168 TENTH ANNUAL REPORT OF 



In viewing tlie cosmical bodies of our system with relation to their 

 densities, they are divided into two great classes — planetary and com- 

 etary bodies, (these last embracing comets proper and shootiug stars,) 

 the former being of dense, and the latter of very attenuated matter ; 

 and so far as our knowledge extends, there is no reason to believe 

 that the density of any comet approaches that of any of the planets. 

 This fact gives some grounds for connecting meteorites with the plan- 

 ets. Among the planets there is also a difference, and a xerj marked 

 one, in their respective densities ; Saturn having a density of 0.'7'7 to 

 0.75, water being 1.0 ; Jupiter 2.00-2.25 ; Mars 3.5-4.1 ; Venus 4.8- 

 5.4 ; Mercury between 1 and 36 ; Uranus 0.8-2.9 ; that of the Earth 

 being 5.67.* If, then, from specific gravity we are to connect meteor- 

 ites to the planets, as their mean density is usually considered about 

 3.0,t they must come within the planetary range of Mars, Earth, and 

 Venus. In the cases of the first and last we can trace no connexion^ 

 from our ignorance of their nature and of the causes that could have 

 detached them. 



This reduces us then to our own 'planet, consisting of two parts — the 

 planet proper with a density of 5.76, and the moon with a density of 

 about 3. 62. 1 On viewing this, we are at once struck with the relation 

 that these bear to the density of meteorites, a relation that even the 

 planets do not bear to each other in their densities. 



As before remarked, I lay no great weight on this view of the den- 

 sity, but call attention to it as agreeing with conclusions arrived at 

 on other grounds. 



The chemical composition is also another strong ground in favor of 

 the lunar origin. This has been so ably insisted on by Berzelius and 

 others, that it would be superfluous to attempt to argue the matter 

 any further here ; but I will simply make a comment on the disregard 

 that astronomers usually have for this argument. In the memoir on 

 the periodic meteors by Sears C. Walker, already quoted from, it is 

 stated, " The chemical objection is not very weighty, for we may as 

 well suppose a uniformity of constituents in cosmical as in lunar 

 substances." From this conclusion it is reasonable to dissent, for as 

 yet we are acquainted w^ith the materials of but two bodies, those of 

 the earth and those of meteorites, and their very dissimilarity of con- 

 stitution is the strongest argument of their belonging to different 

 spheres. In further refutation of this idea it may be asked, is it to 

 be expected that a mass of matter detached from Jupiter, (a planet 

 but little heavier than water,) or from Saturn, (one nearly as light as 

 cork,) or from-Encke's comet, (thinner than air,) would at all accord 

 with each other or with those of the earth ? It is far more rational to 

 suppose that every cosmical body, without necessarily possessing ele- 

 ments different from each other; yet are so constituted that they may 



* For these estimates of the densities of the planets, the author is indebted to 

 Prof. Peh'ce. 



•|- AHhough the average specific gravity of the metallic and stony meteorites is greater, 

 yet the latter exceeding the former in quantity, the number 3.0 is doubtless as nearly 

 correct as can be ascertained. 



J Although the densities of the earth and moon differ, these two bodies may consist 

 of similar materials, for the numbers given represent the density of bodies as wholes ; 

 the solid crust of the earth for a mile in depth cannot average a density of 3.0. 



