PHYSICAL GEOGRAPHY. 



51 



Nearly three fourths of the whole surface of the 

 earlh is covered by water. Indeed, the Pacific Ocean 

 alone covers a greater extent of square miles than 

 the whole dry laud with which we are acquainted. 

 The mean depth of the ocean is estimated to be 

 about three miles, and the mean height of the land 

 above the ocean level, from a mile and a half to two 

 miles ; so that, as De la Beche observes, the present 

 dryland might be distributed over the bottom of the 

 ocean in such a manner that the whole globe would 

 be covered by a mass of waters. 



A very small portion only of the earth's surface 

 is fitted for the habitation of man. Vast regions are 

 covered by polar ice and snow, and by sandy deserts, 

 steril mountains, marshes, rivers, and lakes, so that, 

 it is believed, not more than one fifth of the globe 

 is habitable. The other portions, it is true, are ten- 

 anted by various tribes and species of animated be- 

 ings, " exulting in the pleasures of existence, inde- 

 pendent of human control, and no way subservient 

 to the necessities or caprices of man." 



The waters of the ocean are more or less impreg- 

 nated with salt, there being less at the surface than 

 at some distance below, from the effects of evapo- 

 ration and the fall of rain.* The saline contents 

 vary from 3 to 4 per cent. These consist of com- 

 mon salt {niuriate of soda), sulphate of soda, muinate 

 of magnesia, and muriate of Lime. The Mediterra- 

 nean contains larger proportions of salt than the 

 ocean; which is supposed to be owing to the fact, 

 that the evaporation from its surface is greater than 

 the quantity of fresh water with which it is supplied. 

 This seems to be proved by the circumstance that 

 two curreiits constantly flow into it, one from the 

 Black Sea and the other from the Atlantic. 



The temperature of the earth, which is mainly 



* This is denied by Dr. Marcet, and soniie experiments of Mr 

 ScQiesby render it doubtful. 



