1872.] on Planetary Influence upon Solar Activity. 



211 



grounds for believing that the behaviour of sun-spots with regard to in- 

 crease and diminution, as they pass across the sun's visible disk, is not 

 altogether of an arbitrary nature. From the information which we thenj 

 had, we were led to think that during a period of several months sun-spots 

 will on the whole attain their minimum of size at the centre of the disk ; 

 they will then alter their behaviour so as on the whole to diminish during 

 the whole time of their passage across the disk ; thirdly, their behaviour 

 will be such that they reach a maximum at the centre ; and, lastly, they 

 will be found to increase in size during their whole passage across the. disk. 

 These various types of behaviour appeared to us always to follow one 

 another in the above order ; and in a paper printed for private circulation 

 in 1866, we discussed the matter at considerable length, after having care- 

 fully measured the area of each of the groups observed by Carrington, in 

 order to increase the accuracy of our results. In this paper we obtained 

 nineteen or twenty months as the approximate value of the period of re- 

 currence of the same behaviour. 



2. A recurrence of this kind is rather a deduction from observations 

 more or less probable than an hypothesis ; nevertheless, it appeared to us 

 to connect itself at once with an hypothesis regarding sun-spot activity. 

 u The average size of a spot " (we remarked) " would appear to attain its"^ 

 maximum on that side of the sun which is turned away from Venus, and 

 to have its minimum in the neighbourhood of this planet." In venturing a v 

 remark of this nature, we were aware it might be said " How can a com- 

 paratively small body like one of the planets so far away from the sun 

 cause such enormous disturbances on the sun's surface as we know sun- 

 spots to be ? " It ought, however, we think, to be borne in mind that in 

 sun-spots we have, as a matter of fact, a set of phenomena curiously re- 

 stricted to certain solar latitudes, within which, however, they vary ac- 

 cording to some complicated periodical law, and presenting also periodical 

 variations in their frequency of a strangely complicated nature. Now 

 these phenomena must either be caused by something within the sun's 

 surface, or by something without it. But if we cannot easily imagine 

 bodies so distant as the planets to produce such large effects, we have 

 equal difficulty in imagining any thing beneath the sun's surface that could 

 give rise to phenomena of such a complicated periodicity. Nevertheless, 

 as we have remarked, sun-spots do exist, and obey complicated laws, 

 whether they be caused by something within or something without the sun. 

 Under these circumstances, it does not appear to us unphilosophical to 

 see whether as a matter of fact the behaviour of sun-spots has any reference 

 to planetary positions. There likewise appears to be this advantage 

 establishing a connexion of any kind between the behaviour of sun-spots 

 and the positions of some one prominent planet, that we at once expect a 

 similar result in the case of another planet of nearly equal prominence, and 

 are thus led to use our idea as a working hypothesis. J 



3. We have now a larger number of observations at our disposal than wc 



