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First cost of filtering-bed, exclusive of land. £11,700

Annual expense of raising water in filtering-bed
(From the River Thames, close adjoining,
raised by steam-engine.)

Annual expense of cleansing and renewal
Five per cent. interest on outlay of capital

Total annual cost, exclusive of land

800

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The quantity filtered being 3,136,320 gallons daily, or 1,144,756,800 gallons annually, or at the rate of about 2183 gallons for one penny.

42. The system of cleansing adopted by the Southwark Water Company embraces settling reservoirs, as well as filtering-beds or reservoirs, and some peculiarities in the formation of the former deserve notice. The section, fig. 4, will clearly show the construction. AA are the settling reservoirs, having an area of between four and five acres, and being 13 ft. 6 ins. deep, and faced with gravel. The bed was found to be springy in some places, and there lime was mixed with the gravel, forming an impermeable concrete. The beds are formed with a slight inclination from the sides towards the middle, along which an inverted arch, b, is formed of brickwork in cement, 6 ft. wide, and 3 ft. 6 ins. deep. This invert is an essential improvement, and, with the inclined bed, gives great facilities for cleansing, by sweeping the deposits into the invert, and flushing it away with a current of water from an upper reservoir. The filters are con

Chelsea Works just

structed similarly to those of the described. The series of filtering substances consists of coarse gravel, 1 ft. deep; rough screened gravel,

9 ins. deep; fine screened gravel, 6 ins. deep; hoggin, or fine gravel, 9 ins. deep; and fine wash gray river sand, 3 ft. 6 in. deep. The water is gradually drawn from the settling reservoirs, A A, on to the surface of the sand, on the filter-bed c, and is permitted to percolate through brick culverts, formed with open joints in cement. The filtered water passes from these into close brick tunnels, by which it is conducted into the well of the pumping engine, D.

43. The expense of filtering by this, the Battersea filter, is stated by Mr. Joseph Quick, the engineer for the Works, not to exceed 350l. per annum, the quantity filtered being 2,160,000 gallons per diem, or 66 gallons per superficial foot. At this rate the annual quantity filtered will be 788,400,000 gallons, and the cost about one penny per 9386 gallons. At the Bleaching Works at Dukinfield, 500,000 gallons are filtered daily, at a cost of 1567. per annum, or at the rate of 4874 gallons for one penny.

44. Water which has been subjected to the process of subsidence only still usually contains finely comminuted particles of solid matter, from which the subsequent process of filtration is necessary to cleanse it. The settling reservoirs having answered the double purpose of depositing the grosser solid particles, and of effecting all the chemical softening of the water which can be effected by mere exposure to the atmosphere, the filtering reservoir completes the process of depositing, and sends the water forward in a tolerably pellucid condition. But beyond these processes, and altogether irrespective of any chemical improvement of its constitution, it is found that water which has remained in an exposed reservoir, and subject to the action of light— made so much more effective by the transparency of the filtered water-does, in some states and temperatures

of the atmosphere, betray unequivocal symptoms of vegetable formation within it, and, if the action proceeds, animal life, in the form of minute animalculæ, rapidly succeeds. It has therefore been suggested, that the filtering process could be still further improved, if the water were submitted to a subsequent passage through some filtering medium calculated to detain any such vegetable or insect productions as might be formed on the surface of the filtering-bed, and by chance find their way with the water into the tunnels beneath.

45. When water is drawn from a river having a sandy or gravelly bed in its vicinity, it is comparatively easy and inexpensive to form a natural and highly effective filter. Thus, at Nottingham, the Reservoir, which is formed on the banks of the River Trent, about a mile from the town, is excavated in a stratum of clean gravel and sand, through which the water slowly percolates to a distance of 150 feet from the river. The deposited solid matter thus remains on the bed of the river, from which it is removed by the natural action of the current. The reservoir being exposed to the solar influence, vegetation is sometimes produced, and which is removed at intervals of three weeks in summer, and six weeks in winter, by pumping out the water and sweeping. Besides the reservoir, there is a tunnel filter, which passes through a similar stratum for a considerable distance up the adjacent lands. This tunnel is 4 ft. in diameter, and half-brick thick, laid without mortar or cement, costing about 10s. a foot, including excavation to a depth of 12 feet.

46. An arrangement, somewhat similar to the last described, has been successfully carried out on the River Clyde, a few miles above Glasgow. At the selected spot there is an extensive round bank of sand. A tunnel was constructed in this bank parallel to the

edge of the river, and also to the surface of the water and below the level of the water. This tunnel being constructed of bricks set in mortar below, but bricks without mortar above, received the water, which afterwards percolated into wells, from which it was pumped up for use. Similar natural filters were attempted at other points contiguous to the Clyde, but most of them failed from the interception of springs of water of a harder quality than that from the river. In some cases, also, the natural springs intruded water containing iron, and injurious to the purposes for which the supply was required. Natural filters must, therefore, also be considered with reference to their liability of interruption by natural springs of a different or inferior quality. Beyond this, they should always be designed with a reference to the lowest level to which the river water may fall at any season, or under any circumstances; and this necessity sometimes involves a depth for the pipes, or other constructive difficulties, which altogether mar the economy and advisability of the arrangement. If this last precaution be not adopted, it will happen at the dryest season of the year, when the maximum supply is required, that the reduced level of the river will be below the fixed level of the filtering tunnel, which thus becomes dry and inactive. The only alternative which then remains is to draw the water directly from the river, and thus the filter remains useless at the season when it is most desirable that it should be performing its highest duty. The work of cleaning the tunnels is, moreover, by no means an easy one; and, considering all the circumstances and liabilities of these expedients, it would appear that they are of very limited application.

47. Let us recapitulate the heads of the subjects over which we have already passed.

The

The sources whence the land is supplied with water, without artificial aid, are rain and tidal rivers, as in the Nile, Euphrates, Ganges, Mississippi, &c., besides such springs as rise spontaneously to the surface, either upward, by the pressure of internal reservoirs at a higher level, or at the outcrop of the strata of the earth. The artificial sources are the ocean, rivers, streams, and wells. The quantity of rain which falls over the earth appears to vary with the latitude, the distance from the ocean, the season, and other circumstances, the nature and influence of which we do not yet understand. effect of the rain, as a source of water, and a cause of the necessity for drainage, is limited by the quantity which passes off from the surface of the earth in a state of vapour. The quantity so raised depends upon the temperature which prevails while the process of evaporation is going on. Efficient drainage requires the supply and the discharge of water to be duly regulated, the supply to be sufficient, and not in excess, and the discharge to proceed correlatively. The quantity required for the complete irrigation of a district is determinable by reference to the nature of the soil and the crops, and the position of the district in relation to surrounding tracts of country. The state of soil most favourable to vegetable growth is that of moistness, having water between the particles, but none between the clods or masses of earthy matter. Among the artificial sources of water, that yielded by the ocean requires chemical changes in order to fit it for domestic purposes, and its applicability for those of agriculture is necessarily limited by remoteness. River-supply is

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