WELLS. The term Well Is, generally speaking, applied to any excavation sunk in the ground, for the purpose of obtaining, or of Getting rid of water ; the technical distinction between the two kinds of wells being that the former are known by the simple name of well, the latter by the names of dead orris, or of absorbing wells, accordingly as the water escapes by deep seated, or by superficial permeable bed's. The distinction between Artesian and common wells has already been referred to under ARTESIAN WELLS ; and that between a shaft and a well consists in the fact that shafts are generally dry, and are intended to give access to galleries, tunnels, or subterranean workings, whereas wells aro intended to act as reservoirs of water.
Common wells are made by sinking an excavation through an upper permeable, or partially permeable, stratum, which lies upon an impermeable one, in such a manner as to allow the water falling upon the surface to percolate through, and to accumulate in, the upper stratum. The depth to which it may be necessary to sink a well will then depend on 1, the permeability of the water-bearing stratum ; 2, on its area ; 3, on its thickness ; 4, on the relief of the upholding stratum ; and 5, on the existence of other wells, or places of draught upon the source of supply. The diameter of the well will depend upon the rapidity with which the water will enter from the sides, or bottom, to replace the consumption ; it being always observed that there are a priori advantages in making the well of large dimen sions. in order to obviate the effects of the stagnation of the water, which affects small quantities more rapidly than it does larger ones.
The construction of the sides, and of the margins of the wells, must depend entirely upon the nature of the ground traversed, and upon its greater or lees tendency to cave in.
Generally speaking, the margin of a well intended to supply water for domestic use ie executed of watertight masonry, and the well itself is lined with masonry of the same description to some considerable depth, in order to shut out from the well the infiltrations from land drains, and from surface waters. In England this part of the work is done in brickwork and cement, or in cast iron ; abroad it is commonly done in ashlar masonry. Below tho level to which these waters may
reach, the sides of the wells only require to be lined in such a manner RA to resist the tendency of the sides to collapse; and this object may in many cases be effected by what is called dry steining, or brickwork laid without mortar. It is customary in modern well-sinking to execute the ateining by sinking the ground to the full diameter of the outside of the work to depths of about 6 feet each, and then inserting what is called a wooden cur!, or a frame circular in plan, of from 3 to 6 inches in thickness, at the level of the floor ; upon this curb the steining is then built regularly, as close to the side of the well as possible. The ground is excavated from under the upper curbs, which are maintained in their positions either by the friction upon the side of the steining, or by means of shores, struts, &c. hi traversing running sands, or formations containing much water, close tubes must be used ; and great precautions are required to prevent such sands from blowing, that is to Say, from rising in the excavation. In the lower parts of all wells, whatever may be the nature of the material used for lining the sides, means must be provided for the ingress of the water ; when iron cylinders are used, or close planking of wood is introduced, a series of holes in the circumference will effect this object.
Great care must also be taken that the materials employed be of such nature as not to act upon the quality of the waters; and whilst every precaution must be taken to protect the waters from the light, and from atmospheric impurities, it is desirable that an efficient ventilation should take place in the well. Local considerations of economy must, of course, regulate t.1) a great extent the selection of the materials to be used in ateining a well ; but theoretically it would be preferable to employ none but the hardest fire bricks and Roman cement, or cast iron. Copper tubes aro sometimes used in Artesian wells ; but in ordinary wells the cost of copper pipes of the size required would render the use of that material impossible. Wrought-iron tubes are far too liable to rust, to allow of their being used in wells, without their being galvanised ; cast-iron pipes are therefore most generally used.