28 THE OCEAN 



narrow openings between the islands and 

 against the wind, the tide turns and forces it 

 back. In fact the amount of water that runs 

 back is so inconsiderable, that the level of the 

 Gulf of Mexico remains only about three 

 feet above that of the ocean, as has been 

 proved by careful surveys and observations. 



The trade winds, however, are not the only 

 cause of the Gulf Stream and similar cur- 

 rents, for the water of the ocean is itself con- 

 stantly in motion, owing to the revolution of 

 the earth upon its axis and the variation in 

 temperature between the cold water of the 

 arctic regions and the warm water near the 

 equator. As heated water expands and rises 

 and cold water contracts and sinks there is 

 constantly an interchange of water between the 

 polar and the equatorial regions which causes 

 the warm water in the tropics to flow north 

 and south to replace the colder and heavier 

 water from the arctic and antarctic regions. 

 As the earth revolves upon its axis from west 

 to east and as the water is free to move it slips 

 backward as the earth turns, or in other words 

 moves slowly westward, just as part of the 



