252 



BUTTER-MAKING. 



all the water to the center of the filter-bed, and thus the water 

 gets the full benefit of the filtering process. The total cost of 

 this filtering-can when complete was $11.11. 



Coarse gravel ^o^°'%^^'^'§ 



aSg§?o°g<fe|| 



r^ 







d;-.:: 



Ooaree eri-avel/J^ogo-S^o^ol 



Fig. 152. Fig. 1.53. 



Fig. 152. — Filter-can: 1, overflow; 2, inlet of tap-water; 3, outlet of filtered 

 water. 



Fig. 153. — Cross-section of filter-bed and can: 1, overflow; 2, inlet; 3, out- 

 let of filtered water; 4, perforated galvanized-iron plate; 5, perforated 

 galvanized-iron plate; 6, concave gah'anized-iron plate with hole in 

 center. 



(3) The rate of filtration is necessarily governed by the 

 depth of the filter-bed, the character of the material used, and 

 its fineness. The water passes through the charcoal, coke, 

 and gravel quite rapidly, yet the substances are very strong 

 barriers to the passage of micro-organisms. The sand layer 

 does not admit of so rapid filtration. Fine sand, however, is 

 one of the best filtering substances that can be had. The rate 

 of filtration can be regulated by increasing or decreasing the 



