372 



FILTRATION 



As an example of an upwards and downwards filter, Jit/. 860 may bo exhibited. 

 A B c D is a wooden or metallic cistern, furnished with tho perforated shelf c d mar 

 its under part, upon which a vertical partition is fixed through tho axis of the vessel. 

 A semicircular perforated shelf is placed at a, and a second similar one at b. These 

 Sf ,~ ^ horizontal shelves rest upon brackets in tho sides of 



~ tho cisterns, so that they may be readily lifted out. 

 The space o is filled with coarse sand, J with mode- 

 rately fine, and H with very fine. The foul water is 

 poured into tho chamber E, and presses through 

 o j H and into tho space F, whence it may bo drawn 

 by the stopcock /. 



Fig. 866 represents in section a filtering apparatus, 

 consisting of two concentric chambers; th:-. interior 

 being destined for downwards filtration, and the ex- 

 terior for upwards. Within the larger cistern A, a 

 smaller one B is placed concentrically, with its under 

 part, and is left open from distance to distance, to 

 make a communication between the interior cavity 

 and the exterior annular space. These cavities are 

 filled to tho marked height with sand and gravel. 

 ^ C The inner cylindrical space has fine sand below, then 

 sharper sand with granular charcoal, next coarse 

 .'and, and lastly gravel. Tho annular space has in like manner fine sand below. The 

 foul water is introduced, by the pipe E, the orifice at whoso end is acted upon by a 

 ball-cock with its lever a ; whereby the water is kept always at the same level in the 

 inner vessel. The water sinks through tho sand strata of tho middle vessel, passes out- 

 wards at its bottom into the annular space, thence up through tho sand in it, and, col- 

 lecting above it, is let off by the stopcock on the pipe b. When a muddy deposit forms 

 after some time, it may be easily cleared out. The cord e, running over the pulleys //, 

 being drawn tight, the ball lever will shut up the valve. The stopcock d made fast to 

 the conducting tube B must then be opened, so that the water now overflows into tho 

 866 



annular space at A ; the tube c, in communication with the inner space B, being opened 

 by taking out the stopper h. The water thereby percolates through the sand strata in 

 the reverse direction of its usual course, so as to clear away the impurities in the 

 space B, and to discharge them by the pipe c //. An apparatus of this kind of mode- 

 rate size is capable of filtering a great body of water. It should bo constructed for 

 that purpose of masonry; but upon a small scale it may bo made of stone-ware. 



A convenient .apparatus for filtering oil upwards is represented in fig. 867. g is a n oi I 

 cask, in which tho impure parts of tho oil have accumulated over tho bottom. Imme- 

 diately above this, a pipe a is let in, which communicates with an rlrvafrd water-ciBtarn 

 n. /is the filter (placed on tho lid of the cask), furnished with two perforated shelves, 

 one at c and another at d, which divide tho interior of the filter into three n-m- 

 partmonts. Into the lower space immediately over the shelf c, tho tube b, famished 

 with a stopcock, enters, to establish a communication with the cask ; tho middle ca\ it y 

 c is filled with coarsely-ground c]iaiv>al or other lilti -rip;: materials; and the up|'< -r 

 one has an oduction-pipo I. When the stopcocks of the tnl.es <t and b are opened, tho 

 water passe's from the cistern into the oil cask, oceupn s from its density always t!u> 

 lowest place, and presses the oil upwards, without mixing tho two liquids ; whereby 



