=Differences in level and their results.=—While the surface of the ocean is the common datum plane to which elevations and depressions are referred, it is to be remembered that the sea has “a very complicated undulating surface in consequence of the attraction which the heterogeneous and elevated portions of the lithosphere exercise on the liquid hydrosphere. In the opinion of geodesists, the geoid may in some places depart from the figure of the spheroid by 1000 feet.”[146] These variations in level would, however, not occasion circulation. The differences in level which determine circulation are much more trivial. Every stream which pours fresh water into the sea tends to raise the level of the water where it enters. The waters brought to the ocean by the Amazon, the Mississippi, and other great rivers would appreciably change the level of the sea at their debouchures, if the excess did not promptly flow away. The ready mobility of the water, however, prevents its accumulation, and the discharge of every stream generates widespread movement. This movement is strongest at the debouchure, and weakens with increasing distance from it, though in the case of great streams, such as the Amazon, the movement is traceable, by means of the sediment which the water carries, hundreds of miles out to sea. Changes of level are also brought about by the winds, which pile up water along the shore against which they blow. The level of the water is said to have risen 24 feet at Calcutta on October 5, 1864, as the result of a severe storm. While this is exceptional, a rise of 2 feet is not rare. This piling up of the waters along shore insures a compensating movement (undertow, littoral currents, etc.) in some other direction. Unequal evaporation and precipitation likewise disturb the level of the sea and occasion movement. In the open sea the movements generated by differences of level, like those generated by differences of density, are chiefly slow, creeping movements, but movements which never cease. In bays and gulfs, on the other hand, the surface of the water may be so raised, either as the result of wind, river discharge, or heavy precipitation, as to give rise to strong outward currents. There is little doubt at the present time that the Gulf Stream owes its origin primarily to the difference of level between the Gulf of Mexico and the Atlantic.[147]