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aqueduct

explosives and fired. In the Thirlmere tunnels, driven through very hard Lower Silurian strata, the progress was about°13 yards a week at each face, work being carried on continuously day and night for six days a week. Where the character of the country through which the aqueduct passes is much the same as that from which the supply is derived, the tunnels need not be lined with concrete, &c., more than is absolutely necessary for retaining the water and supporting weak places in the rock; the floor, however, is nearly always so treated. The lining, whether in tunnel or cut-and-cover, may be either of concrete, or brickwork, or of concrete faced with brickwork. To ensure the impermeability of work constructed with these materials is in practice somewhat difficult, and no matter how much care is taken by those supervising the workmen, and even by the workmen themselves, it is impossible to guarantee entire freedom from trouble in this respect. With a wall only about 15 inches thick, any neglect is certain to make the work permeable; frequently the labourers do not distribute the broken stone and fine material of the concrete uniformly, and no matter how excellent the design, the quality of materials, &c., a leak is sure to occur at such places (unless, indeed, the pressure of the outside water is superior and an inflow occurs). A further cause of trouble lies in the water which flows from the strata on to the concrete, and washes away some of the cement upon which the work depends, for its watertightness, before it has time to set. For this reason it is advisable to put in the floor before, and not after, the side-walls and arch have been built, otherwise the only outlet for the water in the strata is through the ground the conditions are those of an artificial river ; the aqueduct on which the floor has to be laid. Each length of about must therefore be carefully graded throughout, so that the 20 feet should be completely constructed before the next fall available between source and termination may bo is begun, the water then having an easy exit at the leadeconomically distributed. This condition requires that ing end. Manholes, by which the aqueduct can be entered, the ground in which the work is built shall be at the are usually placed in the roof at convenient intervals ; thus, proper elevation; if at any point this is not the case, the in the case of the Thirlmere aqueduct, they occur at every aqueduct must be carried on a substructure built up to quarter of a mile. In some parts of America aqueducts are frequently the required level. Such large structures are, howeer, extremely expensive, and require elaborate devices for constructed of wood, being then termed flumes. These maintaining watertightness against the expansion and con- are probably more extensively used in California Timber traction of the masonry due to changes of temperature. than in any other part of the world, for aqueducts. They are now only used where their length is very short, conveying large quantities of water which is as in cases where mountain streams have to be crossed, required for hydraulic mining, for irrigation, for the supply and even these short lengths are avoided by some of towns, and for transporting timber. The flumes are fre engineers, who arrange that the aqueduct shall pass, quently carried along precipitous mountain slopes, and aci oss wherever practicable, under the streams. Where w ide valleys, supported on trestles. In Fresno county, California, valleys interrupt the course of the built aqueduct, or there is a flume 52 miles in length for transporting timber where the absence of high ground prevents the adoption of from the Sierra Nevada Mountains to the plain below; it that type at any part of the route, the cast-iron pipes has a rectangular Y-shaped section, 3 feet 7 inches wide at the top, and 21 inches deep vertically. The boards hereafter referred to are used. The built aqueduct may be either in tunnel, or cut-and- which form the sides are 1-| inches thick, and some of the cover, the latter term denoting the process of cutting the trestlework is 130 feet high. The steepest grade occurs trench, building the floor, side-walls, and roof, where there is a fall of 730 feet in a length of 3000 feet. Masonry an(| covering with earth, the surface of the About 9,000,000 feet of timber were used in the conaqueducts. groun(j ptqng restored as before. For works struction. At San Diego there is a flume 35 miles long conveying water for domestic supply, the aqueduct is in for irrigation and domestic supply, the capacity being 50 these days, in England, always covered. Where, as is feet per second; it has 315 trestle bridges (the longest usually the case, the water is derived from a tract of of which is that across Los Coches Creek, 1794 feet in mountainous country, the tunnel work is sometimes very length and 65 feet in height) and 8 tunnels, and the cost heavy. In the case of the Thirlmere aqueduct, out of the was $900,000. The great bench flume of the Highline first 13 miles the length of the tunnelled portions is 8 canal, Colorado, is 2640 feet in length, 28 feet wide, and miles, the longest tunnel being 3 miles in length. Con- 7 feet deep; the gradient is 5-28 feet per mile, and the ditions of time, and the character of the rock, usually re- discharge 1184 feet per second. As previously stated, the type of aqueduct built of conquire the use of machinery for driving, at any rate in the case of the longer tunnels. For the comparatively small crete, &c., can only be adopted where the ground is tunnels required for aqueducts, two percussion drilling sufficiently elevated to carry it, and where the quantity machines are usually mounted on a carriage, the motive of water to be conveyed makes it more economical than power being derived from compressed air sent up the piping. Where the falling contour is interrupted by valleys tunnel in pipes. The holes when driven are charged with too wide for a masonry structure above the surface of the

than 90 per cent, of the total cost. As a supply of about 50,000,000 gallons a day is available the outlay was justifiable, and the water is in fact very cheaply obtained. Liverpool derives a supply of about 40,000,000 gallons a day from the river Yyrnwy in North Wales, 68 miles distant, and Birmingham is constructing works for impounding water in Radnorshire, and conveying it a distance of 74 miles, the supply being about 75,000,000 gallons a day. In the year 1899 an Act of Parliament was passed authorizing the towns of Derby, Leicester, Sheffield, and Nottingham, jointly to obtain a supply of water from the head waters of the river Derwent in Derbyshire. Leicester is 60 miles distant from this source, and its share of the supply is about 10,000,000 gallons a day. For more than half the distance, however, the aqueduct is common to Derby and Nottingham, which together are entitled to about 16,000,000 gallons a day, and the expense to Leicester is correspondingly reduced. These are the most important cases of long aqueducts in England, and all are subsequent to 1879. It is obvious, therefore, how greatly the design and construction of the aqueduct have grown m importance, and what care must be exercised in order that the supply upon which such large populations depend may not be interrupted, and that the country through which such large volumes of water are conveyed may not be flooded in consequence of the failure of any of the works. Practically only two types of aqueduct are used m England. The one is built of concrete,_ brickwork, &c., the other of cast-iron (or, in special circumstances, Construe- steel) pipes. In the former type the water surlion. face coincides with the hydraulic gradient, and