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close to the towns; but what would have to be allowed for in the extra cost of the mains. Then as to the question of the cost of plant, I may say I took the prices given in the paper from the estimate of a large firm of engineers. The estimate I got was a little lower than the figures I have actually put down; I think them perfectly fair; but these engines are of much higher pressure than are usually employed in water works. I am glad to hear from Mr. Jerram that the Edison system is succeeding so well. I had no idea they were continuing their contract at the same price. The Jablochkoff Company, who light the Thames Embankment, say they give the same light for 13d. that, if produced by gas, would cost 3d. per hour; and I think the authorities of the Strand district have arranged for this Company to light a large portion of their area at this price. It cannot be said electricity is going back : as one gentleman said—“it is rather a thriving infant”; but you must give it time to grow, and not be too hard upon it at the beginning. Then, as to Mr. Pritchard's question about earth currents. I am not myself in favour of earth returns at all, but they are perfectly practicable. I carried out a large installation for the Mersey Docks in Liverpool, and we used the hydraulic mains there to take the return current; I certainly thought in my plans it would be necessary to couple the flanges together by means of copper strips at the joints where lead had been used, which is a very bad conductor, but the work was pushed forward rapidly, and this was not done, and we found the water was itself a good conductor. I should like to call your attention to a piece of wire on the table which was sent me from America. It is rather interesting, because meetings have been held recently to prevent wires being stretched across our streets. It is steel wire, with copper deposited electrically on the surface, the conductivity is very high, and telephonic messages can be sent through it a thousand miles. If this wire was used across streets it would prevent many accidents, because the steel is very strong, and the copper outside prevents oxidation. The other articles on the table are switches, which will probably be used in some districts; also one of the “cut-outs” mentioned in the paper.
Edison uses a bar of lead, but I find a great disadvantage ; it is rather uncertain, and when it melts it flies all over the place: so I take a peculiar kind of tin foil, combined with aluminum. Of course, all these fittings are minor details which will be worked out in time; but I think some attention should be paid to them,
because there is a great deal of work to be done almost before the details can be got into shape.
The PRESIDENT: I am sure we are much indebted to Mr. Hedges for his paper. I hope he will not be disappointed at the shortness of the discussion, or its want of practical character. This subject has not yet got within the scope of our practice, though our attention is necessarily turned to it; and when, at some future time, he meets us, we shall probably discuss the question more fully, more exhaustively, and more practically.
Mr. HEDGES : I am very much obliged by the way in which I have been received at this meeting—the first which I have attended, but I hope it will not be the last.
ABINGDON WATER SUPPLY.
BY GEO. WINSHIP, Assoc. M. INST. C.E., BOROUGH SURVEYOR,
THE Abingdon Waterworks were designed and carried out by Messrs. Bailey Denton & Co., the author of this paper acting as resident engineer. The following description of the source of supply and the reservoir has been furnished by Messrs. Denton & Co. at the request of the author :
“The reservoir, which is situated in the parish of Wootton, about 2 miles from Abingdon, on the road to Oxford, is constructed in the coral rag and calcareous grit formations of the oolitic series overlying the Oxford clay. At no great distance from the reservoir the Kimmeridge clay is exposed to the surface, while overlying the latter the lower greensand caps the neighbouring hills. Thus, within an area of a few square miles, three distinct formations are exposed to the surface, viz. the lower greensand, the Kimmeridge clay, and the calcareous grit and coral rag, from the rifty caverns of which latter the supply of water to the town of Abingdon is obtained.
“The preliminary operations consisted in sinking a trial shaft some 8 feet square through the rock to a depth of 35 or 40 feet from the surface, and in boring 5 inches diameter some 35 feet further into a stratum of soft soapy clay, into which pipes 4 inches diameter, having screw joints, were driven as the boring proceeded.
“ The greater part, if not the whole, of the supply of water, however, was found to spring from a cavern in the rock at the bottom, or just below the bottom of the trial shaft, while very little water was obtained either from the boring below or from the strata above this cavity in the rock.
“During these operations a large quantity of very fine clean sand was pumped out with the water from the aforesaid cavity, so much so that a part of the superincumbent rock caved in, and afterwards caused considerable difficulty when constructing the reservoir.
“While the shaft and boring were proceeding, experimental trials, by continuous pumping by steam power, were made, and observations noted for the purpose of ascertaining the quantity of water that might be depended on for supplying the town, and further to bring the matter to a still closer test, it was determined to make a cutting about 200 yards in length through very hard rock, entering the trial shaft at a depth of 16 feet from the ground surface.
"In this cutting 9-inch pot-pipes were laid and covered in, by means of which the water rising out of the rock was drawn off continuously for several months and discharged over a gauge-board into a neighbouring watercourse.
“The result of these gaugings, as well as of some other springs in the vicinity, was so far satisfactory as to determine the Council of Abingdon, with the express approval of Mr. I. T. Harrison, the Government Inspector, to construct the service reservoir, and to lay the pipes to supply the town, leaving the question of a larger supply, that it was thought might be obtained by adits in the rock or from other sources, for future consideration, should the necessity arise.
“The reservoir was constructed in the following manner :-An excavation was made for about 60 feet on either side of and embracing the original trial shaft. It was 28 feet deep and about 16 feet wide at the surface, tapering slightly towards the bottom. The rock thus excavated and brought to the surface was afterwards broken in a machine into small pieces about the size of a walnut, and with some fine sand (pumped out with the water as before explained) made into concrete blocks in the proportion of 1 part Portland cement to 4 parts broken material and sand. The making of the blocks and the excavation were carried on at the same time, while an engine and chain pump were perpetually at work keeping down the water.
“As soon as the excavation was got out sufficiently the bottom of the reservoir was put in in concrete, an opening being left round the original trial shaft and bore hole so that the water might rise into the reservoir. The side walls were next built
in concrete blocks backed with ordinary concrete, and the whole domed over from end to end by a semi-arch 13 feet diameter, also in concrete blocks with concrete backing.
“A complete reservoir, capable of holding 125,000 gallons or thereabouts, has thus been formed, into which the water rises from the fissures and hollows of the rocky strata below. The capacity of the reservoir is thus partly artificial and partly natural, inasmuch
as the hollows and fissures aforesaid no doubt afford additional capacity besides the 125,000 gallons of the reservoir itself.
“The level of the bottom of the reservoir is 240 feet above Ordnance datum, the highest ground in or about Abingdon being 200 feet.
“The main pipes to Abingdon from the reservoir are 9 inches diameter; they are laid in the cutting made through the rock before alluded to, and enter the reservoir 10 feet above the bottom, The water is drawn out of the lower part of the reservoir by syphon action.
“This arrangement was determined upon to save the expense of making an extra deep cutting about 600 yards in length, which would otherwise have been necessary to admit of the 9-inch pipes entering the reservoir at the bottom. The shorter leg of the syphon is therefore about 9 feet in length, dipping to the bottom of the reservoir, while the longer leg extends to a distance of 600 yards, or thereabouts, and forms, in fact, part of the supplying main to Abingdon.
“When the water in the reservoir rises, as it generally does at night, up to or above the crown of the syphon, it discharges by gravitation simply, but when it sinks below that level syphon action is called into play to a greater or less degree.”
Referring to the excavations, in constructing of the reservoir and laying the pipes, it was found necessary to use both powder and dynamite, but no difficulty to speak of occurred until the portions of rock near the original trial shaft were removed, when a distinct fissure was found, causing a subsidence of the lower rock, over a considerable area, immediately above the bed of rock before alluded to. After a consultation it was decided to sink two cylinders, made of wood, with segmental curbs, and weighed down, while sand and water was excavated and pumped out of the excavation immediately under the side walls of the reservoir. The water was kept at its lowest possible point while these cylinders were being sunk in position; they were then filled solidly with cement concrete and the water allowed to rise and cover them. After allowing time for the consolidation of the concrete, the arches shown upon the plans were sprung from them, and the reservoir completed without further trouble. This part of the work was executed very cleverly by Mr. Henry Potter, the contractor for the works, to whom some credit is justly due.