ADVANCE OF ASTRONOMY. 223 



elated with the names of Faye and Sir J. N'orman 

 Lockyer, that crowds of discrete meteoric bodies 

 drawn together into aggregates by gravitational at- 

 traction, and evolving heat by collisions, may have 

 given rise to nebula?, with further condensation to 

 luminous stars, and eventually to dark planets, whose 

 vitality is at an end unless a collision make it possi- 

 ble for the evolutionary process to recommence. But 

 this remains in the speculative phase. 



The possibility, however, must be borne in mind 

 that some of the existing nebulae may have originated 

 in the collisions of dark sims, and are thus the chil- 

 dren, as it were, of a later generation. " During 

 the short historic period, indeed, there is no record 

 of such an event; still it would seem to be only 

 through the collision of dark suns, of which the 

 number must be increasing, that a temporary reju- 

 venescence of the heavens is possible, and by such 

 ebbings and flowings of stellar life that the inevita- 

 ble end to which evolution in its apparently uncom- 

 pensated progress is carrying us can, even for a 

 little, be delayed. . . . We cannot refuse to admit 

 as possible such an origin for nebula?.'' * 



Tidal Friction. — An interesting recent contribu- 

 tion to the theory of the evolution of planetary sys- 

 tems, and of satellites in particular, has been made 

 by Mr. G. II. Darwin, in his papers on the influence 

 of tidal friction, but the subject is too intricate for 

 discussion within our limits. 



Summary. — A cautious summary forms the last 

 paragraph of Berry's Short History of Astronomy, 

 and this ive venture to quote : — " Speaking generally, 

 7ve may say that the outcome of the nineteenth-cen- 

 tury study of the problem of the early history of the 

 * Sir W. Huggins. Rep. Brit. Ass. for 1891, p. 24. 



