80 



GEOLOGY. 



cally as incompressible. It is also calculated that steel 

 at the centre of the globe would be compressed into a 

 fourth of its bulk, and most rocks into an eighth of their 

 bulk. 



157. Form of the Earth. I have said that the earth is 

 nearly round. -Its deviation from a perfectly spherical 

 shape is very slight that is, compared with its whole 

 bulk. Though it bulges out at the equator 13^ miles, 

 this, in a ball eight thousand miles in diameter, is but a 

 small matter. This can be shown by the diagram, Fig. 

 31. Let the curved line PEPE represent the circum- 



\**L 



Fig. 31. 



ference of the earth as it is with the line running through 

 the poles, PP. But this is not a true circle, for the di- 

 ameter EE is longer than the diameter PP. The true 

 circle described by the diameter PP revolving on the 



