SECT. iii.J STABILITY OF THE SYSTEM. 25 



the sun : the same may be said of Ceres and Pallas ; and 

 the inclination of the orbits of Astraea, Iris, Flora, and Metis 

 differ by a very small quantity. However, these planetary 

 atoms have no influence on the motions of the larger planets, 

 for Jupiter has a diameter of 90,000 miles, while that of 

 Pallas, his nearest neighbour, is only 97 miles, little more 

 than the distance from London to Dover. The diameter of 

 Mars, on the other side of the small planets, is 4100 miles, 

 and that of the earth 7916 miles, so that the telescopic group 

 are too minute to disturb the others. M. Le Verrier found 

 another zone of instability between Venus and the sun, on 

 the border of which Mercury is revolving, the inclination of 

 whose orbit to the plane of the ecliptic is about 7, which is 

 more than that of any of the large planets. Neptune's orbit 

 is, no doubt, as stable as that of any other of the large planets, 

 as the inclination is very small, but he will have periodical 

 variations of very long duration from the reciprocal attrac- 

 tion between him and Uranus, one especially of an enormous 

 duration, similar to those of Jupiter and Saturn, and, like 

 them, depending on the time of his revolution round the 

 sun, being nearly twice as long as that of Saturn. Mr. Adams 

 has computed that Neptune produces a periodical perturba- 

 tion in the motion of Uranus, whose duration is about 6800 

 years. 



The equilibrium of the system, however, would be de- 

 ranged, if the planets moved in a resisting medium 

 (N. 78) sufficiently dense to diminish their tangential 

 velocity, for then both the excentricities and the major 

 axes of the orbits would vary with the time, so that the 

 stability of the system Would be ultimately destroyed. 

 The existence of an ethereal fluid is now proved : and, 

 although it is so extremely rare that hitherto its effects on 

 the motions of the planets have been altogether insensible, 

 there can be no doubt that, in the immensity of time, it 

 will modify the forms of the planetary orbits, and may at 

 last even cause the destruction of our system, which in itself 



