Mr. R. P. Greg on the Periodicity of the Solar Spots, 273 



days 8 hours, say 25*3 days ; then in the time that the sun makes 

 160 revolutions on his axis, Mercury will make almost exactly 

 46 revolutions in his orbit, and Venus 18 ; and this singular 

 conjunction will recur at periods of about 11 years and 31 days, 

 or about 11*08 years, which does not differ materially from the 

 solar-spot cycle of 11*1 years. The statement is as follows : — 

 Revolutions. Time of Revolution. Days. 



Sun ... . 160 (on axis) x 25-30 =4048 

 Mercury . . 46 (in orbit) x 87*97 =4047 

 Venus ... 18 „ x 224*70 =4045 



Earth . . . 11*08,, x 365*25 =4047 



" It appears, then, that in periods of about 11*08 years a con- 

 junction of Mercury and Venus takes place with the same side 

 of the sun presented to them. This phenomenon may occur at 

 several places in the ecliptic, one of which happens to be at 

 the perihelion of the orbit of Mercury, and also within a few 

 days of his passing the ascending node. Now the orbit of Mer- 

 cury is considerably more eccentric than that of any of the other 

 planets — as much as one-fifth of his semi-major axis; there 

 must therefore be a considerable increase in his attractive 

 energy at that particular point of his orbit ; this, added to the 

 fact that he is very nearly in a line with the sun's equator 

 (being nearly in the node), combined also with the conjunction 

 of Venus, would tend to increase the amount of attraction 

 acting on the sun's surface at that time. This singular con- 

 j unction will sometimes be strengthened by that of some of the 

 other planets ; but this will only be a casual occurrence. For 

 instance, at the last conjunction, which happened in March 

 last, Jupiter was less than 30° from that point of the ecliptic ; 

 he should therefore have some influence. At the next return 

 of the period Jupiter will be almost, if not quite, in exact con- 

 junction with Mercury and Venus; this will take place in April 

 1871. The following statement shows the times when this 

 phenomenon has last occurred : — 



November 1815. 



December 1826. 



January 1838. 



February 1849. 



March 1860. 



" These years, I believe, do not exactly agree with the years of 

 maximum spots, but yet so nearly as to challenge further atten- 

 tion to the coincidence, and induce one to think that the pheno- 

 menon will have something to do with them ; if so, I should be 

 inclined to think the occurrence of spots, in a lesser degree, 

 should be more frequent, viz. at intervals of 88 days, which is 

 the time in which Mercury goes from his perihelion to his peri- 



