Kirk wood.] O^A. [Sept. L9, 



The Cosmogony of Laplace. By Daniel Kirkwood, LL.D., Blooming 



ton, Indiana. 



{Read before the American Philosophical Society, Sept. IQth, 1*79.) 



Laplace's celebrated nebular hypothesis was first distinctly stated in his 

 Systeme du Monde.* The reasoning by which it is there sustained is gen- 

 eral, and it does not appear that the author made any efforl to test his 

 theory by analysis. The law of the conservation of energy was then un- 

 discovered, and hence data, which now seem available for a critical exami- 

 nation, were entirely wanting. Let us consider the hypothesis in some of 

 its obvious aspects. 



1. It is assumed by Laplace that nebulous rings were abandoned only in 

 the vicinity of the present orbits of the planets. While I have for many 

 years believed that the matter of the solar system originally existed in a 

 gaseous condition, and hence that a nebular hypothesis in somt form must 

 furnish the true explanation of the planetary motions, I have more than 

 once ventured the opinion that this assumption of Laplace is wholly un- 

 warranted. I make a single quotation from the Monthly Notices of the 

 Royal Astronomical Society for January, 1869 : 



"The known facts in regard to the zone of minor planets, as well as the 

 phenomena of Saturn's rings, seem to demand a modification of the nebu- 

 lar hypothesis as generally held. No reason has ever been assigned why 

 the solar nebula should not have abandoned rings at distances intermediate 

 between the present orbits of the planets. On the contrary, it seems highly 

 probable that, after first reaching the point at which gravity was counter- 

 balanced by the centrifugal force arising from the rotation of the contract- 

 ing spheroid, a continuous succession of narrow rings would be thrown off 

 in close proximity to each other, and revolving in different periods accord- 

 ing to Kepler's third law." 



The view thus expressed in 1808 has never been called in question, and 

 I have seen no reason to modify it. The ring theory thus seems to require 

 that alter matter began to be thrown oil' at the equator of the revolving 

 mass, the process should have been almost continuous until the nebula be- 

 came transformed into a close system of rings presenting the appearance 

 of a thin plate or disk. The theory would thus fail to account for the 

 formation of the solar system as it actually exists. 



2. But even if we adopt Laplace's theory of ring formation, we at once 

 encounter difficulties no less serious. It is obvious, on the slightesl exam- 

 ination, that the mutual attraction of different portions of a zone could 

 have very little influence in bringing its molecules together around a com- 

 mon nucleus. Laplace, it is true, supposed the fragments of a ring to have 

 been thus collected into a single planet. " Almost always," he says, 

 "each ring of vapors ought lobe divided into several masses, which, be- 

 ing moved with velocities that differ little from each other, should continue 

 to revolve at the same distance from the sun. These masses should assume 



Published at Paris in 1813. 



