268 



CHURNING AND WASHING BUTTER 



total cost of this filtering-can when complete was $11.11. Since 

 the time when this can was constructed prices have advanced 

 considerably. 



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

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



FIG. 100. 



FIG. 101. 



FIG. 100. Filter-can; i, overflow; 2, inlet of tap-water; 3, outlet of filtered water. 



FIG. 101. Cross-section of filter-bed and can: i, overflow; 2, inlet; 3, outlet of 

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

 iron plate; 6, concave galvanized-iron plate with hole in center. 



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 such rapid filtration. Fine sand, however, is one of the 

 best filtering substances that can be had. The rate of filtra- 

 tion can be regulated by increasing or decreasing the depth of the 



