INTRODUCTION TO MOVEMENTS OF THE OCEAN We have already (Art. 48) noticed the remarkable uniformity in the composition of sea water throughout the ocean—horizontally, from the equator to the poles ; and ver tically, from the surface to the bottom. The composition of sea water certainly varies slightly with the latitude, depth, degree of evaporation, and precipitation, but the difference is so small that, generally speaking, the salts of the sea are equally distributed throughout the whole mass. Now, warm water has a greater dissolving capacity than cold water, consequently the warm waters of the tropical seas, bulk for bulk, hold a greater amount of saline ingredients in solution than the cold waters of high latitudes. Naturally, then, the waters of the Torrid Zone should be considerably salter than those of the higher Temperate and Frigid Zones. But the difference is not so marked as we might expect, being in fact merely fractional, and, as we have said, scarcely affects the generally uniform saltness of the ocean. And although several causes are almost constantly at work, tending to produce considerable local differences—such as active evaporation in one place, and heavy precipitation in another—yet the sea is very nearly as salt in the latter as in the former. The uniform salinity
of the ocean must therefore be due to its incessant motion, by which its waters are thoroughly mingled together.
The movements of the ocean are naturally divisible into three classes, according to the manner in which they are produced. Thus aerial currents produce disturbances of tho surface waters of the ocean in the form of waves ; while the sun and moon, but more especially the latter, by attracting the mass of water, unequally, produce a periodical "bulging out of the water, thus forming the great tidal wave ; while other movements, due partly to the prevailing winds, and partly to the differences of temperature and density, constitute what are known as currents. We shall therefore treat—(1) of the atmospheric movements, or waves; (2) of the sidereal move ments, or tides; (3) of the proper movements, or currents; appending a few remarks on general oceanic circulation.