Kirkwood.] ^^ [April 2, 



liam A. ISTorton, of Yale College, in Ills "Treatise on Astronomy," pp. 

 434 — 436, presents a brief but valuable discussion of the same subject. 

 An inspection, however, of Table III., shows that writers generally have 

 given undue weiglit to Saturn's influence. Again, although Mercury's 

 action at aphelion is but feeble, and even at his mean distance, less than 

 that of Venus or Jupiter, his perturbing power at 'perihelion is the 

 greatest of all planets — a fact which certainly demands consideration in 

 any theory which refers the origin of solar spots to planetary agency. 

 In short, after giving the subject much study and attention, I deem it 

 impossible, with the numbers given in table III., and witJiout the intro- 

 duction of any modifying cause, to establish a general correspondence 

 between the different svm-spot periods and those of regvilarly recurring 

 planetary configurations. 



(11.) But the hypothesis that a particidar portion of the sun's surface 

 is more favorable to spot formation — or, in other words, more susceptible 

 to planetary influence — than others, will, it is believed, obviate all diffi- 

 culty. Is there, then, any independent probability of the truth of this 

 hypothesis? It is well known that the formation of spots occurs chiefly 

 between particular parallels of latitude, and that the numbers are greater 

 in the northern than in the southern hemisphere. It seems, therefore, at 

 least not improbable that a like difference may exist in regard to longi- 

 tude. " Sommering directs attention to the fact, that there are certain 

 meridian belts on the sun' s disk, in which he had never observed a solar- 

 spot for many years together."* Buys-Ballot, of Utrecht, has found, 

 from an elaborate discussion of a great number of meteorological ob- 

 servations, that there is a short period of variation in the amount of solar 

 heat received by our planet ; the period from maximum to maximum 

 coinciding, at least approximately, with that of the sun's rotation with 

 respect to the earth. Sir William Herschel also believed that one side 

 of the sun, on account of some peculiarity in its physical constitution, 

 was less adapted to radiate light and heat than the other. 



(12.) On the hypothesis which we have ventured to suggest, the sun- 

 spot period would be equal to the interval between two conjunctions of 

 the disturbing planets on the heliographic meridian (designated by M) of 

 that part of the surface most susceptible to their influence. It Avould 

 depend, therefore, on the ratio of the sun' s period of rotation to the inter- 

 val between two consecutive conjunctions of such planets. Or, as Mer- 

 cury's influence is extremely variable, a maximum would be produced 

 by this planet's perihelion passage, when the most susceptible part of the 

 sun's surface had the same, or nearly the same, heliocentric longitude. 

 In order, then, to test this hypothesis, we must first inquire what is the 

 most probable period of the sun's rotation? 



(13.) On account of the ^j?*ojpg?' motion of the solar spots, the time of 

 the sun's rotation as determined by their apfarent motion across the 

 disk, varies from about 25 to 29 days. The f roper motion of the spots 



* Humboldt's Cosmos, Vol. IV., p- 378. 



