100 



DOMESTIC WATER SUPPLIES FOR THE FARM 



If the screen is clogged by sand or clay that cannot be re- 

 moved by ordinary pumping, water is sometimes forced down the 

 well to carry the material out into the surrounding soil. Another 



method of avoiding the difficulty is to 

 drive the pipe without screen, and 

 with a loosely fitting drive point. On 

 reaching water, the pipe is withdrawn 

 a short distance, while the screen, 

 which has been previously lowered 

 through the casing, is simultaneously 

 pressed upon the shoe or drive point 

 which remains behind. The withdrawal 

 of the outer casing is stopped as the 

 top of the screen is reached, and the 

 well left in substantially the same 

 condition (except that there has been 

 no clogging of the screen due to driv- 

 ing) as if the screen had been sunk in 

 the usual way. 



An advantage of screens inserted 

 in this manner . is that they may, if 



FIG. 44. Diagram showing for- 

 mation of sand filter through 

 pumping. 



the materials are fairly coherent, be withdrawn for cleaning or 

 replacement, but there is the disadvantage that no tests of quan- 

 tity of water can be made during the process of sinking. 



A simpler method is to use a pipe that slips down over the 

 screen and rests on the point during the process of driving. The 

 pipe can be withdrawn a little at a time and the well tested by 

 pumping, after which, if water is not found, driving may be 

 resumed. The danger, under this method, lies in the fact that in 

 pushing forward the casing over the screen the latter is likely to 

 be torn and ruined by pebbles ground into it by the advancing 

 pipe (Fig. 43, i). 



Depths of Driven Wells. Driven wells are best adapted to 

 sandy or similar materials where the water rises to within 25 feet 

 of the surface, but they are, nevertheless, very common in many 



