, be divided into eight and one half parts, and let one of these
be assigned to the thickness of a column. Let the base, including the
plinth, be fixed at half the thickness, and let the proportions of the
capital be as shown in the third book. If the column is to be
Corinthian, let its shaft and base be proportioned as in the Ionic, but
its capital, as has been written in the fourth book. In the stylobates,
let the increase made there by means of the "scamilli impares" be taken
from the description written above in the third book. Let the
architraves, coronae, and all the rest be developed, in proportion to
the columns, from what has been written in the foregoing books.

5. The space in the middle, between the colonnades and open to the sky,
ought to be embellished with green things; for walking in the open air
is very healthy, particularly for the eyes, since the refined and
rarefied air that comes from green things, finding its way in because of
the physical exercise, gives a clean-cut image, and, by clearing away
the gross humours from the eyes, leaves the sight keen and the image
distinct. Besides, as the body gets warm with exercise in walking, this
air, by sucking out the humours from the frame, diminishes their
superabundance, and disperses and thus reduces that superfluity which is
more than the body can bear.

6. That this is so may be seen from the fact that misty vapours never
arise from springs of water which are under cover, nor even from watery
marshes which are underground; but in uncovered places which are open to
the sky, when the rising sun begins to act upon the world with its
heat, it brings out the vapour from damp and watery spots, and rolls it
in masses upwards. Therefore, if it appears that in places open to the
sky the more noxious humours are sucked out of the body by the air, as
they obviously are from the earth in the form of mists, I think there is
no doubt that cities should be provided with the roomiest and most
ornamented walks, laid out under the free and open sky.

7. That they may be always dry and not muddy, the following is to be
done. Let them be dug down and cleared out to the lowest possible depth.
At the right and left construct covered drains, and in their walls,
which are directed towards the walks, lay earthen pipes with their lower
ends inclined into the drains. Having finished these, fill up the place
with charcoal, and then strew sand over the walks and level them off.
H