Chap. XY1II.
SUPPLY OF TOWNS WITH WATER.
1215
the water and length of the line of pipes : as the distance increases, the power must either beincreased or the velocity reduced ; the shorter the distance, the less the power required toovercome the friction; if it he therefore necessary to exert a great power to force the waterto the extremities of an extensive district, that they may be properly supplied, it is veryevident that the power exerted near the source, not being required to overcome so great anamount of friction as at the extremities, will increase the velocity of the water through theorifices near the source ; consequently, therefore, if some arrangement were not made to pre-vent it, the houses so situated would have a superabundant supply, whilst those at a distancewould obtain a very small quantity, if any : when, however, those near the source have re-ceived their supply, the cocks on the services are shut down, and the water in the mainswill pass on to supply the extremities : that each may have an equal supply, those near thesource have the communication opened with the main for a shorter time than those at adistance, in proportion to the velocity with which the water is delivered. In addition tothis, on every line of mains and services, orifices about 2 inches in diameter are made atcertain distances, filled up with fire-plugs, which get the whole force of the water simplyby closing the service-cocks, in such a manner that in case of fire or other accident it maybe concentrated at the spot required.
Supposing, then, water to be required for 20,OCX) houses, and the height to which it mustbe raised at the works is such that a 20-inch main would be sufficient to give the supplyaccording to the foregoing system ; it could not be so, if the water were constantly on inall the pipes, both mains and services ; for if the size of the lead pipes to supply the housesbe upon an average J inch in diameter, the aggregate areas of 20,000 half-inch pipes wouldbe equal to 27| square feet, and a main of 71 inches in diameter would be required at thesource to supply the town instead of 20 inches, and for side streets, containing 100 houseseach, pipes of 5 inches in diameter, instead of 3 or 4 inches : this is an extreme case, butone that it may be necessary to provide against, because if the water is always on, the housesmay be all supplied at one time, and even, trusting to the chances of only one-half thatnumber, the main must be 48 inches in diameter at the source. In addition to thenecessity for this extraordinary outlay, in the first instance, the quantity of water usedwould be enormous ; and consequently, the expense of raising a sufficient supply be in-creased ill proportion, and the object sought, that of having a strong pressure of water in themains, would be defeated by the very means proposed to insure it ; for, as the water in thepipes would be always on, so would the draught by the houses be constant, and the presentpower of shutting off the supply from the side streets, and applying the full force to theparticular locality requiring it, would be destroyed: again, if separate mains were laid fromthe works to be used as fire-mains only, the expense would be nearly the same as that ofthe system of pipes for supplying the inhabitants, and this item, which is the most impor-tant portion of the outlay in waterworks, would be nearly doubled.
Mr. T. Hawkesley, in his evidence taken before the Commissioners, states, that if thewater for the supply of the metropolis were taken from the Thames, in its full purity,above Windsor, and filters or subsiding beds erected there, the transmission of 500 gallonsof water per second might be effected by two mains, each of 60 inches in diameter. He
calculates the resistance from friction from this formula for long pipes, P =
Q 3 / ,HOfi 5
which P represents the horse-power necessary to overcome the friction, / the length of thepipe in inches, Q, the quantity of water to be delivered in one second, and d the diameterof the pipe. Hence it appears that the resistance arising from friction in pipes of thegiven size, and 25 miles long, would require less than 450 horse-power, beyond the forceemployed to raise the water into an elevated reservoir: if this reservoir were situated at aheight of 220 feet, the steam power required to raise the water would be about 2000nominal horses, and the total power in transmitting and raising 500 gallons per secondwould amount to less than 2500 horses. The cost of the main pipes would be about1,000,000/. sterling; of the engines and machinery, with some reserve of power, about150,000/., and of the tanks and reservoirs probably 200,000/.
Height that a jet of water rose from the mains and services belonging to the SouthwarkWater Company, under a fixed pressure of 120 feet. The first trial was made in UnionStreet, between High Street and Gravel Lane, Borough, 31st January 184-4, over anextent of 800 yards of 7-inch main, and through the Fire Brigade stand-pipes, hose, andjets. This 7-inch main is connected to the 9-inch main in the High Street, Borough,which, after a run of 500 yards, is joined to 200 yards of 12-inch main, and then continuedby 550 yards of 15-inch main to the great main, leading from the company’s works atBattersea, making a total distance of 5500 yards from the place where the experiment wasmade.
One 2^ inch stand-pipe, with 40 feA of hose, and | inch jet, rose 50 feet.
Two c 2\ — 40 — l — 45
Three 2^ ■— 40 — ^ — 40