Engineering Wonders of the World
Volume III

344 ENGINEERING WONDERS OF THE WORLD. ontside the 8-inch casing pipe, and defied all efforts to check it. rush of water completely ruined the well, which had finally to be abandoned as un- manageable, being left to flow as a permanent spring. Overflowing wells will always occur when a water-bearing rock receives rain or snow at a considerable elevation, and dips thence below some impermeable stratum up through which the water cannot escape as a natural spring. When such an artesian basin is tapped by many wells, these much diminish the stream flow from the outcrop or other point of drain- age. Ultimately an increase in the number of the wells reduces the head of water in the rock, and diminishes the flow. London, a comparatively small basin of limited outcrop areas, is a striking example of this process. Not a single well now overflows to the north of the Thames within several miles of th© river, so great has been the pumping draught of the many wells over the Metropolitan area ; and the once overflowing wells south of the river have now all to be pumped. Great as is the importance of a good water supply in a country blessed, as England is, with a good annual rainfall, it is doubly great in a region where, during part of the year, Ultimately the heavy - rivers and streams dry up, and at the best only a few pools remain. In a previous article (vol. ii., p. 312 foil.) have been noticed the artesian wells of Australia, which are as re- PUMPING FROM AN ARTESIAN WELL. markable for their depth as for their produc- tiveness. The latter quality is due to the fact that they overflow naturally. The aver- age yield is about 700,000 gallons a day. The deepest bore-hole in the country, that at Bimerah, goes down 5,046 feet, or nearly a mile. Only those who have actually bored an artesian well in a thirsty land can appreciate the importance of the work, and the wide- spread interest aroused by it. Steadily the long line of tools eats its way down into the ground ; slowly rises the débris detached. Five hundred feet are pierced, but still no sign of water. A thousand feet, and only dry rock. But the engineer does not lose heart, knowing that if only he perseveres the chances are heavily in his favour. A depth of 1,500 feet is at last reached. How much further will the hard dry shale continue ? At last the experienced workman becomes conscious of a change. He feels that he is in another kind of rock. Water creeps sluggishly up the bore-hole, and dribbles over the protector flange of the lining tube—the first sign of success. The men, greatly encouraged, work on, and the water-flow gains strength. The dribble is replaced by a foun- tain, 2, 3, 4, 5, 6 inches high, darkened by the muddy débris. The advance becomes moro and more rapid, and in due course an 8-inch jet rises 3 clear feet above the top of the tubing. Now for a test. A 500-gaIlon tank is filled in one minute. Multiply that quantity by 1,440,