ASTRONOMY. 22 



have proved to life in the rest of the creation ; and, most 

 perhaps of all, in plants. The habitable earth, and its 

 beautiful variety, might have been destroyed, by a simple 

 mischance in the axis of rotation.* 



(*) III. All this, however, proceeds upon a supposition 

 of the earth having been formed at first an oblate spheriod. 

 There is another supposition ; and, perhaps, our limited 

 information will not enable us to decide between them. 

 The second supposition is, that the earth, being a mixed 

 mass, somewhat fluid, took, as it might do, its present form, 

 by the joint action of the mutual gravitation of its parts and 

 its rotatory motion. This, as we have said, is a point in 

 the history of the earth, which our observations are not 

 sufficient to determine. For a very small depth below the 

 surface (but extremely small, less, perhaps, than an eight 

 thousandth part,t compared with the depth of the centre) 

 we find vestiges of ancient fluidity. But this fluidity must 

 hav€ gone down many hundred times further than we can 

 penetrate, to enable the earth to take its present oblate 

 form ; and, whether any traces of this kind exist to that 

 depth, we are ignorant. Calculations were made a few 

 years ago of the mean density of the earth, by comparing 

 the force of its attraction with the force of attraction of a 

 rock of granite, the bulk of which could be ascertained ; 

 and the upshot of the calculation was, that the earth upon 

 an average, through its whole sphere, has twice the density 



*^The earth being an oblate spheroid, we may suppose it to be cut by 

 a plane passing through A B, Fig 3, Plate XXXIX, which may rep- 

 rent its axis, and the common section of this plane with the spheroid 

 will be an ellipse like A D B E ; of this ellipse A B will be an axis ; 

 and, from the property of the curve, it will also be the shortest line 

 which can be drawn through the centre C. If now the diameter D E 

 be drawn at right angles to A B, it will be the longest line which can 

 be drawn in the ellipse, and it will represent a diameter of the equator. 

 As the plane passing through A B is not confined to one situation more 

 than another, D E may represent any "one of the longest axes of the 

 spheroid," and will, as well as A B, always be a " permanent axis of 

 rotation." But if any other diameter, as G H, is taken, the earth 

 could not continue to revolve permanently about it. Paxton. 



t The "deep St. John," one of the deepest mines in the Hartz, was 

 found by M. Deluc to sink 1359 feet. This was in 1778; and it may, 

 since that time, have been carried lower, but probably not to the 

 depth of the mine of Valenciana in New Spain, the bottom of which, 

 according to Humboldt, is 1681 feet below the surface. Now the di- 

 ameter of the earth being about 7912 miles, "the eight-thousandth 

 part of the depth of the centre " must be 2611 feet, or nearly 

 half a mile. Ibid, 



u 



