1212

Theory and practice of engineering.

Book II.

with clean gravel, about T 3 fl of an inch in diameter; this is followed by five other layers ofgravel, each of the same depth, and each succeeding layer a little finer than the previousone, the last being coarse sand : over this is placed 2 feet depth of clean, sharp, fine sand,similar to that used in hour-glasses, but a very little coarser ; about6 or 8 inches deep of the fine sand nearest the top is mixed with animalcharcoal, ground to the size of coarse meal, each particle about ^ of an .• i

inch in diameter. A longitudinal drain or pipe, N, runs between the i

filter and the pure water basin, communicating with both; on each of ■ I

Fig. 1902. FILTER AT PAISLEY.

_

the openings between the pipe and the filter is a stop-cock to close the communicationwhen necessary ; there are also two drains, to carry off the foul water when the filtersare being cleaned, and another to prevent the water from rising too high : when the filteris complete, its action is as follows : the sluice R and the valve S are opened, and thewater permitted to flow through the filter into the drain N below, until it becomes quiteclear. This will take twoor three days when first setto work, unless very greatpains are taken to washthe gravel and sand be-fore they are put into thefilter, which will now flowcopiously for some weeks,and when the quantitypassing begins to de-crease, the stop-cocks areshut, and the valves Sraised. The water thenenters below, filling all thedrains, and having a head pressure of several feet it will force its way up through thesand to the top, and in its passage raise the scales or particles of mud which have been

Tfs

rrs

-—

-n -

Fig. 1903.

FILTER AT PAISLEY.

Fig. 1904.

FILTER AT PAISLEY: SECTION* OF SIDB.

deposited in the downward passage, and carry them into the foul water drain below. If thesand of the surface be stirred by a fine-toothed rake, after the water has been raised above it,and a little additional water admitted on the top, through the conduit, it will facilitate theoperation of cleaning, as the mud is always deposited on the very surface of the sand.