352 SEE— ORIGIN OF ZONE OF ASTEROIDS. [November 4, 



nomena of the solar system, by an argument so brief and so much 

 to the point that even a layman may grasp it without difficulty. 



1. It was pointed out by Oppolzer in 1880^ that the resistance 

 to two homogeneous spheres revolving in a discontinuous medium 

 of cosmical dust is inversely as their radii, and therefore relatively 

 very large for a small body and very small for a large one.' The 

 secular effect of such a cause, therefore, is to make the small body 

 approach the sun very rapidly, while it scarcely modifies the mean 

 distance of the large body, the latter change being so small that it 

 often may be neglected entirely. 



2. Now if we contemplate the arrangement of the orbits of the 

 asteroids in the solar system, we find them grouped almost entirely 

 within Jupiter's orbit, in accordance with the mathematical investiga- 

 tions of Newton and Callandreau, and moreover spread over the 

 entire zone from Jupiter to Mars, and even extending beyond these 

 limits. Thus Eros has a mean distance slightly less than that of 

 Mars, while the Achilles group of asteroids projects beyond the orbit 

 of Jupiter. How much wider the zone may hereafter be found to 

 be, it is difficult to predict. 



3. Since the asteroids have been thrown just within Jupiter's 

 orbit by the successive actions of that great planet, and subsequently 

 had their mean distances so decreased with the lapse of ages as to 

 carry them down to the orbit of Alars, it follows that this spreading 

 of the asteroids over such a wide zone affords a clear and unmis- 

 takable illustration of the eft"ects of resistance and collisions — these 

 small bodies having approached the sun much more rapidly than 

 the giant planet Jupiter which gathered them in. No other in- 

 terpretation can be given to the great width of the asteroid zone. 

 For the perturbative action of Jupiter could throw the asteroids but 

 slightly within his own orbit, and the great decrease in the mean 

 distances of many of them must be accounted for in some other way. 

 We conclude, therefore, that the more rapid dropping of the 

 asteroids towards the sun illustrates the secular effects of resistance 

 in the form of cosmical matter, such as meteoric swarms and comets, 



^ Cf. A. y., 2314 and 2319. 



" Cf. my " Researches," Vol. II., p "293. 



