Kirk wood., "J-^ Sept. Itf, 



8. The difficulty here presented is one of no small importance. If re- 

 moved, however, we are immediately met by another perhaps still more 

 formidable. Assuming the increase of Neptune's radius to have been uni- 

 form during the time required for the accumulation of the ring around a 

 single nucleus, the daily superficial deposit would be less than one-sixtieth 

 of an inch ; the density being equal to the present density of the planet. 

 This extremely slow transformation of the nebulous zones would develop 

 little heat ; so lhat the planets would he nearly cold during the process of 

 their formation. Laplace's theory, therefore, obviously fails to account 

 tor the origin of satellites. 



4. It is easy to show that the period of a rotating nebula in the process 

 of condensation would vary as the square of the radius. If the solar neb- 

 ula, therefore, rotated once in 164.6 years when it tilled the orbit of Nep- 

 tune, its period when it had contracted to the orbit of Uranus ought to 

 have been 67 years; at the orbit of Saturn, 16.7 years ; at that of Jupiter, 

 4.94 years, &c, etc. This obvious inconsistency with Kepler's third law* 

 has been noticed by astronomers, and recourse lias been had to the addi- 

 tional supposition that the rate of variation of density from surface to cen- 

 tre was continually changing through all the cycles of planetary forma- 

 tion.! Till this latter hypothesis — invented to sustain the hypothesis of 

 Laplace— shall itself have been placed on a basis of facts, the super- 

 stiucture must be regarded as eminently unstable. 



Conclusion. 



It has been shown (1) that the hypothesis of Laplace gives no explana- 

 tion of the immense intervals between the planetary orbits ; (2) that, apait 

 from this objection, the periods required for the formation of planets from 

 nebulous rings arc greater than the probable age of the solar system ; 

 (3) that it fails to account for the origin of satellites ; and (4) that it is ap- 

 parently incompatible with a known physical law. The conclusion seems 

 inevitable that this celebrated hypothesis must yet be abandoned, or that 

 its principal features must be essentially modified. 



* Let ?•, »■', and 1, I' represent t lie radii and periods of rotation of the solar 

 nebula at two different epochs; then t : l':: r? : r'i. But by Kepler's third 



law, t : t' : : r% : r'?. 



t See the able and interesting memoir on the Nebular Hypothesis by Prof. 

 David Trowbridge, in the Am. Journal of Science for Nov., 1S61. 



