WAVE-WORN CLIFFS AND PEBBLE-BEACH AT ETRETAT, FRANCE.
(FROM A PAINTING BY WILLIAM P. W. DANA.)
But these geographical distinctions are merely convenient methods of speech. After all, there is only one ocean “poured round all,” and its particles are incessantly changed in place and remingled by means of a world-wide system of tides and currents, the effect of which is to keep sea-water everywhere uniform in character and perfectly pure and healthful.
IN MID-OCEAN: A GREAT WAVE.
CHAPTER II
WAVES, TIDES, AND CURRENTS
Now that we have studied the ancient ocean, it is time to study its present characteristics and understand the great and important part it plays in the world.
A very striking thing about the ocean is its flatness. Being water, it seeks always to find its level; and we commonly assume that it everywhere does so, and take the sea-level as the standard from which to calculate all heights above or depths below its surface; that is, we assume that every part of the surface of the ocean when calm and at mean tide is exactly the same distance from the center of the globe. This, however, is not wholly true. Careful observation has shown that the Pacific is several feet lower on the western shore of the Isthmus of Darien than is the Atlantic on its eastern shore—a fact due, no doubt, to the crowding of water by the Gulf Stream into the Caribbean Sea. The Mediterranean is known to be somewhat higher than the Atlantic, and other differences exist in similar places elsewhere.