The first explanation was, that the rising of the river was occasioned by the prevalence of northerly winds on the Mediterranean at that time of the year, which drove back the waters at the mouth of the river, and so caused the accumulation of the water in the upper parts of the valley. Herodotus thought that this was not a satisfactory explanation; for sometimes, as he said, these northerly winds did not blow, and yet the rising of the river took place none the less when the appointed season came. Besides, there were other rivers similarly situated in respect to the influence of prevailing winds at sea in driving in the waters at their mouths, which were, nevertheless, not subject to inundations like the Nile. The second theory was, that the Nile took its rise, not, like other rivers, in inland lakes, or among inland mountains, but in some remote and unknown ocean on the other side of the continent, which ocean the advocates of this theory supposed might be subject to some great annual ebb and flow; and from this it might result that at stated periods an unusual tide of waters might be poured into the channel of the river. This, however, could not be true, for the waters of the inundation were fresh, not salt, which proved that they were not furnished by any ocean. A third hypothesis was, that the rising of the water was occasioned by the melting of the snows in summer on the mountains from which the sources of the river came. Against this supposition Herodotus found more numerous and more satisfactory reasons even than he had advanced against the others. In the first place the river came from the south--a direction in which the heat increased in intensity with every league, as far as travelers had explored it; and beyond those limits, they supposed that the burning sun made the country uninhabitable. It was preposterous to suppose that there could be snow and ice there. Then, besides, the Nile had been ascended to a great distance, and reports from the natives had been brought down from regions still more remote, and no tidings had ever been brought of ice and snow. It was unreasonable, therefore, to suppose that the inundations could arise from such a cause.