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compensation water given by the Corporation amounts to about seven times the amount of the former dry weather flow.

Now, with regard to the lake or reservoir itself, I must first say a few words, for the benefit of those who are ignorant of the subject, as to its object. The dry weather flow of the river Vyrnwy, at the point where it has been cut off for Liverpool, is less than two million gallons per day. In heavy flood the flow at the same point has been found to be, by actual measurement, at the rate of more than one thousand million gallons per day, being equal to about five hundred times the flow in times of drought. The rainfall (which produces the rivers) varies not only from month to month, but it also varies greatly from year to year. Thus there are periods of dry years and periods of wet years, so that it is not easy to ascertain in the Vyrnwy or any other such district, without gaugings extending over many years, the amount of rain upon which reliance can be placed in calculating the net volume of water that can be obtained. A mean rainfall and a minimum rainfall having been determined or assumed, storage space is required of such capacity as to obtain for the use of the town or district to be supplied the highest average volume per day that the watershed is capable of yielding. For instance, I have stated that the minimum dry weather flow of the river Vyrnwy is less than two million gallons per day, and in dry years this rate of flow may continue for many weeks. But, by impounding the floods in a reservoir, the watershed is made capable of yielding day by day, over a succession of dry years, a continuous supply throughout the longest drought of no less than fifty-three and a half million gallons per day. The reservoir does not store the whole of the flood waters, for it is not practicable, within reasonable limits of expenditure, to make reservoirs large enough to store the excessive floods of wet years, and hence provision has to be made to permit such floods to pass harmlessly away to the river below the embankment. This is one of the difficulties of reservoir building,

. and more accidents have happened from insufficient overflows, or by-washes, than from any other cause.

The Vyrnwy reservoir or lake consists simply of the valley as formed by nature, with the lower end blocked by a dam. At the site of the old village of Llanwddyn the valley is more than half a mile in width, but about two miles lower down, to the S.E., the valley becomes a narrow ravine, on the steep sides of which the rock crops out at the surface. Across this narrow neck a masonry wall has been built to a height of 84 feet above the river bed, the effect of which is to hold back the water and make a lake nearly five miles long. The length of the wall at the top is 390 yards, and the contents of the lake when full between twelve and thirteen thousand million gallons. The top water level is 825 feet above the ordnance datum, the highest ground in the city of Liverpool being 220 feet above the same datum. In order to ensure a sound, watertight, foundation for the masonry wall, a trench was cut across the valley from side to side, through the alluvial deposits and glacial drift down to the solid rock *-the Caradoc beds of the lower silurian formation. The trench at the widest part was over 120 feet in width, and its greatest depth in the middle of the valley was about 55 feet. On this rock foundation the building of the wall was commenced in October, 1882. The process of building was necessarily slow, owing to the mass of material to be prepared and carried and manipulated, and owing also to the severity of the long winters. With the exception of the stone and sand, every ounce of material used in the work, such as Portland cement, bricks, timber, iron, machinery, plant, coal, all had to be carted over ten miles of hilly Welsh roads from the nearest Cambrian railway station at Llanfyllin. The difficulties connected with labour were considerable, the difficulty of getting suitable workmen, and the difficulty of housing and feeding them when got.

* The glacial drift contained boulders five or six tons weight, and blocks of rock of forty or fifty tons, dislodged by glacial action.

The stone used for building was similar in character to the rock foundation, and was obtained from a quarry opened for the purpose at a distance of about one mile and a quarter from the wall. Between the

Between the quarry and the wall rails were laid, worked by locomotives, and the wagons of stone were brought either on to the wall, or to a convenient position at the side of it for lifting by cranes. About 800,000 tons of earth and rock had to be quarried, but necessarily much of this was waste, the weight of the finished masonry in the wall being about 510,000 tons.

I have referred to the difficulty experienced in disposing of big floods. The erection of a masonry wall at Lake Vyrnwy, instead of the earth embankments usual in this country, permitted of an important departure in the method of dealing with overflows. Instead of a costly and difficult by-wash at the side of the valley, according to the ordinary practice in reservoir building, the wall itself has been utilized as an overflow, and it has been shaped so that whenever the water rises above the top water level, it falls in a graceful curve over the back of the wall, and passes away into the river.

The total width of this overflow is 744 feet. I shall give further details with respect to the construction of the wall when I come to exhibit the slides.

The water of the River Vyrnwy having been collected into the reservoir the next thing to be done was to bring it

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to Liverpool. The elevation of the lake is, as I have shown, such as to give ample fall to permit of the water flowing by gravitation along properly constructed channels to the banks of the Mersey.

At a distance of nearly three-quarters of a mile from the masonry wall a strong watertight tower has been built on the northern side of the lake, and carried up from below the old valley bottom to a level considerably above the highest point to which the water can rise. This tower forms the entrance gate to the aqueduct.

From the bottom of the tower there is a culvert built under the bed of the lake, and terminating in a brick-lined shaft at a point on the lake side about thirty chains north of the masonry wall, where the aqueduct proper begins. From this shaft a tunnel, two and a quarter miles long, has been driven through the hills that separate the Vyrnwy valley from the watershed of the river Tanat. This tunnel is seven feet in diameter, and has a fall of two feet per mile. At the outlet end there is a small masonry well or basin, from which cast iron pipes have been laid along the Hirnant valley, through the villages of Pen-y-Bont and Llanrhaidr-yn-Mochnant, to a small reservoir or balancing tank at Parc Uchaf, about one mile and a half east of Llanrhaidr. The cast iron pipes, of which the aqueduct chiefly consists, vary in internal diameter from 3 feet 2 inches to 3 feet 6 inches, according to the gradient to which they are laid, and they vary in thickness from one to two inches, according to the pressure they have to resist. They are made in lengths of twelve feet, and are joined together by the spigot, or plain end, of one pipe being placed in the socket of the next, and molten lead run in, and caulked by hammer and chisel. The proper laying of large pipes, such as these, requires a considerable amount of practical experience, skill, and judgment.

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The carrying of the heavy tubes through hilly districts and over bad roads is no easy matter, and when, as sometimes happens, a dozen or more miles of such roads have been traversed, there is often the still more difficult task of crossing fields, and ploughed lands, and occasional bogs before the pipe line is reached.

Two leading considerations determine the position that the pipes are to occupy in the ground. They must not be so deep as to be liable to be crushed by the superincumbent weight of earth, and they must have sufficient cover for protection against frost and for the purposes of agriculture. All the pipes are laid to a regular gradient. To use a theological term, they are always either on the down grade or on the up grade. When on the down grade provision must be made at the lowest point for sluicing to the most convenient water course.

When on the up grade provision has to be made at the highest point for the escape of air. In addition to the sluices and air cocks there are numerous appliances along the line, such as stop valves for shutting off the water, self-acting valves for automatically stopping the flow in the event of a sudden fracture, and other incidental machinery.

From the relieving tank, near Llanrhaidr, the water takes a fresh start for Liverpool, still in 42 in. cast iron pipes, for a distance of six miles, when its progress is barred by the Cynynion hills. These hills are too high for the water to be carried over, and they have therefore to be tunnelled. The first tunnel ends in the narrow valley of the river Morda, which is crossed by a syphon pipe; the second tunnel discharges into a small reservoir built on an elevated site outside of the town of Oswestry. The total length of these tunnels is one mile and three-quarters, and they are both, like the Hirnant tunnel, 7 ft. in diameter. From this reservoir the water is drawn through

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