1. In a previous communication by us to this Society, an Abstract of which was published in the Proceedings, vol. xiv. p. 59, we showed some grounds for believing that the behaviour of sun-spots with regard to increase and diminution, as they pass across the sun’s visible disk, is not altogether of an arbitrary nature. From the information which we then 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 recurrence 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. “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 Yenus, and to have its minimum in the neighbourhood of this planet.” In venturing a 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 restricted to certain solar latitudes, within which, however, they vary according 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 in 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.
II. Further investigations on planetary influence upon solar activity