The Manuscripts of Leonardo da Vinci, Vol. 4

On the 22nd day of April, 1508, I began this original: "chominciai questo"

and it is a collection of many papers which I have
copied here, hoping then to put them in their
proper places, in order, according to the subjects
of which they treat. I believe that before
I reach the end of this work,
I may have to repeat the same thing
multiple times; therefore, reader, do not blame me,
because the subjects are many and the memory
cannot retain them all and say, "This
I do not want to write because I already wrote it."
And if I wished to avoid this, I would fall into the
danger of not writing it at all due to the
long interval already mentioned.

If you wish to treat the subject of water,
first experience, and then reason original: "pria l'esperietia e poi la ragio",
must demonstrate why such things happen
in such a way. And this is the rule
that must be observed by those who investigate
natural effects.

And although nature begins from
reason and ends in experience,
it is necessary for us to do the opposite:
that is, beginning, as I said
above, from experience, and with that,
investigating the reason.

on the motion of water

Water by itself does not move
unless it descends, fleeing the center
of the world Leonardo uses "center of the world" to refer to the Earth's center of gravity., unless it is moved by
an external force such as winds,
or vapors, or the heat of the sun, or by
the flight from a vacuum vacuum: Leonardo refers to the "horror vacui," the idea that nature resists empty space, which explains how siphons draw water upward, as is seen in
instruments for drawing water upward.

Air thickens or becomes rarefied,
and it thickens the closer it gets to
the earth, and it rarefies the further it moves
away from the earth. And this happens because of the weight
of the moist vapors that are mixed into it,
no differently than how the water of the sea is thicker
at the bottom than at the surface, because of the
heavy weight of the salt that is
mixed into it. And such weight on the earth is
greater than above said earth; and for
this reason, the air on the earth will be thicker
than above it. And such weight will be
greater on the earth because the air
compresses itself between the weight
of the vapors and the earth; and this it does not
do above, because on one side it is
bounded by air less thick than itself,
and on the other, it is bounded by vapor
thicker than itself. Thus, this
air will be rarer high up than on the earth.

The diagram on the right side of Folio 1v illustrates the density of atmospheric layers relative to the Earth's surface.