One of the natural phenomena most puzzling to the Greeks and Romans was the inundation of the Nile.[135] Herodotus in his famous book on Egypt had given a lengthy account of the Nile and what it meant to Egypt. He had called Egypt the “gift of the Nile,” for he understood the alluvial character of the country. His theory as to the cause of the flood—he held that the normal height of the river was its flood height but that the etesian winds, by driving the sun southward out of its course in winter, caused the sun to dry up the headwaters of the stream—was less successful than his description of the features of the flood itself. Seneca also gives a long and extremely picturesque description of the inundation[136] and sets forth various older explanations of its origin, all of which he tries to refute without presenting an opinion of his own. He tells how, starting in the upper reaches of the river, the flood travels downstream and arrives in Egypt about midsummer; how it adds to the fertility of the country by its deposits of silt; and how—here Seneca repeats the crisp phrase of Herodotus—Egypt is the creation of its stream. Among the various theories which he comments upon and refutes it is rather significant to find one which had been propounded by Anaxagoras and which is now recognized, in part at least, as the right explanation: that the high water is caused by the melting of the snows on the Ethiopian mountains. Seneca said that there were twenty proofs available to refute this hypothesis.[137] Another view which Seneca rejected was that the flood was caused by the etesian winds backing up the water, a theory fated to reappear in many medieval books, among them the _Expositio in hexaemeron_ of Peter Abelard.[138] Pliny discussed the Nile and its peculiarities.[139] Like Herodotus, he believed that it rises in the western part of Africa and reaches the Sudan and Upper Egypt only after a series of long subterranean journeys. He described the flood, giving statistics of the various heights of the water on the nilometer and explaining which heights meant plenty and which meant famine. He shows a lack of critical sense in his remarks on the causes of the high water; for he held that two theories are equally worthy of credence, the theory of the etesian wind, which we have just examined, and the true explanation that the floods are due to summer rains in Ethiopia. THE LANDS