Engineering Wonders of the World
Volume III

336 ENGINEERING WONDERS OF THE WORLD. province of Artois; though later knowledge tells us that the bored well has been known to the Chinese for many centuries, so that the wells of Artois were at most but bored on a re-discovered method long familiar to the Chinese. In a district where the water in the ground naturally rises above the surface when set free by a bore-hole, the artesian well with a diameter of only three or four inches is practicable. Though the artesian well was primarily bored only where water was confidently anti- cipated to overflow the surface, the original signification of the term is now almost lost, and any well, bored, in place of being dug, is now quite commonly called artesian. Neces- sarily, such a bored well must be large enough to contain a single barrel pump of a size sufficient to raise the quantity of water required. Every drop of water that exists in the ground comes originally from the atmosphere. A very usual estimate of what happens to the rain which falls upon the Rainfall. , ,, 4 earth s surface is that one- third of it runs off promptly into the streams and rivers ; one-third is dried up by the sun and air ; and one-third sinks into the ground and subsequently appears as springs, or finds its way into the sea below water-level. It is obvious that all the fissures and porous rocks of the earth’s surface, where accessible to rainfall, must be filled with water at least to sea-level, for the ground cannot possibly be drained by gravitation to a level lower than that of the sea. Over great parts of the earth’s surface the ground is filled to much higher levels, and springs are found issuing from the ground even near mountain tops. The formation of a spring is simple. Rain sinking into the earth descends until it encounters an impermeable stratum. The water thus checked in its downward path flows along this stratum until it reaches the surface, and finds its way out through some opening. Where rocks are soluble, as are chalk and limestone, large underground water passages often exist, and rivers disappear entirely below ground in many cases where the rocks in which they flow Subterranean _ . _ . Streams, are drained at some lower point. The Mole in Surrey is an example of a river which thus burrows beneath the sur- face ; and the streams of the Derbyshire limestone may often be heard tinkling below their dry mossy beds in summer time, when the rocks are not filled to their customary winter’s level. It has occasionally happened that hard and much-fissured rocks have yielded water from wells, and living creatures have been found in it. But, as a rule, the water T ’ . ’ . Dug Wells. which penetrates to any depth below the surface must pass through a con- siderable thickness of surface soil. This thor- oughly filters out all living germs, so that, as a rule, water from wells is of the highest organic purity. It contains only soluble minerals, such as carbonate or sulphate of lime, the two principal agents which render water hard. But otherwise the water contains nothing unsafe. Now, when a well is of large size, as it must be when dug, its water may be seriously endangered by the entrance of foreign bodies. Surface drainage soaks down behind the brick lining, and is often an un- suspected cause of danger ; and in many ways the direct communication with the surface is a danger. Dug wells are always prone to run dry. They cannot be carried below water- level except by the assistance of powerful pumps. When a well is dug at a period of high-water level, it invariably runs dry sooner or later, and the writer has walked on the dry bottom of many a well and heading in the chalk. Then is the time to deepen the well to the low-water level, for years may elapse before a drought occurs so severe as to cause this deeper well to run dry. The water-level is always rising or falling, and