Chap. VIII.
553
Few subjects have more perplexed the civil engineer than the theory of filtration, andmany thousand pounds have been uselessly expended to obtain pure and wholesome water.The two processes generally adopted are, first, either that which acts entirely on thesurface, in the same manner as passing water through filtering paper, or by a layer ornumber of layers of various qualities of sand or other material.
By the first of these methods minute impurities may be retained, and as the filtrationproceeds, the quantity of water that passes is comparatively small; its passage is graduallylessened by the deposit of impure matter upon the filtering surface, and the pores throughwhich it passes become stopped. By the second the sand is usually selected of differentsized grains, and the mud or foreign matter which the water holds mechanically in thecourse of time fills up the vacuities that exist between the particles, destroys its porousquality, and prevents its use as a filter. Water passing through sand, therefore, depositsits impurities between the grains, and if these are not frequently changed, the mass willbecome impermeable, and cease to do the work required. Such a filter would thereforeanswer admirably for a short time, or until the whole of the interstices were closed, and ex-periment has proved that the quantity of water passed through diminishes each succeedingweek. A filter should be so constructed, that the material used should be unchanged, im-putrescent, and without any alteration in its mechanical structure, allowing the water at alltimes to pass freely.
M. Maurras's system of filtration is effected by means of a water-tight iron box, about5 feet 6 inches square, with a filtering surface within it equal to 60 superficial feet, which,with a 12 feet 6 inches head of water, it is calculated will filtrate 150,000 gallons in thespace of 24 hours.
'Hie water is introduced at the top, and makes its exit at the bottom, after having passedthrough several layers or beds of coarse and fine sand ; it at first percolates a coarse sand,which lies both at top and bottom, then a fine sand, and afterwards meets in the centre ofthe box, where it makes its exit.
The principal part of the arrangement, however, consists of a number of horizontal closediron boxes, the upper and lower sides of which are pierced with small holes, for the passageof the water under pressure : these are placed one over the other, and between them arealternate layers of coarser and finer sand, through which the water is first made to pass ; theyarc tightly packed with riddled sand, which cannot escape through the fine holes made forthe passage of the water, in consequence of its large grain, whilst its interstices are suffi-ciently small to allow the finer sand of the filtering beds between the boxes to enter and beretained, and although that of the smallest grain has been used under a pressure of a columnof water of 60 feet, it has been found that none has escaped.
The machine is cleansed by reversing the current of the water, which, by an arrangementof sluice cocks, is effected very easily ; when the water is admitted at bottom and forcedupwards, a violent agitation takes place throughout the pores of the sand, and the accu-mulation of deposited matter is thoroughly dislodged and carried away.
The filtering sand, of various degrees of fineness, cannot escape, and can be changed atpleasure.
In most parts of England water is obtained from wells, some of which are Artesian. Ofthe quantity of water which descends in rain in our latitude, one-third has been estimatedas passing off by evaporation, the remaining two-thirds being required for the support ofanimal and vegetable life, and to supply the subterraneous springs. Rain water either runsoff upon the surface or percolates the strata, being received in the fissures or vaults of theearth, where it forms subterraneous reservoirs.
"Where beds of gravel rest upon a substratum of clay, the lower portions contain water ;if the clay is thin, and has fissures or openings in it, the water passes through, and continuesto descend till it meets some other layer, which will retain it.
Some wells are supplied by water descending in the strata, others by its ascent frombelow by means of hydrostatic pressure, which is the case with Artesian wells, or perpe-tually flowing springs: these are numerous in the neighbourhood of London , where theyare formed by penetrating the chalk or the plastic clay formation. At Sheerness , the sandystrata of the plastic clay formation was reached after boring through the London clay330 feet, and in many districts the chalk has been pierced to a considerable depth beneaththe clay, and abundance of water obtained, which is generally perfectly clear and bright, forby its passage through the various strata, it is deprived of all that it held mechanically, asw r ell as other impurities, which are taken up by one earth or the other.
The district called the London Basin mav be considered as a continuous seam of chalk,varying in thickness, and sometimes covered with sand and gravel alternating with plasticclay, over which is a thick stratum of London clay. Under the chalk basin is a substratumof clay, through which water will not pass, and consequently a large supply is always to befound in it. The surface of the water in this subterranean reservoir does not stand at oneuniform level, but rises in a distance of 14 miles, as between Watford and the highest springin the chalk hills, as much as 300 feet. The molecular attraction of the particles of chalk