the intense heat always melts into air all the clouds which are borne thither by the winds.  And if you chose to say that such rivers, as increase in July and August, come from the snows which melt in May and June from the sun’s approach to the snows on the mountains of Scythia [Footnote 9:  Scythia means here, as in Ancient Geography, the whole of the Northern part of Asia as far as India.], and that such meltings come down into certain valleys and form lakes, into which they enter by springs and subterranean caves to issue forth again at the sources of the Nile, this is false; because Scythia is lower than the sources of the Nile, and, besides, Scythia is only 400 miles from the Black sea and the sources of the Nile are 3000 miles distant from the sea of Egypt into which its waters flow.

The tide in estuaries.

971.

Book 9, of the meeting of rivers and of their ebb and flow.  The cause is the same in the sea, where it is caused by the straits of Gibraltar; and again it is caused by whirlpools.

[3] If two rivers meet together to form a straight line, and then below two right angles take their course together, the flow and ebb will happen now in one river and now in the other above their confluence, and principally if the outlet for their united volume is no swifter than when they were separate.  Here occur 4 instances.

[Footnote:  The first two lines of this passage have already been given as No. 957.  In the margin, near line 3 of this passage, the text given as No. 919 is written.]

On the alterations, caused in the courses of rivers by their confluence (972-974).

972.

When a smaller river pours its waters into a larger one, and that larger one flows from the opposite direction, the course of the smaller river will bend up against the approach of the larger river; and this happens because, when the larger river fills up all its bed with water, it makes an eddy in front of the mouth of the other river, and so carries the water poured in by the smaller river with its own.  When the smaller river pours its waters into the larger one, which runs across the current at the mouth of the smaller river, its waters will bend with the downward movement of the larger river. [Footnote:  In the original sketches the word Arno is written at the spot here marked A, at R.  Rifredi, and at M.  Mugnone.]

973.

When the fulness of rivers is diminished, then the acute angles formed at the junction of their branches become shorter at the sides and wider at the point; like the current a n and the current d n, which unite in n when the river is at its greatest fulness.  I say, that when it is in this condition if, before the fullest time, d n was lower than a n, at the time of fulness d n will be full of sand and mud.  When the water d n falls, it will carry away the mud and remain with a lower bottom, and the channel a n finding itself the higher, will fling its waters into the lower, d n, and will wash away all the point of the sand-spit b n c, and thus the angle a c d will remain larger than the angle a n d and the sides shorter, as I said before.