4 GEOLOGY. ^ 



mon center of gravity or some central body, as are the planets to-day. 

 In other words the constituents are infinitesimal planetoids or planetesb 

 mals. Under this hypothesis the dynamical system was essentially the 

 same at the outset as it is now; the evolution has consisted in the 

 aggregation of the planetesimals into planets and satellites. Dynamic- 

 ally considered, it differs more from the other two hypotheses than they 

 do from one another, but it is liable to be confused with the second, 

 from which, however, it departs fundamentally. These three hypothe- 

 ses will be considered in detail. 



I. The Laplacian or " Nebular" Hypothesis. 1 



During the last century the Laplacian hypothesis of the origin 

 of the solar system was almost universally accepted by men of science, 

 and geological theories respecting the early states of the earth and 

 respecting many of its subsequent events were built upon it, and these 

 views remain largely prevalent to-day. 



The postulated nebula. — The Laplacian hypothesis assumes thai 

 the matter of the solar system was once in an extremely heated gase- 

 ous condition and formed a vast spheroid extending beyond the outer- 

 most planet and rotating in the same direction as the present system. 

 How this condition of things arose was not definitely postulated by 

 Laplace. It has since been referred to the collision of two large bodies, 

 and to other agencies. The extreme tenuity of the gaseous spheroid 

 thus postulated is to be noted, for computation makes its average 

 density but a small fraction of a millionth of the density of our atmos- 

 phere at the surface of the earth, while its outer density was very 

 much less. 2 The hypothesis assumes that this extreme degree of 

 rarefaction was maintained by intense heat. 



Formation of rings. — The hypothesis holds that the gradual loss 

 of this heat by radiation must have resulted in contraction; and that 



1 A somewhat similar hypothesis was advanced earlier by the philosopher Kant 

 and a cruder one still earlier by Thomas Wright. Swedenborg also made a suggest 

 tion in the same line. These earlier attempts, however, commanded little attention 

 and the wide acceptance which the hypothesis gained. later was due mainly to the 

 elegant elaboration given it by the mathematical genius of Laplace, and to the great 

 influence of his name which it properly bears. The hypothesis is here sketched in 

 the slightly modified forms usually held at present. 



2 Kelvin gives the average density of a gaseous nebula having a radius forty times 

 that of the earth's orbit, as 1/570,000,000 of that of common air at the ordinary tem- 

 perature of 10° C. (Pop. Lect. and Add. I, p. 419). F. R. Moulton finds the average 

 density when the nebula extended to the orbit of Neptune, 1/240,000,000 of average 



