Even in gravitation works there are various methods of solving this question, for the water may bo led either in closed pipes or in open or is covered conduits. Open conduits may be resorted to when the district they traverse is not likely to furnish any elements of a nature to alter the purity of the water ; but if small local watercourses should be likely to pour into the stream, or if ti,o latter should be exposed to the danger of contamination from the dust, smoke, or other impurities of a town atmosphere, the conduit must be covered. The advantage of a pipe-conduit consists precisely in the protection it affords against such accidental sources of contamination, and, generally speaking, it will be found to be cheaper than a masonry conduit of ordinary dimensions Moreover, the facility the pipes furnish for the execution of reversed syphons dispenses with the necessity for the construction of costly aqueducts. It is essential to make the dimensions of the pipe some. what larger than would theoretically be necessary to convey the quan. City of water required to be delivered ; because there is a danger, in almost all cases, of the formation of an incrustation in the interior of the pipes, which would practically diminish their sectional area ; and it is essential, also, to provide means for occasionally examining and clearing the pipes. In all descriptions of conduits it is necessary tc provide stop-gates, or valves, and overflows, so as to arrest the passage of the water in case of accident. The material generally used for pipe. conduits is cast-iron ; but glazed stone-ware is often employed with Humes, when the water does not flow under great pressure. The Romans frequently used lead-pipes for this purpose; but as a general rule they preferred covered aqueducts, and the Continental engineer still adhere to this system. In the great Liverpool gravitation water works, and in the new supply to Glasgow from Loch Katrine, cast-iron pipes have been exclusively used where any great differences of level have been encountered.
In the majority of cases at the present day, the water brought from a distance for the supply of a town, is filtered before distribution ; and this is effected in positions as close as possible to the pumping station, or to the entry of the distributing mains, unless the water should be so pure naturally as to allow this precaution to be dispensed with. There are numerous systems of filtration in use in England, but the one most generally adopted is to cause the water to pass through a succession of layers of sand and dravell (increasing in coarseness from above downwards) to a series of drains leading to a covered reservoir. The total thickness of the filtering media is usually made from six to seven feet, and the number of layers about eight or nine, the head of water over the upper surface of the sand being about two feet. Engineers, however, are not unanimous in their opinions as to the best relations between the depth of the filtering media and the head of water over them ; for in some eases the depth of the sand and gravel is made only 4 feet 6 inches, and the working head four feet, instead of the dimensions quoted above ; but evidently the degree of purity of the water itself must materially affect this question. It may even
happen that the quality of the water may require the introduction of some ingredient in the filtering media which should be able to exercise a chemical influence upon the water itself ; and in some cases it has been proposed to mix animal charcoal with the upper layers of sand, in order to remove any organic matters; or to mix the magnetic oxide of iron with the sand when peaty matters are present; or even to apply Dr. Clark's process when the bicarbonate of lime is present in notable excess. Local considerations must guide the engineer in his choice of the various systems; but however the water may be treated in this stage of the distribution, it is absolutely necessary that it should not be exposed to the air at all after having been filtered, and that it should he either at once passed into the distributing mains, or be stored in covered reservoirs until wanted for use. In the construc tion of the latter, the only important observation to be made is that they must be executed in such a way as to protect the water against the action of light and of the sun's rays; local facilities for the supply of building materials must regulate the precise details of the construc tion itself. In some peculiar cases it is desirable to provide, in addi. tion to the filters and pure water reservoirs, depositing basins, in which the heavier materials, in mechanical suspension in the waters of the natural source of supply, may deposit themselves; but, again, this detail admits of no universal law ; and it is even to be observed that the tendency of the London engineers is rather towards the suppression of both depositing basins and storeage reservoirs. They make the filters of sufficient area and thickness to complete rapidly and effec tually the separation of the impurities; and they pump the filtered water at once into the, distributing mains. Where storeage reservoirs aro used, their dimensions may vary between the calamity required for three days' consumption, when the source of supply is liable to occa sional interruptions; and the capacity required for a few hours' supply when the machinery Is only subject to ordinary periods of rest or repair. In the latter case it may be necessary, however, to make the machinery in duplicate, or at least to keep a stock of duplicate parts. Care must be taken to provide for all such service reservoirs, valve pits, overflow pipes, waste weirs, scouring pits, and other conveniences for examina tion, cleansing, and repair.
Unless in the cases of gravitation works, it is necessary, in all modern domestic water supplies, to raise the water to such heights as to allow of its distribution in the loftiest rooms of the houses situated on the highest ground in the respective towns, by some description of ma chinery. When the quantity to be raised is small, and there is a watercourse of considerable volume, and great constancy of flow, in the neighbourhood, it may often be advisable to resort to hydraulic power, and to use one of the forms of water-wheel hereafter described.