OF THE FLOW AND EBB.

All seas have their flow and ebb in the same period, but they seem
to vary because the days do not begin at the same time throughout
the universe; in such wise as that when it is midday in our
hemisphere, it is midnight in the opposite hemisphere; and at the
Eastern boundary of the two hemispheres the night begins which
follows on the day, and at the Western boundary of these hemispheres
begins the day, which follows the night from the opposite side.
Hence it is to be inferred that the above mentioned swelling and
diminution in the height of the seas, although they take place in
one and the same space of time, are seen to vary from the above
mentioned causes. The waters are then withdrawn into the fissures
which start from the depths of the sea and which ramify inside the
body of the earth, corresponding to the sources of rivers, which are
constantly taking from the bottom of the sea the water which has
flowed into it. A sea of water is incessantly being drawn off from
the surface of the sea. And if you should think that the moon,
rising at the Eastern end of the Mediterranean sea must there begin
to attract to herself the waters of the sea, it would follow that we
must at once see the effect of it at the Eastern end of that sea.
Again, as the Mediterranean sea is about the eighth part of the
circumference of the aqueous sphere, being 3000 miles long, while
the flow and ebb only occur 4 times in 24 hours, these results would
not agree with the time of 24 hours, unless this Mediterranean sea
were six thousand miles in length; because if such a superabundance
of water had to pass through the straits of Gibraltar in running
behind the moon, the rush of the water through that strait would be
so great, and would rise to such a height, that beyond the straits
it would for many miles rush so violently into the ocean as to cause
floods and tremendous seething, so that it would be impossible to
pass through. This agitated ocean would afterwards return the waters
it had received with equal fury to the place they had come from, so
that no one ever could pass through those straits. Now experience
shows that at every hour they are passed in safety, but when the
wind sets in the same direction as the current, the strong ebb
increases [Footnote 23: In attempting to get out of the
Mediterranean, vessels are sometimes detained for a considerable
time; not merely by the causes mentioned by Leonardo but by the
constant current flowing eastwards through the middle of the straits
of Gibraltar.]. The sea does not raise the water that has issued
from the straits, but it checks them and this retards the tide; then
it makes up with furious haste for the time it has lost until the
end of the ebb movement.