=Coverings of Concrete.=--The use of concrete for the coverings of wells, cisterns and springs has become a recognized form of the best construction. It is not more expensive than other good materials and when properly executed it forms an imperishable protection and gives a neat appearance. The spring cover in Fig. 126, and the cistern top in Fig. 127 are illustrations of its application. =Artesian Wells.=--Artesian wells are made by boring into the earth until the drill reaches the artesian stratum, the internal pressure forces the water through the opening to the surface. They are usually small in diameter and often of great depth. In some areas the artesian flow is found a few feet below the surface, but generally it is much deeper and 3000 feet is not an unusual depth. The pressure and amount of flow from these wells is sometimes sufficient to permit the water being used for the generation of power. Small waterwheels are not uncommonly driven in this way and the power used for the generation of electricity for lighting and running small household appliances. =Driven Wells.=--In localities where the nature of the soil gives opportunity, wells are made by driving a pipe to the required depth. Wells of this character are usually made in places where the water-bearing soil is of sand or gravel. The pipe terminates in a sand-point such as that of Fig. 128. This sand-point is a perforated pipe with a pointed end, that facilitates driving. The perforations, as shown in the point _P_, form a strainer which allows the water to enter the pipe but prevents the sand from filling the opening. [Illustration: FIG. 126.--Concrete cover for a spring.] [Illustration: FIG. 127.--Concrete cistern top.] In the use of driven wells, the water-bearing soil must be sufficiently open to allow the water to flow into the pipe as fast as the pump takes it away. =Bored Wells.=--In many localities the water-bearing stratum is of such nature as to give a ready flow of water but yet not sufficient to permit of the use of a sand-strainer; if, however, the opening is somewhat enlarged, the water will enter with sufficient rapidity to supply a pump. In such cases bored wells are quite generally used. They are made by boring a hole of the required size with an earth auger. These wells are made of any size up to 2 feet in diameter. They are often called tubular wells because they are lined with iron tubing or tile, to prevent the earth from refilling the hole. [Illustration: FIG. 128.--Driven well with a sand-point strainer.] =Cleaning of Wells.=--Very few dug wells are so constructed as to exclude dust and washings from the ground. It is, therefore, necessary that they be occasionally cleaned. Accumulations from these causes may be sufficient to hinder the entrance of the water to the well and thus lessen its capacity.