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
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.
8. Furthermore, our ancestors in establishing these works provided cities with storehouses for an indispensable material. The fact is that in sieges everything else is easier to procure than is wood. Salt can easily be brought in beforehand; corn can be got together quickly by the State or by individuals, and if it gives out, the defence may be maintained on cabbage, meat, or beans; water can be had by digging wells, or when there are sudden falls of rain, by collecting it from the tiles. But a stock of wood, which is absolutely necessary for cooking food, is a difficult and troublesome thing to provide; for it is slow to gather and a good deal is consumed.
9. On such occasions, therefore, these walks are thrown open, and a definite allowance granted to each inhabitant according to tribes. Thus these uncovered walks insure two excellent things: first, health in time of peace; secondly, safety in time of war. Hence, walks that are developed on these principles, and built