272 FIRST YEAR SCIENCE 



sphere. Most of the maps of the world do not represent 

 the 'southern hemisphere below latitude 60, which is 

 about the latitude of Petrograd (St. Petersburg) in the 

 northern hemisphere. Thus the equator is usually con- 

 siderably below the center of the map. 



125. Divisions of the Sea. Although most of the surface 

 water of the earth is connected, yet for many purposes it 

 is better to put this water area into somewhat arbitrary 

 divisions. We thus speak of the Atlantic, the Pacific, 

 the Indian ocean, each of which may be divided by the 

 equator into a northern and a southern part, and the 

 Arctic and Antarctic oceans which surround either pole. 

 Sometimes a division is made from the parallel 40 south 

 and this great body of water, almost without land boun- 

 daries is called the Southern Ocean. 



The boundaries of these oceans are irregular in shape, 

 but with the exception of the great Southern Ocean and 

 of the Arctic Ocean, which is really an inclosed sea, 

 they narrow toward the north. They have a number of 

 partially landlocked seas connected with them. In some 

 instances these penetrate far into the land, as in the cases 

 of the Mediterranean Sea and Gulf of Mexico. The sur- 

 face of the sea is level, unstable, easily moved and always 

 rising and falling in rapid and changeful undulations. 



126. Continental Shelf. Around the border of the con- 

 tinents and of those islands which are near the continents, 

 there extends, in some cases to a distance of two or three 

 hundred miles, a gradually deepening ocean floor. This 

 gradually deepening border is called the continental shelf. 

 When this floor has reached the depth of about 600 feet, 

 the gradual slant suddenly changes into a quick descent 

 to the depths of the ocean, two or three miles. 



Upon this shelf lie the great continental islands, like 

 the British Isles and the East Indies. It is this that fur- 



