Besides the regular periodical currents produced in the ocean by the tides, various others arise from different causes.[31] The waters of the sea may be put in motion by an external impulse, by a difference in temperature and saltness, by the periodical meeting of the polar ice, or by the inequality of evaporation that takes place in different latitudes. Sometimes several of these causes concur in producing the same effect; at others, their actions are opposed to one another, and their effects wholly or partially destroyed. Some of those currents constantly follow the same direction, others are subject to periodical changes, whilst a third class are more accidental. The most regular and extensive current on the globe is that which constantly flows from east to west, between the tropics, and extends on each side of the equator to about the thirtieth degree of latitude. This vast current necessarily results from the attraction of the heavenly bodies, the diurnal motion of the earth, and the direction of the trade winds. Its existence is incontestibly proved by the fact, that vessels sailing to the westward, are always ahead of their reckoning; that is, their real situation, as determined by observations of the heavenly bodies, is always found to be west of that estimated from the rate of which the vessel is supposed to sail, as impelled by the wind alone. This difference of situation is occasioned by the general movement of the waters in the same direction, and is, consequently, the proper measure of the current. This is the reason why navigators, in sailing from Europe to America and the West India Islands, make the latitude of the Canaries, and then shape their course in the direction of the wind and current across the Atlantic. A general current also flows from the poles towards the equator. This arises from the increased evaporation in the equatorial regions, and the augmented temperature of the waters, which render them specifically lighter than those of the ocean in higher latitudes, as well as from the increased supplies produced by the melting of the polar ice; all of which render these currents necessary to maintain the equilibrium of this perpetually circulating fluid. Their existence and effects are fully attested by the enormous masses of polar ice, which they convey into the more temperate regions of the ocean, and which sometimes float as low as forty degrees of latitude.