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. 
2 ” 
os 1 
H fe 0,20, 
a Coarse gravel SEson dee x 
a Bo 
faai 
ca] i 
B8S58800°9 
Oo Oo. 
Fine sand 
Gravel 
Coarse gravel RS ogee 4 
& ee a haa 
3 3 
t 
18-———__—> 
Fig. 152. Fig. 153. 
Fig. 152.—Filter-can: 1, overflow; 2, inlet of tap-water; 3, outlet of filtered 
water. 
Fic. 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 galvanized-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 
