2G8 



criURxrxG and washing butter 



tolal cost of this filtcring-can when complete was Sri.ii. Since 

 the time when this can was C(jnslructed prices have advanced 

 considerably. 



(3) The rate f)f filtration is necessarily governed Ijy the depth 

 of the filter-l>ed, the character of the material used, and its 



S^i- 



Fk; 



Coai-se gTa%fl " 



r^ 





Fig lor. 



Fig. 100. — Filter-can; i, overflow; 2, inlet of tap-water; ,5, outlet of filtered water. 



Fif.. lor. — Cro^s-section of filter-bed and can; r, overllow; 2, inlet; 3, outlet of 

 filtered water; 4, [)erforated K^ilvanized-iron plate; 5, perforated galvanized- 

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



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

 gravel quite rapidly, yet the substances are \-er}- 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 b\' iiK reasing or decreasing the depth of the 



