The tremendous fury of the wind driven by the falling in of the
hills on the caves within--by the falling of the hills which served
as roofs to these caverns.

A stone flung through the air leaves on the eye which sees it the
impression of its motion, and the same effect is produced by the
drops of water which fall from the clouds when it [16] rains.

[17] A mountain falling on a town, will fling up dust in the form of
clouds; but the colour of this dust will differ from that of the
clouds. Where the rain is thickest let the colour of the dust be
less conspicuous and where the dust is thickest let the rain be less
conspicuous. And where the rain is mingled with the wind and with
the dust the clouds created by the rain must be more transparent
than those of dust [alone]. And when flames of fire are mingled with
clouds of smoke and water very opaque and dark clouds will be formed
[Footnote 26-28: Compare Pl. XL, 1--the drawing in Indian ink on the
left hand side, which seems to be a reminiscence of his observations
of an eruption (see his remarks on Mount Etna in Vol II).]. And the
rest of this subject will be treated in detail in the book on
painting.

[Footnote: See the sketches and text on Pl. XXXVIII, No. 1. Lines
1-16 are there given on the left hand side, 17-30 on the right. The
four lines at the bottom on the right are given as No. 472. Above
these texts, which are written backwards, there are in the original
sixteen lines in a larger writing from left to right, but only half
of this is here visible. They treat of the physical laws of motion
of air and water. It does not seem to me that there is any reason
for concluding that this writing from left to right is spurious.
Compare with it the facsimile of the rough copy of Leonardo's letter
to Ludovico il Moro in Vol. II.]