Herodotus: II 19-31. On the Nile River

Concerning the nature of the river, I was not able to gain any information either from the priests or from others. I was particularly anxious to learn from them why the Nile, at the commencement of the summer solstice, begins to rise, and continues to increase for a hundred days- and why, as soon as that number is past, it forthwith retires and contracts its stream, continuing low during the whole of the winter until the summer solstice comes round again. On none of these points could I obtain any explanation from the inhabitants, though I made every inquiry, wishing to know what was commonly reported- they could neither tell me what special virtue the Nile has which makes it so opposite in its nature to all other streams (contradiction)

Some of the Greeks, however, wishing to get a reputation for cleverness (public discussion), have offered explanations of the phenomena of the river, for which they have accounted in three different ways. Two of these I do not think it worth while to speak of, further than simply to mention what they are. One pretends that the Etesian winds cause the rise of the river by preventing the Nile-water from running off into the sea. But in the first place it has often happened, when the Etesian winds did not blow, that the Nile has risen according to its usual pattern; and further, if the Etesian winds produced the effect, the other rivers which flow in a direction opposite to those winds ought to present the same phenomena as the Nile, and the more so as they are all smaller streams, and have a weaker current. But these rivers, of which there are many both in Syria and Libya, are entirely unlike the Nile in this respect (observation, contradiction).

The second opinion is even more unscientific than the one just mentioned, and also, if I may so say, more marvellous....

The third explanation, which is very much more plausible than either of the others, is positively the furthest from the truth. It is, that the inundation of the Nile is caused by the melting of snows. Now, as the Nile flows out of Libya, through Ethiopia, into Egypt, how is it possible that it can be formed of melted snow, running, as it does, from the hottest regions of the world into cooler countries? (observation, contradiction). Many are the proofs whereby any one capable of reasoning on the subject may be convinced that it is most unlikely this should be the case. The first and strongest argument is furnished by the winds, which always blow hot from these regions (observation, analogy, contradiction). The second is that rain and frost are unknown there. Now whenever snow falls, it must of necessity rain within five days;.so that, if there were snow, there must be rain also in those parts. Thirdly, it is certain that the natives of the country are black with the heat, that the kites and the swallows remain there the whole year, and that the cranes, when they fly from the rigours of a Scythian winter, flock thither to pass the cold season. If then, in the country whence the Nile has its source, or in that through which it flows, there fell ever so little snow, it is absolutely impossible that any of these circumstances could take place. (contradiction)

Perhaps, after censuring all the opinions that have been put forward on this obscure subject, one ought to propose some theory of one's own (public discussion) . I will therefore proceed to explain what I think to be the reason of the Nile's swelling in the summer time. During the winter, the sun is driven out of its usual course by the storms, and removes to the upper parts of Libya (modern Sudan). This is the whole secret in the fewest possible words; for it stands to reason that the country to which the sun approaches the nearest, and which it passes most directly over, will be scantest of water, and that there the streams which feed the rivers will shrink the most.

To explain, however, more at length, the case is this. The sun, in its passage across the upper parts of Libya, affects them in the following way. As the air in those regions is constantly clear, and the country warm through the absence of cold winds, the sun in its passage across them acts upon them exactly as it wont to act elsewhere in summer (pattern), when its path is in the middle of heaven- that is, it attracts the water. ... The Nile, not deriving any of its bulk from rains, and being in winter subject to the attraction of the sun, naturally runs at that season, unlike all other streams, with a less burthen of water than in the summer time. For in summer it is exposed to attraction equally with all other rivers, but in winter it suffers alone. The sun, therefore, I regard as the sole cause of the phenomenon. (Non-theological explanation)