442 



THE FARMER'S AND 



lower part of the tube, and rising upward to 5, is complete- 

 ly purified, and drops into a vessel placed below. If 

 found necessary, a >iece of linen or muslin may be tied 

 over the mouth, 5, to prevent any particle of sand coming 

 over. A tube of this kind, about three inches in diameter, 

 ' will filter about three quarts of water in an hour. The 

 longer the leg a, the more rapidly it will filter, from the 

 pressure of the water. The sand should be made pretty 

 compact, for the slower the passage of the water, the 

 more it will be purified. 



Upon this principle, an improved mode of filtering has 

 been effected in cisterns, namely, 

 by forcing the water to ascend 

 through the filter, instead of de- 

 scending. Here the cistern has 

 two partitions, a and b. That 

 at a does not reach quite to the 

 bottom, and the other has an apparatus at b. In the 

 middle division, a piece of perforated metal, wood, or 

 stone, or a cloth, is fixed a little above the bottom ; on 

 this is placed a layer of small pebbles, then coarse sand 

 and layers of charcoal, then finer sand and charcoal, the 

 whole being covered by another cloth, also fixed just be- 

 low the aperture b. The water to be filtered is put into 

 the division a ; it then passes below the first partition, and 

 by its pressure rises through the perforated plate c, and 

 likewise, through the pebbles, sand, and charcoal, and 

 passing through the cloth above it, runs through an aper- 

 ture in the partition b into the last division, from which it 

 is drawn by a cock as it is wanted. 



An easy method of cleaning the filtering materials is by 

 making the water pass through them in a contrary direc- 

 tion to what it does when filtering. For example, suppo- 

 sing, as we have just stated, the division a is that which 

 receives the foul water, and b that which receives the 



