In the West, near to Flanders, the sea rises and decreases every 6
hours about 20 braccia, and 22 when the moon is in its favour; but
20 braccia is the general rule, and this rule, as it is evident,
cannot have the moon for its cause. This variation in the increase
and decrease of the sea every 6 hours may arise from the damming up
of the waters, which are poured into the Mediterranean by the
quantity of rivers from Africa, Asia and Europe, which flow into
that sea, and the waters which are given to it by those rivers; it
pours them to the ocean through the straits of Gibraltar, between
Abila and Calpe [Footnote 5: _Abila_, Lat. _Abyla_, Gr. , now
Sierra _Ximiera_ near Ceuta; _Calpe_, Lat. _Calpe_. Gr., now
Gibraltar. Leonardo here uses the ancient names of the rocks, which
were known as the Pillars of Hercules.]. That ocean extends to the
island of England and others farther North, and it becomes dammed up
and kept high in various gulfs. These, being seas of which the
surface is remote from the centre of the earth, have acquired a
weight, which as it is greater than the force of the incoming waters
which cause it, gives this water an impetus in the contrary
direction to that in which it came and it is borne back to meet the
waters coming out of the straits; and this it does most against the
straits of Gibraltar; these, so long as this goes on, remain dammed
up and all the water which is poured out meanwhile by the
aforementioned rivers, is pent up [in the Mediterranean]; and this
might be assigned as the cause of its flow and ebb, as is shown in
the 21st of the 4th of my theory.

III.

SUBTERRANEAN WATER COURSES.

Theory of the circulation of the waters (961. 962).