On the motion of air (996--999).
996.
That the return eddies of wind at the mouth of certain valleys
strike upon the waters and scoop them out in a great hollow, whirl
the water into the air in the form of a column, and of the colour of
a cloud. And I saw this thing happen on a sand bank in the Arno,
where the sand was hollowed out to a greater depth than the stature
of a man; and with it the gravel was whirled round and flung about
for a great space; it appeared in the air in the form of a great
bell-tower; and the top spread like the branches of a pine tree, and
then it bent at the contact of the direct wind, which passed over
from the mountains.
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