Kitap 5
9.6
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.
9.7
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. Hence, on account of the porous nature of the charcoal and the insertion of the pipes into the drains, quantities of water will be conducted away, and the walks will thus be rendered perfectly dry and without moisture.
