476

FILTRATION.

open part of the apparatus in communication with the lowest space a. From the com*partment fni a small air tube l runs upwards. The filtering substance consists at bottomof pebbles, in the middle of gravel, and at the top of fine sand, which may be mixed withcoarsely ground bone black, or covered with a layer of the same. The water to be filter-ed being poured into the cistern at lop, fills through the tube 6 d the inferior compartmenta> from which the hydrostatic pressure forces the water upward through the perforatedshelf, and the filtering materials. The pure water collects in the space h h, while the airescapes by the small tube /, as the liquid enters. The stopcock i serves to draw off thefiltered water. As the motion of the fluid in the filter is slow, the particles suspended init have time to subside by their own gravity ; hence there collects over the upper shelf atd. as well as over the under one at a i a precipitate or deposite which may be washed outof the latter .cavity by means of the stopcock m.

As an example of an upwards and downwards filter, fig. 392 may be exhibited, a b CD

A

392

D

391

is a wooden or metallic cistern fur-nished with the perforated shelfc d near its under part, upon whicha vertical partition is fixed throughthe axis of the vessel. A semi-circular perforated shelf is place-'at a, and a second similar one atb. These horizontal shelves restupon brackets in the sides of thecisterns, so that they may be read-ily lifted out. The space g isfilled with coarse sand, j with mod-erately fine, and H with very fine-The foul water is poured into thechamber e, and presses throughg j h and into the space f ; whenceit may be drawn by the stop-cock /.

Fig. 393 represents in section a filtering apparatus consisting of two concentricchambers; the interior being destined for downwards filtration, and the exterior \°upwards. Within the larger cistern a, a smaller one b is placed concentrically, with

under part, and is left open fr 0IT1make a

the iil '

exterioi*

distance to distance, tocommunication betweentenor cavity and the . .annular space. These caviUJlied height

space

are filled to the mar

avel.

The

with sand and

inner cylindrical space has DJVjsand below, then sharperwith granular charcoal, n ,coarse sand, and lastly .j. flThe annular space has mmanner fine sand below,foul water is introduced by ,pipe E, the orifice at whos ,? coC l<is acted upon by a nawith its lever a ; where 5^^water is kept always 1 ,e vesse l,level in the inner vessel. The water sinks through the sand strata of the mi ^ it,passes outwards at its bottom into the annular space, thence up through the depositand collecting above it, is let off by the stopcock on the pipe b. When a mu > U ueysforms after some time, it may be easily cleared out. The cord e, running ov® 1 \j c f astf f 9 being drawn tight, the ball lever will shut up the valve. The stopcock . nt0 theto the conducting tube e must then be opened, so that the water now oveino) 0 p e nedannular space at a ; the tube c, in communication with the inner space b, e j j n theby taking out the stopper k. The water thereby percolates through the san s e andreverse direction of its usual course, so as to clear away the impurities in the si ca p a bleto discharge them by the pipe c h. An apparatus of this kind of moderate size IIjaS onry 5of filtering a great body of water. It should be constructed for that purpose 01but upon a small scale it may be made of stone-ware. ~ ; s an oil

A convenient apparatus for filtering oil upwards is represented in fig- m S Imme-cask, in which the impure parts of the oil have accumulated over the hot ■ ^ c i s tern

diately abo^e this, a pipe a is let in, which communicates with an elevated