290 FIRST YEAR SCIENCE 



land habitable, the utility of the great water surfaces 

 becomes apparent. From the evaporation of the ocean 

 surface comes nearly all the water which supplies man, 

 land animals and plants. 



It is not only true that all streams eventually run to the 

 sea but it is also true that all their water comes from 

 the sea. Other things being equal, the smaller the 

 surface for evaporation the less the water supplied to 

 the land. Besides supplying the land with water, the 

 ocean has a great effect on its climate. The animals of 

 the sea also furnish food for thousands. A large part 

 of the earth's population is now, and always has been, 

 located not far from the shore of the ocean. 



In early times before the advent of railways almost all 

 commerce was carried on over the sea. Even now this is 

 the cheaper way of transportation. Modern methods of 

 conveyance have enabled man to live with comfort at a 

 considerable distance from the ocean, but the dry interiors 

 of continents still remain sparsely inhabited. All com- 

 mercial nations must have an outlet to the sea and to 

 obtain it much blood and treasure have often been spent. 

 Man for his commerce has even broken through the bar- 

 riers by which nature has separated seas and oceans, as in 

 the case of the Suez and Panama canals. 



Summary. The sea occupies nearly three quarters of 

 the area of the globe. It is usually spoken of as divided 

 into oceans, the Atlantic, Pacific, Indian, Arctic and Ant- 

 arctic. Although some parts of the ocean are nearly six 

 miles deep, the most interesting and most extensively in- 

 habited part of it is that above the continental shelf. This 

 shelf is the gradual slope from the edge of the different 

 continents to a depth of about six hundred feet. Beyond 

 this the descent becomes rapid. 



