Seneca’s account of the Nile derived from travellers and previous writers gives a clear summary of what was then known about the river, but of more interest is his discussion of the opinions that had been propounded before his time as to the cause of the annual rise. He first quotes the view of Anaxagoras, shared by the Greek tragedians and widely accepted, that this rise was due to the melting of snow on the uplands of Ethiopia. This idea he cogently combats by adducing various kinds of evidence of the great warmth of the climate in those southern regions. Some of these proofs, indeed, are exaggerations, as where he affirms that silver is unsoldered or melted. But one of his proofs, drawn from the habits of the animals of the country, is worthy of notice. He remarks that no hibernating creature is found there, and that even in midwinter the serpent is seen above ground. He argues that in Africa, as in Europe, melting snow would swell the rivers in spring and early summer, whereas the Nile flood continues to rise later during four months. In a subsequent part of this treatise (235) allusion is made to an explanation which had been given of the rise of the Nile, that it is due not to the fall of rain from above but to the outflow of water from within the earth, and it is in connection with this opinion that he cites the experience of Nero’s two centurions above referred to, as if he were disposed to believe that what these explorers saw was really a vast body of water issuing from underground. The opinion of Thales is next criticised that the Etesian or northerly winds drive the waters of the Mediterranean against the mouths of the Nile and consequently pond back the waters of the river. This view was of course entirely erroneous, but though Seneca rejects it, he does not seem to have quite understood it, for he argues that, coming from the same quarter as the winds, the Nile water should not have been turbid, but clear and blue, like that of the sea. In commenting upon the futile support given by Euthymenes of Marseilles to the idea of Thales, Seneca throws light on the wide extent to which the coasts of the outer sea had then been made known by trading vessels. In rejecting another explanation proposed by Oenopides of Chios, the author shows that he is aware of the fact that caves and wells are warm in winter and cool in summer, and that he has partly divined the reason, when he states that in winter they are warm since they do not admit the frosty air from without and in summer they feel cold because the warm air from outside has not penetrated into their recesses. He returns to this subject in Book VI. (241).[115] [115] The various ancient interpretations of the cause of the Nile’s annual rise are succinctly given by Lucretius (_De Rer. Nat._ vi. 712–37), but he does not indicate a preference for any one in particular, though he devotes most space to the influence of the Etesian winds.