TYPES OF WELLS 73 



iron casing open at the bottom. In weak water beds, in soft 

 materials and in quicksands, either perforated casings or casings 

 equipped with long screens are necessary to permit the entrance 

 of the required amount of water. In many of the harder rocks 

 the walls will stand without caving and casings are therefore un- 

 necessary, the water entering at any point without hindrance. 



Where strong water beds exist storage is unnecessary, the 

 water entering from the rock as fast as the pumps demand. 

 \Yhere the supply is derived from weaker beds, especially those 

 having only small seeps, storage is an essential factor and the 

 type and size of the well are of great importance. The volume of 

 tubular wells of equal depth varies with the square of their di- 

 ameters; hence, a 6-inch well will hold nine times as much water 

 as a 2-inch well of the same depth, and a 3-foot well thirty-six 

 times as much as a 6-inch well. Dug wells are therefore of ad- 

 vantage in clays and similar materials where the water enters 

 more slowly than it can be lifted by the pumps, for they permit 

 accumulations that may tide over periods in which the amount 

 used is greater than the supply. For the deeper rock waters dug 

 wells are impracticable and small-bore drilled wells must be used 

 even where the supplies are slight. To get the best results the 

 wells are generally made as large as can be afforded and sunk con- 

 siderably below the point of entrance of the water, to afford the 

 necessary storage. 



Relation of Depth of Water to Type of Well. --The depth of 

 the water is a factor of importance in the determination of the 

 type of well to be sunk. A dug well, for instance, is suitable only 

 when the water is within 30 or 40 feet of the surface, although 

 many deeper dug wells exist. A punched well is commonly 

 limited to depths of 50 feet, and a bored well is with difficulty 

 carried to depths greater than 100 feet. Driven wells are most 

 suitable at depths of less than 150 feet, although they are some- 

 times successfully extended to depths of 250 to 300 feet, or even 

 to 400 or 500 feet or more, where the conditions are favorable. 

 Jet wells are usually sunk only where it is not necessary to go 



