Ear/ha Axis of Ru/ation. 101 



density of 2'5, that conunoiily adopted, and yet sufficiently near that 

 density for the purpose intended. 



Laplace estimated the mean density of our planet as 1'55, the 

 solid surface being considered as 1, hence taking the interior density 

 higher than that of the external parts. We see, looking at such mi- 

 neral substances as form masses of rock, that they are all oxides ; but 

 of the depth to which these oxides may descend we know nothings 

 Unless we suppose them oxides from the beginning, that is, from 

 the time the matter of our earth may have been gathered together as 

 a body revolving around the sun, an hypothesis for which it would 

 appear difficult to find much reason, the various metals, such as sili- 

 ciuni, aluminum, calcium, and the rest, became oxides from coming 

 in contact with oxygen. We have sufficient oxygen in our atmo- 

 sphere, supporting the animal and vegetable life which now exists, 

 and which probably also during a long lapse of geological time has 

 existed on the earth's surface, to permit the assumption that in an 

 early state of our globe oxygen may readily have been far more 

 abundant among the gaseous portion of the matter formins our 

 planet, including its atmosphere, than at present, when animal and 

 vegetable life is adjusted to the quantity remaining. 



As far as we are acquainted with the substances forming our globe, 

 we may have an oxidized solid crust, supporting in parts a compa- 

 ratively thin and irregularly-disposed covering of saline water, and 

 enveloped by a gaseous covering, the interior not composed of oxides, 

 but more or less homogeneous, allowing for the effects of any heat, 

 which may be supposed to remain in it, and for the densities due to 

 the gravitation towards its centre of all the particles of matter of 

 which the earth is composed. 



W^hen we have to consider any changes in the earth's axis of 

 rotation due to the absence of homogeneity in its component parts, 

 we have also to regard the probability of this want of homogeneity 

 extending to a depth at which it would have any appreciable value. 

 As far as the distribution on the face of the earth of the igneous rocks 

 is known to us, — rocks whence, with the exception chiefly of lime- 

 stone deposits (many of which have been accumulated by means of 

 animal lite), so many others have been formed, — we do not find any 

 accumulation of masses of very dift'erent density in one part more 

 than another, so as to have produced very marked differences in 

 density on at least the surface of our spheroid. On the contrary, 

 we find the probable distribution of granite and granitic rocks with 

 the same density, very uniform in various parts of the earth's surface, 

 and their abrasion has furnished abundant materials for other rocks. 

 The like happens with the heavier compounds of hornblendic and 

 felspathic substances, and the strata derived fi'oni them. Masses of 

 limestone are indeed here and there more irregularly distributed ; 

 but as the limestones do not differ much from the gi'anitcs in specific 

 gravity, no great effects would follow their unequal distribution, more 



