The Greeks and Romans certainly had no satisfactory understanding of the general circulation of the atmosphere. Only with the maritime voyages since the fifteenth century have we come to know the distribution of belts of prevailing winds and calms. Aristotle said that the etesian, or north, winds blow from the cold countries full of water and snow under the Great Bear; and that the south wind originates at, but not south of, the Tropic of Cancer;[90] this is the nearest he came to giving a theory of atmospheric circulation. Megasthenes had heard of the monsoons of the Indian Ocean; Pliny described the use made of them by sailors in going out to India,[91] but he made no attempt to explain the general areas of westerlies or trades. On the other hand, Aristotle,[92] Seneca,[93] and Pliny[94] all recognized and discussed at considerable length the influences of wind on weather; for example, the fact that the etesians, though they bring clear skies to Italy, deluge Ethiopia and India with rain—a conception which contains a shadow of truth.[95] Auster, the south wind, was supposed to bring rain to Italy. THE WATER ELEMENT Since water was one of the four—or, according to Aristotle, five—elements that were supposed to make up the universe, the ancient authorities looked upon the ocean as necessarily as old as the earth itself. Seneca thought that the Nile and the Ister (Danube) are of equal age with the primordial ocean, because of remarkable characteristics which differentiate them from all other streams.[96] THE SEA: ITS SALINITY, DEPTH, CURRENTS, AND TIDES We must note what features of the sea interested the Greeks and Romans. These were primarily its saltness, its depth, its currents, and its tides. The problem of why the sea is salt gave rise to a good deal of theorizing. That the evaporation of the lighter fresh water leaves behind the heavy salt water was well understood, but in the further solution of the problem opinions diverged widely. Aristotle thought that the salt was the result of combustion;[97] that it was an ashlike substance first carried into the air by the exhalations from the earth and then deposited in the sea by rainfall—particularly by the autumn rains that accompany the south winds blowing from hot, dry districts where the process of combustion is most active. Pliny believed that the salt came partly from dry vapors intermingled with the sea waters and partly from the nature of the earth, which tends to impregnate the sea with salt.[98]