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.

[Footnote: Above the first sketch we find, in the original, this
note: "_Sopra il pote rubaconte alla torricella_"; and by the
second, which represents a pier of a bridge, "_Sotto l'ospedal del
ceppo._"]