50 



PHYSICAL GEOGRAPHY. 



bleaching cotton, linen, paper, &c. When melted 

 with borax, it always gives a purple tinge, by which 

 it maybe distinguished from iron and other substan- 

 ces. The other metals occur so rarely, and are of 

 so little importance, that we shall pass them over; 

 referring the reader, however, to Cieaveland, Corn- 

 stock., Dana, and other authors who have treated of 

 mineralogy, for any information in relation to them 



CHAPTER IV. 



PHYSICAL GEOGRAPHY OF THE EARTH. 



Shape of the Earth. — Mean Density . — Ocean — Mean depth. — 

 Sahne Contents. — Mediterranean. — Temperature of Earth.— 

 Reason for Supposing it has undergone a Change. — Central 

 Heat. — Facts to Support such a Theory. — Influence of Chmaic 

 on the Animal and Vegetable Kingdom. — Height oi Mount- 



" ains. — Highest Land in Asia. — hi Europe. — In America. — 

 Shape of Hills and Mountains. 



The figure of the earth has been compared to that 

 of an orange, being a flattened spheroid. The equa- 

 torial diameter is about 7924 miles. 



The polar axis 7898 " 



Difference 26 " 

 It can be demonstrated that a fluid body, possess- 

 ed of rotatory motion in space, would assume a sim- 

 ilar shape. 



The mean density of the earth has been variously 

 estimated, but, from experiments performed by dif- 

 ferent philosophers, it is now believed to be about 

 five times greater than that of water, and, conse^ 

 quently, double that of the mineral crust of ouj: 

 globjB, 



