50 



PHYSICAL GEOGRAPHY. 



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

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

 it may be 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 Cleaveland, 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. — 

 Saline Contents.— Mediterranean. — Temperature of Earth. — 

 Reason for Supposing it has undergone a Change. — Central 

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

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

 ains. — Highest Land in Asia. — In 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 our 

 globe. 



