There have been various reasons suggested for this increase of the river. Of these, however, the most probable are, either that its waters are driven back by the Etesian winds[3535], which are blowing at this season of the year from an opposite direction, and that the sea which lies beyond is driven into the mouths of the river; or else that its waters are swollen by the summer rains of Æthiopia[3536], which fall from the clouds conveyed thither by the Etesian winds from other parts of the earth. Timæus the mathematician has alleged a reason of an occult nature: he says that the source of the river is known by the name of Phiala, and that the stream buries itself in channels underground, where it sends forth vapours generated by the heat among the steaming rocks amid which it conceals itself; but that, during the days of the inundation, in consequence of the sun approaching nearer to the earth, the waters are drawn forth by the influence of his heat, and on being thus exposed to the air, overflow; after which, in order that it may not be utterly dried up, the stream hides itself once more. He says that this takes place at the rising of the Dog-Star, when the sun enters the sign of Leo, and stands in a vertical position over the source of the river, at which time at that spot there is no shadow thrown. Most authors, however, are of opinion, on the contrary, that the river flows in greater volume when the sun takes his departure for the north, which he does when he enters the signs of Cancer and Leo, because its waters then are not dried up to so great an extent; while on the other hand, when he returns towards the south pole and re-enters Capricorn, its waters are absorbed by the heat, and consequently flow in less abundance. If there is any one inclined to be of opinion, with Timæus, that the waters of the river may be drawn out of the earth by the heat, it will be as well for him to bear in mind the fact, that the absence of shadow is a phænomenon which lasts continuously[3537] in these regions.