(3) Artesian and Driven Wells.—The use of machinery has made it possible to drill wells of great depth. Artesian wells are those in which the well penetrates to porous layers of rock lying between im pervious clayey layers. The layers must be tilted sufficiently, so that part of the porous layer will reach the surface at a point higher than the top of the well. In that case, the water will flow out of the well and even gush out, as at Louisville, Kentucky, where, if un hindered, it spouts up 170 feet. One such well at Lillers in France has been flowing steadily for nearly 800 years. Artesian water from great depths is always warm. A well 2050 feet deep at Charleston, South Carolina, for example, has a temperature of F.
Artesian wells are especially important in dry regions like the Sahara Desert, where they support many oases. The French have there tapped deep sources of water derived from rain that falls many hundred miles away. Driven wells, which penetrate deep into the ground but do not strike water that rises, arc also highly important in dry regions, since they give a water supply which does not dry up. They are very expensive to operate, however, since it costs a good deal to pump water from such great depths by means of gasoline or elec tricity. In the southwestern United States such wells are common, but they are much more feasible on cattle ranches, where only a small supply of water is needed, than on irrigated farms where a large supply is required. Moreover, the water from such wells is apt to contain a large percentage of dissolved minerals, and thus is good neither for men nor for plants.
City Water Systems.—The most complicated methods of obtaining and distributing water are employed in great cities. No matter what may be the source of the water, a city must have an extensive system of large water pipes or mains, and of minor pipes running to every street and house. Filtration plants are also needed in many cases, and a well-developed sewage system is always planned in connection with the water system in every up-to-date city. Each city ought also to have a reservoir sufficiently large and located high enough so that in case of sudden demands such as fires, or in case the mains are broken, there will be enough water for an emergency.
The sources of city water are very various. Some cities like Pittsburgh pump water out of rivers and have to spend much money in purifying the water and in raising it high enough to supply the hilly parts of town. Others, like. Chicago, get water from lakes close at
hand, and have the same problem of purification and pumping, al though the cost of pumping is slight because the city lies so close to lake level. In other cases like New York, the city spends an enor mous sum in building great reservoirs far away among the hills. The Ashokan Reservoir lies among the Catskill Mountains 85 miles from New York, and its water is brought to the city by a great aque duct which goes under the Hudson River in a tunnel of great depth. Although the first cost of such a reservoir and aqueduct is enormous, the later cost is slight. Little expense is needed for maintenance, purification is unnecessary, since the reservoir is protected from con tamination, and the water flows by gravity without being pumped. Some of New York's sky-scrapers, however, are so high that for a long time they had to maintain their own pumping plants in order to raise the water to the upper stories. Los Angeles, being located in a region where there is a long dry season, has to bring its water much farther than New York. It taps the Owens River on the east side of the Si erras, and brings the water through an aqueduct about 250 miles long, crossing some of the mountains in a tunnel.
Cities also get water from artesian and driven wells. Although London's supply comes chiefly from the rivers Thames and Lea, it likewise has a huge system of artesian wells driven into the underlying chalk. So numerous are these wells and so great the demand of London for water that the water table has been permanently lowered over a large area. Before Brooklyn shared New York's water supply it had a similar experience on a smaller scale.
Perhaps the most unusual method of getting a water supply is that of Baku and Aden. Both cities are located in regions so dry that sufficient fresh water cannot be secured. Hence the only re course is to piece out the meager supply with distilled sea water. This is inexpensive at Baku because of the abundance of oil, but at Aden, where coal must be brought from a distance, the water supply is unusually costly.
Water as a Source of Power.—Water furnishes the cheapest kind of power. In order easily to use this power the water must flow regularly at all seasons and must descend rapidly to provid good "head." Hence three conditions are favorable to the develop- --'---.