GRADATION OF DENSITIES. 27 



strained by the centrifugal ; while this latter force, attaining 

 its maximum at the equator, meets and wards off the gravi- 

 tating particles in their rush toward the center, and thus the 

 two forces finally settle into an exact equipoise, of which the 

 oblately spheroidal form of the planet is an equally exact 

 expression. 



These considerations seam to sufficiently prove that the 

 earth (before shown to haye been originally in a state of 

 igneous, if not gaseous fluid) was formed by the predominat- 

 ing force of attraction, and hence contraction, acting upon 

 materials in a rarer state, and reducing them to their present 

 dense form. The attractive and contractive operation must, 

 of course, have proceeded through a progressive series of 

 analogous stages, which somewhere must have had a begin- 

 ning ; and we can not conceive of any possible beginning short 

 of the greatest possible diffusion a state of diffusion which, 

 originally applying to the materials of all planets, must have 

 brought them all into' the form of one common vapory 

 mass. 



Though this argument, in proof of the nebular theory, 

 seems hitherto to have generally escaped the notice of 

 astronomical writers, it is one which, nevertheless, deserves to 

 be pondered and borne in mind. 



2. Another argument for the same theory, is derived from 

 the regular gradations of densities of planets, from innermost 

 to outermost. Thus it is stated, on the basis of mathematical 

 calculations, that Mercury must be about the weight of so 

 much lead ; Venus is nearly six times the weight of so much 

 water ; the Earth, as a whole, is four and a half times the 

 weight of water ; Mars is a little over three times the weight 

 of water ; Jupiter is a small fraction over the weight of so 

 much water ; Saturn is less than half that specific weight, or 



