187!t.| '>->) [Kirkwopd. 



a spheroidal form, with a rotary motion in the direction of that of their rev- 

 olution, because their inferior particles have a less real velocity than the 

 superior; they have, therefore, constituted so many planets in a state of 

 vapor. But if one of them was sufficiently powerful to unite successively 

 by its attraction all the others about its centre, the ring of vapors would be 

 changed into one sole spheroidal mass, circulating about the sun, with a 

 motion of rotation in the same direction with that of revolution." 



In regard to the mutual attraction here referred to, it may be remarked, 

 that two parts of the Neptunian ring on opposite sides of the sun 

 could produce no sensible perturbation of each other's motion. If, more- 

 over, the fragments of any ring were distributed around the orbit with ap- 

 proximate uniformity, their mutually disturbing effects would nearly 

 destroy each other. That this state of things should have obtained in the 

 case of some of the eight principal planets is extremely probable. The 

 theory, therefore, of planetary aggregation by the attraction between dif- 

 ferent parts of the rings, requires an indefinite antiquity of the solar sys- 

 tem. Let us suppose, then, that the planet-forming process was due to the 

 different velocities of the fragments into which a ring had been broken up. 

 Take, for example, the ring which was transformed into Neptune. Let us 

 assume that two fragments, A and B, differed in longitude by 180°, and 

 that the mean distance of the centre of gravity of A from the sun's centre 

 exceeded that of B by 1000 miles. It is then easy to show that the corres- 

 ponding difference of their angular velocities would not bring tiiem to- 

 gether around the same nucleus in 15. millions of years. But even after 

 all the fragments had thus been collected, other millions of years — assuming 

 with Laplace that the united mass was still in the gaseous form — would be 

 required for the process of condensation. The supposition we have made 

 is not an extravagant one. In Laplace's cosmogony, therefore, hundreds 

 of millions of years are involved in the separate history of a single planet. 

 Is so great an implied age of the solar system admissible? 



According to Helmholtz, whose theory is now generally accepted, the 

 sun's heat is but the transformed motion of its parts condensed or drawn 

 together by the force of gravitation. Now, the law of the conservation of 

 energy enables us to calculate the age of the sun, knowing (1) the amount 

 of solar heat radiated in a given time, and (2) the amount produced by a 

 given contraction of the sun's mass. It has thus been found that conden- 

 sation from the distance of the nearest fixed stars to the sun's present vol- 

 ume, would have kept up a supply of heat equal to the present for about 

 twenty millions of years. This estimate, it will be understood, is based on 

 the assumption that the sun's density is uniform from centre to surface. 

 If, as is altogether probable, the density increases towards the centre, the 

 age of the sun may be considerably greater.* 



* " On the only hypothesis science will now allow us to make respecting the 

 source of the solar heat, the earth was, twenty millions of years ago, enveloped 

 in the fiery amosphere of the sun."— Prof. Simon Xcwcomb in the N. A. Review, 

 for July, 1876. 



puoc. amkr. rnir.os. soc. xviii. 104. 2p. printed xov. 7. 1879. 



