The most ingenious and highly involved form of ancient water-raising machine was a water-wheel. The method of operating a water-wheel depended much on the region where used. In Egypt, along the Nile, oxen were employed for this purpose. In China, coolies were found more satisfactory even in raising large quantities of water for irrigation purposes, which they did by walking a simple form of treadmill on the outer edges of the water-wheel. The Romans, slow at originating, but, like the Japanese, quick to recognize the value of anything new and adapt it to their purposes, borrowed the idea of the water-wheel from the Greeks or Egyptians, but made it automatic when used in streams and rivers by adding paddles that dipped into the running water and were moved by the current of the stream. Water-wheels operated by oxen were in use at Cairo up to the twelfth century, where they raised water vertically a distance of 80 feet from the Nile to an aqueduct that supplied the citadel of Cairo. Our present elaborate system of water distribution was of humble origin. It was not a rapid growth, but a gradual evolution. Its four principal stages were: First, distribution from natural sources by water carriers; second, aqueducts conveying water to communities where a system of sub-conduits or aqueducts conveyed the water from the main aqueduct to reservoirs at different points in a city; third, a system of distributing mains through which water was furnished to householders at certain hours only during the day; and fourth, our present system of continuous supply at all hours of the day and night. In the first stages of water distribution, water was carried on the backs of water carriers in earthenware jars constructed especially for the purpose, or in goat or other animal skins properly tanned and sewed to hold water. While this method of water distribution is of great antiquity, it is still practiced in most tropical countries, and to this day water carriers, some with the burdens on their backs, others with goatskins of water on donkeys' backs or with jars of water in two-wheeled carts, may be seen plying their trade in Mexican and Egyptian cities. The earliest record we have of any effort to supply a community with water conveyed in tunnels or aqueducts from a great distance, dates from the year 727 B. C. King Hezekiah or Ezekias, who reigned in Jerusalem at that time, was much troubled over the poor quality of water furnished to the city and undertook to provide a better supply. [Illustration: Pool of Siloam] [Illustration: Pool of Solomon]