ARTESIAN WELLS are perpendicular perforations or borings, through which water rises from various depths, according to cir cumstances, above the surface of the soil. Such perforations have been named Artesian wells, from the opinion that they were first used in the district of Artois in France. They are seldom more than a few inches in dia meter, and are made by means of the usual boring instruments. Their action is due to the constant endeavour of water to seek its level.
Let us suppose, for example, that there is an undulating tract of country, with a town placed in a hollow or valley. Near the sur face is a thick bed of rock or clay, either im • pervious to water, or through which it perco lates with difficulty. Beneath this is a sandy or porous stratum which crops out' or ap pears at the surface, on certain heights beyond the limits of the town. Beneath this stratum, again, is a close or impervious one. Under these supposed conditions the rain-water, falling on the heights, is absorbed by the ' crop out,' and filters down through the sandy stratum entirely beneath the town : unable to leave that sandy film or layer, be cause the strata above and below are nearly impervious to water. If, under these condi tions, a perforation be made near the town, deep enough to reach the sandy stratum, the water which saturates this sand, the water will rise through the hole—not merely to a level with the town, but will often spout up far above that level, according to the height of the ground where the 'out-crop' of the sandy stratum occurs.
This is the principle of the Artesian well, let it be carried out how it may. The geolo gist has to determine whether a watery stra tum probably exists : the well-borer has to devise the mode of reaching it.
One of the most celebrated Artesian wells is that at Grenelle, near Paris. It was com menced in 1833, and not finished till 1811. Geologists were aware, from the nature of the formation beneath and around Paris, that any watery sandy stratum must necessarily be at •a great depth ; but the urgent want of water for the supply of the south-western portion of Paris, determined the authorities to brave all difficulties in the attempt. The boring pro ceeded slowly, interrupted from time to time by disasters which severely taxed the patience of the engineer. At length, in February 1841, the boring rods suddenly penetrated into the watery stratum ; and a stream of pure and limpid water rushed up to the light of day, from the subterranean depths where it had been so long buried. The depth to which the boring was carried exceeded 1800 feet.
In 1837 the inhabitants of Southampton, requiring an improved supply of water, caused an experimental boring to be made on an elevated common about two miles north of the town. The result being deemed satisfac
tory, measures were taken for the sinking of a shaft. It proved an enormous work. The well- diggers penetrated through 78 feet of alluvium, 300 feet of stiff clay, 100 feet of plastic clay, and about 100 feet of chalk : encountering much difficulty in forming a substantial open shaft. Having thus reached a depth of about 580 feet, the boring rods were used, and were driven to a further depth of more than 700 feet ; so that the total depth attained exceeds 1300 feet. The labour and expense were ultimately rewarded ; for a flow of 50,000 gallons of water per day was ob tained. The expense was nearly 20,000/.
The Artesian Wells of Grenelle in France, and Southampton in England, may be taken as types of these works on a large scale. Nearly all the others which have been formed are of less magnitude, and have involved less expense.
In a paper read before the Institute of British Architects by Doctor Buckland, iu 1849, he drew attention to the fact that a well is not properly artesian unless the water over flows at the surface. Thirty years ago there were several such wells in and about London ; but such has been the exhaustion of the springs within this district, that there is now hardly a single well of such a kind, although there are about 300 which are called artesian, chiefly from the mode in which they are sunk by boring instead of digging. The water now obtained in London wells does not rise to within many feet of the surface, and machinery is often necessary to make it avail able. In the artesian well which supplies the fountains in Trafalgar Square the water does not reach within forty feet of the surface, whence it is pumped up by a steam engine : the supply is good, but the process of obtain ing it is costly. The level of the water in the London wells is gradually sinking, as the number of wells increase. Doctor Buck land estimated the present average level in the London wells at CO feet below the surface, and he thinks that in twenty years it will have sunk to 120 feet. One of the great porter breweries has now to pump up water from a depth of 188 feet, in a well which a few years ago maintained a level 05 feet below the surface.
These observations, coming from a high authority, have had some influence on the re cent discussions concerning the supply of London with water. Whatever may be the plan ultimately adopted, it does not seem that the artesian system would be fitting for such an enormous supply as would be necessary.
Tho mode of digging or boring for Artesian Wells is described elsewhere. [Boun.m.]