PHYSICAL GEOGRAPHY. 
51 
Nearly three fourths of the whole surface of the 
earth is covered by water. Indeed, the Pacific Ocean 
alone covers a greater extent of square miles than 
the whole dry land 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 (muriate of soda), sulphate of soda, muriate 
of 77iagnesia, 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 currents 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 some experimentg of Mr 
Scoresby render it doubtful. 
