104 DOMESTIC WATER SUPPLIES FOR THE FARM 



water is forced downward through the drill pipe and hollow bit 

 (Fig. 47), and loosens the material about the bottom of the well, 

 the finer portion being carried to the surface by the current 

 ascending between the drill pipe and the casing. 

 The bit is turned slowly during the process to 

 increase the rapidity of sinking and insure a 

 straight hole. As fast as the bit advances, it is 

 followed by the casing, which sometimes, as where 

 a paddy or expansion drill (Fig. 48) has previ- 



ously been used to ream out a hole larger than 

 FIG. 47. Hollow bit , . . , , 



used in jet process. tne casing, sinks under its own weight. More 



commonly, however, a drive weight (Figs. 45, 46) 

 is used to force it down. The same size casing is, 

 as a rule, used from top to bottom of the well. 



Hard layers are penetrated by substituting 

 an ordinary drill for the hollow bit, the former 

 being lifted and dropped as in the standard per- 

 cussion methods. In clayey or other coherent 

 materials the walls will sometime stand alone, 

 and the casing may not be inserted until after 

 the hole has been jetted to its full depth. FIG. 48. Paddy or 



Depth, Size and Care of Jet Wells. - Jet " 



wells are usually sunk only when it is necessary 

 to go less than 100 feet, but this is a limitation of practice rather 

 than a limitation of possibility, for wells of the jet type have been 

 successfully sunk in California to depths of over 600 feet, and by 

 reducing the size of casing at a point from 500 to 600 feet below 

 the surface, considerably greater depths may be attained. Com- 

 mon diameters of the casings for depths up to 150 feet are 2 and 

 3 inches; for wells that are to be sunk to depths of 400 to 600. 

 feet, 4-inch casing is most common. 



The jet wells are not usually provided with strainers, hence 

 they avoid many of the troubles due to the incrustation of the 

 screens. Incrustations within the pipe and corrosion of casings 

 are treated as in the case of driven wells. 



