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in the regions of comparatively moderate Tain in this country the storage to effect this object should vary from 20,000 or 30,000 to 50,000 or 60,000 cubic feet for each acre of collecting ground, the smaller quantity being about sufficient for an available rainfall of perhaps 18 in., and the larger for one of about 36 to 40 in.'* 80,000 cubic feet per acre of collecting ground are provided at Lough Island Reavy; 60,000 at the Gorbals reservoirs, Glasgow; 49,000 at Rivington Pike, and 34,000 at Manchester; at the last, the whole fall not being impounded.
I proceed, neglecting further details on this subject, which belongs rather to the province of the civil than the mechanical engineer, to give an example of the carrying out of these views, of the utility and importance of reservoirs in districts abounding with waterfalls, and where mills are numerous and depending in whole or in part on a steady and regular supply of water.
In 1836 I was called upon to report upon the best means of regulating the water supply upon the river Bann, which, from its excessive variations of flow, was a source of great inconvenience to the manufacturers on its banks. The river Bann rises among the lofty bare summits of the Mourne mountains, in the north-east of Ireland, where there is a heavy rainfall, and in consequence devastating floods frequently poured down its channel, carrying bridges, embankments, and other obstructions before them. On the other hand, during the summer months the ordinary supply of water was totally inadequate to the demands of the mills; whilst the flourishing state of the linen trade called for an extended application of power in a district where steam was not available as a motive power, unless at great cost. Hence, in co-operation with Mr. Bateman, the ground was surveyed and two reservoirs erected in the upper part of the river, by which these evils were removed, and a continuous and adequate supply of water rendered available.
* Report of the British Association, 'On the Supply of Water to Towns.' By J. F. Bateman, C.E. 1858.
Lough Island Reavy, the site selected for the principal reservoir, was a natural lake, bounded on the north and south by land of considerable elevation, which, although having a comparatively small extent of drainage (3,300 acres ultimately), was supplied by good feeders, which, united to the surplus waters of the river Muddock, would fill the reservoir at least once or twice a year. The original Burface of the Lough, fig. 84, was 92£ acres in extent; on this it was proposed to raise a depth of 35 feet more water, by the aid of embankments, and to draw off at a depth of 40 feet under that height. The area thus enlarged would be 253 statute acres, and the capacity of the reservoir is 287,278,200 cubic feet.
Corbet Lough was the' second site, and, although at first abandoned from its. proximity to the town of Banbridge, was afterwards adopted. At a small expenditure for embankments, Corbet Lough was raised 18 feet above its summer level,'so as to cover 74£ acres, and to have a capacity of 46,783,440 cubic feet of water.
A third site was selected further up amongst the mountains, but at this part the works were never executed.
It is understood that 12 cubic feet of water per second falling one foot will, in its best application on a water-wheel, afford a force equivalent to 33,000 lbs. raised one foot high per minute, or one-horse power. Now, supposing the reservoirs to discharge 40 cubic feet of water per second, the fall from the lowest point of outlet at Lough Island Reavy to the tail water of the lowest mill on the Bann being 350 feet, we have a total force of 1,166 horses available for mill purposes ; or in other words, the mill-owners will derive an average advantage of 3*3 horse power for every foot of fall. This, it must be observed, is not a supposed quantity, but the result of certain data taken by calculation from the waters of the Bann. It must be noticed further that this supply of 2,400 cubic feet per minute is not the whole power. The calculations are for one-half, the river supplying the remainder, except in extremely low water, when the demands from the reservoir may be increased to meet the emergency.
From the estimates made at the time, the expenditure to secure this result would be
£ s. d.
For Lough Island Reavy 12,600 0 0
„ Corbet Lough 3,512 0 0
At Lough Island Reavy it was necessary to construct four embankments, marked A, B, C and D, in fig. 84.
The principal SW. side . . . . 137,400 cubic yards.
Small do. .... 17,400
NW. end . . . . 5,200
„ E. end .... 99,781 ;,
Total . 259,781
The substratum of the valley being water-tight, the footing for the puddle was easily obtained by sinking a trench into the water-tight stratum, whence the puddle wall was carried up vertically with the bank to the required height. It was 12 feet in width, at 40 feet below the top, diminishing to 8 feet wide at the summit. A layer of peat was brought up on the inside of the puddle, and a similar layer on the face of the slope. Above the peat a layer of three feet of gravel was laid, and on that the stone pitching forming the inner side of the bank. The inner slopes of the embankments were 2J horizontal to 1 vertical, and 3 horizontal to 1 vertical. The outer slopes 2 horizontal to 1 vertical, and 2£ horizontal to 1 vertical. The discharge pipes, two in number, each 18 in. in diameter, were placed at the bottom of a stone culvert, at the lowest part of the embankment, with suitable discharge valves, &c. The rainfall for the district amounted to from 72 to 74 in. annually, of which at least 48 in. found its way to the reservoirs.
Fig. 84 is a plan of the original disposition of Lough Reavy and its feeders. The original area of the lake is shaded, and its present area is indicated by the dotted line connecting the embankments A, B, C and D. The diversions of roads and new feeders rendered necessary are also indicated.
Fig. 85 represents a section of the embankment of the Belmont reservoir, which will sufficiently explain the arrangement of the culvert and discharge pipe a a, with the top and discharge valves v v, in the valve-house T, which in works of this
Paet I. G
"kind is always under lock and key. The water entering the pipe through the tunnel b b, flows out into the well c, and gauge basin d, where, as it passes over the gauge or dam board g, its quantity may be ascertained. The construction of the regulating discharge valve is shown in figs. 86 and 87. Fig. 86 is an elevation of the valve at the side at which the water flows in, and fig. 87 a cross section. A is the valve case, closed below and fitted with a bonnet b at top; the valve v works up and down in the valve box, against a brass facing c, and is confined by a guard d behind; the adjustment of the valve is affected by a valve spindle/, of wrought iron, cased in gun metal, so as to slide freely in the stuffing box g, and is worked by a fly-wheel h, and screw above. By means of this fly-wheel the valve may be adjusted to any required opening.
Of late years Mr. Bateman has introduced an ingenious valve admirably adapted for the discharge of reservoirs of great depth, where the amount of pressure upon the valve is an impediment to its employment. To remedy this evil, the valve is divided into three parts; first, the small valve by which about ^ of the area is opened; secondly, the intermediate valve of about \ the total area ; lastly, the large valve unclosing the remainder. It will be seen that the small valve is drawn first, and is followed by the second, and ultimately by the largest, after the pressure is removed or partially neutralised.
The pressure of water against the side of an embankment is enormous in most instances, and varies upon any part in the