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tide rises to any great extent, it is found desirable to have two speeds of wheel work, one to be used at low water, and the more powerful combination to act against the rising tide. It is usually not necessary to raise the water more than 3 or 4 ft. above the surface to be drained, and that only when the river is filled by longcontinued rains or floods from the upland. If the main drains be 7 ft. deep, and the floats dip 5 ft. below the surface of the water, 1 ft. in depth will be left below them to admit the passage of weeds or other matters, and the water will yet be kept 18 in. below the surface of the land. If the wheel dips 5 ft. below the drainwater level, and the level of the water in the river is 5 ft. above that in the drain, the wheel will be said to have "10 ft. head and dip," and should be 28 or 30 ft. in diameter. For a dip of 5 ft. and head of 10 ft., that is," a head and dip of 15 ft.," Mr. Glyn used wheels of 35 ft. to 40 ft. in diameter. A wheel of 40 ft. diameter, and situated on the ten-mile bank near Littleport in the Isle of Ely, is driven by an engine of 80-horse power. The largest quantity of water discharged by one engine is from Deeping Fen, near Spalding. This fen comprises 25,000 acres, drained by two engines of 80 and 60-horse power. The 80-horse power engine works a wheel 28 ft. diameter, with float boards 5 ft. by 5 ft., and moving with a mean velocity of 6 ft. per second. When the engine has its full dip, the section of the stream is 27 ft., and the quantity discharged per second is 165 cubic feet, equal to more than 4 tons. These two engines were erected in 1825, before which time the district had been kept in a half cultivated condition (being sometimes wholly under water) by 44 windmills. 77. Recurring to Fig. 10, p. 51, the districts there illustrated will require methods of drainage determined

by the inclination of the surface. If this be comparatively level, the drains may be generally cut with beds parallel, or nearly so, to the surface, and arranged to deliver into one or more main drains having lower beds, but still above the low-water level of the river or receiving channel, and from which the water can be let off when the tide is down by providing sluices suitable for the purpose. If the surface undulate, the main drains must be laid in the hollows, and the feeders be distributed over the higher parts, and made to communicate with the mains. Small sluices fixed at intervals, both in the main and minor drains, will, by intercepting the water, permit an accumulation when desired for flooding or irrigating the higher lands. Figs. 23 and 24 show a plan and section of a district of this character. AA is the river or receiving channel; BB, the principal main drain; and c C and D D two other main drains

Fig. 23.

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A

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delivering into it; each of the mains receiving the drainage from the feeders or minor drains. Fig. 24 is a section supposed to be taken on the line z z on the plan. Two imperative rules require to be observed in these arrangements, viz., that all the junctions shall be curved, and that no two feeders shall enter the main drain at opposite points. If these rules are neglected, the currents will be interrupted at these points, and mischief may arise from flooding when the drains become filled in wet seasons. It is also advisable, if the ground be of a loose texture, to guard the junctions with a few rough stones piled together in the form of a retaining wall; or, for greater permanence, concreted with lime and gravel, as shown in the plan and sections, figs. 25, 26, and 27, of which, fig. 25 is a plan, fig. 26,

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Fig. 26.

Fig. 27.

a section through the ordinary drain taken on the line Y y; and fig. 27, a section through the guard-walls, taken on the line x x.

78. If the general inclination of the surface of the district be considerable, it is often desirable to form catch-water drains, or series of drains at different elevations, communicating with each lower one successively by falls. By this method great facilities are obtained for regulating the management of the waters, so that any required quantity can be retained to compensate for seasons of drought; while, moreover, the falls are applicable as water-power, and may be used for a variety of purposes. Fig. 28 is a plan, and Fig. 29, a section of a district drained in this manner. A A, B B, and c c, are the main or catch-water drains, each of which receives the drainage from the minor drains or feeders connected with it, and delivers it to the next lower main, through the channels a a, b b, and cc, each of which has sluices fitted to it, while the water forms a series of falls at the points marked y. Or the water from the superior levels may be received in reservoirs constructed for the purpose and in the places of the catch-water drains, and there disposed of for agricultural, manufacturing, or domestic purposes.

79. In fig. 11, p. 52, we have sketched an inland

B

Fig. 28.

Fig. 29.

B

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body of water, or lake, which receives the drainage of the adjacent districts, and to these, thus situated, the same methods of draining as those just described are generally applicable. The formation of lakes upon the surface of our globe appears to have resulted from three causes, viz., the outcropping of internal springs or sources of water; subterranean communication with

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