On the Evolution of the Earth-Moon System. 427 



I venture to predict that at some future time practical astro- 

 nomers will no longer be content to eliminate variations of 

 level merely by taking means of results, but will regard cor- 

 rections derived from a special instrument as necessary to each 

 astronomical observation. 



XL VII. New Views of Mr. George H. Darwin's Theory 

 of the Evolution of the Earth-Moon System, considered as to 

 its bearing on the question of the Duration of Geological Time. 

 By the Rev. Samuel Haughton, M.D., Fellow of Trinity 

 College, Dublin*. 



IT has- been tacitly assumed, even so far back as the times 

 of Newton and Clairaut, that the earth and planets have 

 passed through a liquid condition (owing to former great 

 heat) before assuming the solid condition which some, at least, 

 of them now possess. 



Laplace, in his nebular hypothesis, also assumes the former 

 existence of this liquid condition; and it is openly asserted by 

 all geologists who believe that the earth consists of a solid 

 crust (more or less thick), reposing upon a fluid or viscous 

 nucleus. 



It has been proved by Sir William Thomson, following out 

 the views of the late Mr. Hopkins, that the present condition 

 of the earth, taken as a whole, is such that it must be re- 

 garded as being more rigid than glass or steel, possibly more 

 rigid than any terrestrial substance under the surface-condi- 

 tions of pressure. 



The following considerations show that it may be fairly 

 doubted whether the earth or any other planet ever existed in 

 a fluid condition. 



1. The possibility of the equilibrium of the rings of Saturn, 

 on the supposition that they are either solid or liquid, has been 

 more than doubted, and the most probable hypothesis respect- 

 ing them is, that they consist of swarms of discrete meteoric 

 stones. 



2. It is difficult to understand the low specific gravity of 

 Jupiter and the other outer planets, on the supposition that 

 they are either solid or liquid; for we know of no substance 

 light enough to form them f . If the outer planets consist of 



* From the ' American Journal of Science ' for November 1882. Head 

 before the Mathematical Section of the American Association for the 

 Advancement of Science, at Montreal, August 1882. 



t The force of this argument could not be felt before the revelations of 

 the spectroscope, because at that time there was no proof that the whole 

 universe was composed of the same simple substances, and those very- 

 limited in number. 



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