6 



OUTLINES OF GEOLOGY. 



of fluid, (the ocean,) and that this is distributed with tolera- 

 ble uniformity throughout the whole surface, and is found in 

 every varying latitude. Now, were the earth more oblate 

 than is consistent with the equilibrium of a fluid mass revolv- 

 ing upon an axis, the ocean would have accumulated at the 

 poles ; and were it less oblate than would satisfy that condi- 

 tion, the waters would form a fluid zone on each side of the 

 equator. 



6. We become acquainted with the density of the earth 

 by comparing its attraction with the mutual attraction of 

 bodies near its surface. In this way it has been ascertained 

 that the mean density of the earth does not differ much from 

 five times that of water. Now, as a very large portion of 

 the surface is actually covered with water, and as the great- 

 er part of the remaining surface is composed of earthy 

 matter, whose density is not much more than 2, it may be 

 at once inferred, that the density of the earth increases from 

 the surface towards the centre. 



7. The amount of the earth's oblateness leads to a simi- 

 lar result. Were the earth of uniform density throughout, 

 the oblateness would be ^ibr Were it infinitely dense at 

 the centre, and infinitely rare at the surface, the oblateness 

 would be 5^g- . Now, as the oblateness is 3-^, which lies 

 between these limits, the density must decrease from the 

 surface towards the centre. 



II. 



OF THE TEMPERATURE OF THE CRUST OF THE EARTH. 



8. The temperature of the surface of the earth, although 

 rarely exactly the same with, never differs more than a .few 

 degrees from, that of the portion of the atmosphere which is 



