



1-015 



+ 35 



175,747 



1-032 



+ 49 



171,140 



1-059 



+ 63 



162,541 



1-115 



5. From the above Table it appears that the average behaviour of spots, 

 as far as can be judged from the information at present attainable, is not 

 quite symmetrical as regards the centre of the disk. Without attempting 

 at present to enter into an explanation of this remarkable phenomenon, we 

 may point to it as a confirmation of our view previously stated, that most 

 spots are accompanied by a wall-shaped surrounding of facula. Observations 

 show that on the whole the life-history of the facula begins and ends earlier 

 than that of the spot which it surrounds, and that throughout a gradual 

 subsidence or dissipation of this elevated mural appendage seems to be taking 

 place. But such a diminution of the wall discloses more of the spot itself; 

 and hence the spot-areas measured in the eastern half of the hemisphere 



VOL. XX. It 



