SPECIAL PROBLEMS 115 



iron casings. In such instances, perforated wooden pipe wound 

 with iron wire is often used. 



In the California or stovepipe type of wells the casing itself 

 serves as a screen. In drilling, a record of water-bearing strata 

 is kept, and, on completion, a perforator is lowered into the pipe 

 and vertical slits are cut in the thin sheet-steel casing at the desired 

 points. Practically the entire length of casing is sometimes con- 

 verted to a screen where the well is in a continuous water-bearing 

 stratum. 



In clayey materials, quicksands, etc., one of the several types 

 of patented screens or strainers is commonly used. The Layne 

 strainer (Fig. 50 a) consists of a perforated tube wound with metal 

 strips (or wire) of triangular cross section, flat side outward, the 

 exterior presenting a flat surface to the sand, reducing the danger 

 of clogging and prolonging the life of the well. The Cook strainer, 

 another popular form (Fig. 506), consists of a seamless brass tube 

 cut with horizontal slits of varying widths to suit different ma- 

 terials. The outside of the pipe is smooth, but the slits expand 

 towards the inside to permit the free entrance of water while re- 

 ducing the possibility of clogging to a minimum. 



Setting the Casing. In water wells of the drilled type ex- 

 tending into rock it is almost always desirable to make a tight 

 joint between the casing and the rock, either at the surface of 

 the latter or a few feet lower down, the object being to prevent 

 the entrance into the well of surface waters or polluting matter. 



Many drillers count on securing a water-tight contact with 

 the rock by simply driving the casing firmly against the surfaiv. 

 but it is needless to say that the efficacy of such a contact is not to 

 be relied upon. To secure a tight contact the size of the hole 

 should be reduced by substituting a smaller bit. This results in 

 the formation of tapering walls at the point of change, against 

 which the casing may be firmly set. 



"Casing Off." Where a stratum of caving materials is pen- 

 etrated or where a flow of objectionable water is encountered it 

 must be "cased off." Since casing from the surface to the point 



