S. Hmighton — Evolution of the Earth-Moon SysUm . 3 



is be decided a few generations hence. In all ordinary cas 

 binocular vision the judgment is cumulative, the conclush 

 ckly reached being a product not only of difference in t 

 r ree of attention given at the same moment to objects se 

 direct and by indirect vision respectively, but also to var 

 i in attention given to different points directly viewed 

 cession, to the muscular sense while free play is given 

 eyes, and to all the elements available in monocular visio 

 eh I have grouped together under the name of physical 



[Read before nation for the 



Advancement of Science, at Montreal. August, 1 882.] 



It has been tacitly assumed, even so far back as the times of 

 Newton and Churaut, 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 



Laplace, in his nebular hypothesis, also assumes the former 



existence of this liquid condition, and it is openly asserted bv 

 — 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 regarded as 

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

 a >.— • n< >ti ia! su 1 »stance under the surface conditions 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 

 wore than doubted, and the most probable hypothesis respect- 

 ] ng them is, that they consist of swarms .1 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 



