310 The Nebular Hypothesis of Laplace. 



THE NEBULAR HYPOTHESIS OF LAPLACE 

 EXPLAINED BY DELAUNAY. 



Recent investigations into the nature of nebulas have strongly 

 revived public interest in what is called the " Nebular Hypo- 

 thesis/' and as fresh facts, which we may expect to be made 

 known by further research, promise to supply arguments for 

 and against this theory of the origin of suns and planets, it is 

 well that clear ideas should be entertained concerning the 

 views which Laplace held. Professed students of astronomy 

 are sufficiently acquainted with them ; but to the general scien- 

 tific reader, the following explanation, given by M. Delaunay, 

 in his Gouts Elementaire d'Astronomie, 4th edit., will be 

 acceptable. He tells us that " in adopting the ideas of 

 Herschel concerning the progressive condensation of nebulas, 

 and their transformation into stars, and in applying these ideas 

 to our planetary system, Laplace arrived at the most satis- 

 factory mode of explaining their formation. No peculiarity 

 that observation has disclosed relative to planets or their 

 satellites, has escaped the ingenious explanation which he has 

 developed at the close of his Exposition du systeme du Monde, 

 of which we shall proceed to give an idea/' 



" Laplace supposes that in the beginning, the sun, and all 

 the bodies circulating round him formed a single nebula, 

 animated by a movement of rotation about a line passing* 

 through its centre, and extending to the orbit of the most 

 distant planet, or beyond it. He further conceives that by 

 reason of a progressive cooling, larger and larger portions of 

 nebulous matter were condensed at its centre, so as to form a 

 nucleus, the mass of which gradually enlarged. Starting from 

 this supposition, he demonstrated that in the course of time 

 the nebula would be reduced to the state in which we actually 

 find the planetary system.'" 



11 In proportion as the cooling caused the condensation ot 

 new parts of the nebula, the materials so condensed would fall 

 towards the centre just as drops of rain fall through the 

 condensation of the vapour contained in our atmosphere. But 

 this fall of condensed matter could not take place without 

 occasioning an acceleration of the velocity with which the 

 entire nebula turned upon its axis. Matters condensed and 

 falling towards the centre of the nebula would acquire a move- 

 ment of rotation about its axis more rapid than that of the 

 rest of the mass. Then the friction of different parts of the 

 nebula, one against another, would accelerate the motions of 

 those turning less rapidly, and would lessen the rapidity of 

 those moving with greater velocit} r , and after a certain lapse of 



