Engineering Wonders of the World
Volume III

ARTESIAN WELLS, AND HOW THEY ARE BORED. 345 and the total daily flow is ascertained—720,000 gallons—quite a nice little river, which will slake the thirst of thousands of sheep, cattle, and horses, and enable many stock owners to weather a severe drought; for the subterranean sources of supply are affected not at all by the lack of rain in the district which they supply. Until the engineer came along with his tools inexhaustible supplies flowed within a few hundred yards of doomed flocks, to escape perhaps to the ocean bed somewhere in the Great Bight. Now this bad state of things has been removed in great part by the steel tubes which connect the pent-up subterranean reservoirs with the upper world. One of the principal defects of a bore-hole from which the water does not flow naturally is that the water supply to be obtained from it is limited, not by the diameter The Air Lift. . , . . . , , , or the bore-hole, but by the capacity of the pump that can be put inside it. Thus a 6-inch hole, 170 feet deep, with its water supply coming all the way from the bottom of this length of bore, will deliver 500 gallons per minute under a head of 10 feet. That is to say, if the water would rise to 13 feet over the surface, and the lining pipe be cut off at 3 feet above the surface, it will yield the above amount. But inside a 6-inch pipe the largest practicable pump is only about 5 inches diameter, and its yield would not exceed 2,000 gallons per hour when worked comfortably. Unless a well can be pumped from the surface, its supply is thus much curtailed. But when the water-level is not too far below the surface in comparison with the total depth of the well, a very full yield can be obtained by means of compressed air. To carry this out, the rising main is inserted down the bore-hole to about three times the distance which the water-level stands below the surface, or is likely to stand when the pumping is in operation at a given rate previ- ously fixed as the result of a pumping test. DIAGRAM TO SHOW THE PRINCIPLE OF THE “ AIR LIFT ” APPARATUS USED FOR RAISING WATER WHERE PUMPING IS IMPRACTICABLE. (By permission of Messrs. C. Isler and Co.) I Sometimes the rising main stands inside a slightly larger air pipe, and sometimes the air supply pipe passes down inside the rising main, or it is carried down as a separate small pipe alongside of it. (See illustration.) The lower end of the air pipe opens by one or more openings into the foot of the rising main. When air is pumped down it escapes into the rising main, and converts the whole column of water into foam or into an alternation of water and plugs of air. The result is that there is less water in the rising main from its foot-piece to its surface outlet than there is between the surface of the water in the bore- hole and the foot of the rising main. Thus the external column exerts a greater pressure than the internal aerated column, and the result is that the water flows continuously into the foot of the main, is aerated, and rises to the point of discharge. Obviously, if the water-level is far below the surface, the total depth of the