The demand for water increases, but the supply is not supplemented.

Years ago the utmost was made of the sources from which water is drawn; no pains have been spared to extract every possible drop of water from the heart of the hills within a considerable distance, and to convey it undiminished by evaporation to the city.

This is done by underground channels called _kanats_, which are excavated with great ingenuity and skill, and are marvels of industry.

This system prevails all over Persia, and existence as well as the fertility of the soil mainly depends on the water-supply thus obtained.

The sandy expanse round Yezd in the desert of South-eastern Persia has been made literally to blossom like the rose by means of these subterranean channels, some of which are tunnelled for a distance of thirty miles.