1869.] 



97 



[Kirkwood. 



tinuous curve, lie finds in it a series of small undulations succeeding each 

 other at an average interval of 7.65 months,"* or 233 days. 



(7.) The 21-Day Period. — The same astronomer thinks he has detected 

 a short period of variation corresponding to the sun's time of rotation 

 v^ith respect to the earth, or about 27 days. 



(8.) The 584r-Dai/ Period. — De La Rue, Stevrart and Lcevs^y, have found 

 a period varying between 18 and 20 months ; the mean being about 584 

 days.f Other periods of maxima and minima will probably be detected ; 

 but those we have enumerated are perhaps the only ones sufficiently well 

 established to justify any attempt at explanation. 



§ //. — Discussion of the Phenomena. 



(9.) That the solar spots are produced in some way by the planetary 

 disturbance of the photosphere, is now generally admitted. As yet, 

 however, the manner in which this infliience is exerted, can be little more 

 than matter of conjecture. If the action is analogous to that of the moon 

 on the earth, the relative disturbing power of the different members of 

 the system will be as follows : 



TABLE IIL 

 Relative Influence op the Planets on the Sun's Surface. 



This table is derived from the formula 



where S represents the disturbing power of a planet, m, its mass, 

 and a, its distance. 



(10.) The connection between the number of sun-spots and the posi- 

 tions of the planets was noticed by Wolf as long since as 1858. In the 

 Gomptes Eendus, for January, 1859, he published a formula in which the 

 number of sun-spots was made to depend on the difierent configurations 

 of Venus, the Earth, Jupiter and Saturn. In the learned and interesting 

 memoir — previously referred to — of De La Rue, Stewart and Lcewy, the 

 causal connection between the positions of Venus and Jupiter and the 

 behaviour of sun-spots seems to be clearly established. Professor Wil- 



* Sir John Herschel, Quart. Jour. Sci., Vol. I., p. 22S, April, 1864. 

 t Am. Journ. of Sci. and Arts, for March, 1867. 

 A. P. S. — VOL. XI — M 



