PART IV 

 THE OCEAN 



CHAPTER XIX 



THE OCEAN AS A WHOLE 



The oceans lie in the great depressions in the earth's surface 

 (p. 3). These depressions are called basins, but they have little 

 resemblance to the homely vessel which bears this name. If 

 with a string four feet long as a radius, we draw on the blackboard 

 an arc three feet long, it will represent about an eighth" of a circle. 

 This may be taken to represent the width of the Atlantic Ocean 

 between the United States and Europe. If the top of the chalk 

 line stands for the surface of the ocean, another line representing 

 the bottom of the ocean could not well be drawn below it with a 

 common crayon, without exaggerating the depth of the water. 

 Fig. 262 may help to give us some conception of the real shape of 

 an ocean basin, which is, in general, convex upward. 



The sea-level. The surface of the sea is very different from 

 that of the land. The latter is uneven, while the former is essen- 

 tially even. We speak of the surface of the sea as if it were level, 

 but it is in reality a curved surface, and its curvature is nearly that 

 of a sphere, somewhat flattened at the poles'. 



What the physical geography of the sea includes. The physi- 

 cal geography of the sea includes many things. Among them are 

 (1) the distribution of its waters, (2) its depth at all points, (3) 

 the topography of its bottom, (4) the composition of its water, 

 (5) its color, (6) its temperature at the surface and beneath it, (7) 

 its movements, (8) its life, and (9) the material of its bottom. 



The physical geography of the sea has become known in various 

 ways. The composition of its waters is known by chemical analysis. 



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