The Sunspot Periodicity.—Many attempts have been made to correlate the 11 year period of spot variation with the 11.86 year period of Jupiter's revolution. The latter, as it stands, differs too widely, and it is necessary to combine it with some other period. Prof. T. J. J. See, in a special number of Astr. Nachr., vol. 216, attempts to combine it with 9.93 years, which is the period in which Jupiter gains a semi-revolution upon Saturn. He weights these two periods in the ratio 1.828 to 1, this ratio being the square root of that of Jupiter's mass to Saturn's mass. The result is 11.18 years, which is close to the sunspot period. But it is to be noted that while the 11.86 year period depends wholly on Jupiter, that of 9.93 years depends on both planets, so that the appropriateness of the above ratio is far from clear; apart from this the resulting period of two wave motions does not depend on the ratio of their amplitudes, but on the time that one takes to gain a revolution on the other. For example, the period from spring tides to spring tides is a semi-lunation, and this would not be altered by an alteration in the relative heights of solar and lunar tides.