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these works is shown in Fig. 19, in which a represents the embankment of earth; B, a solid wall or dyke of puddle; c, a facing wall of masonry; D, the high-water level of the sea or bay, and E, the natural bed. The form of the front wall must be adapted to resist the action of the waves, and the embankment must have an internal slope, according to the nature of the materials of which it is composed: for ordinary materials, a base of 1.5 to a perpendicular height of 1 will insure the necessary stability and firmness.

67. If the entire embankment be formed of loose stones, with occasional facing only of laid masonry, as in the case of the celebrated breakwater at Plymouth, a form of less steepness must be adopted for the river front of the embankment. By way of illustration we may refer to fig. 20, which shows a section of the

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Plymouth breakwater. The line A A shows the level of high water spring tides; B B, low water spring tides; cc, original bottom, varying from 40 to 45 feet below low water mark; D, the fore shore; E, sea slope; F, top, 45 ft. wide. The mass of the work is composed of limestone, from the Overton quarries, distant four miles from the spot. The stone is raised in blocks varying from one quarter to ten tons and upward in weight, which are promiscuously thrown into the sea, care being taken that the greater number of the large blocks are thrown

upon the outer or sea slope, and that the whole are so mixed together as to render the mass as solid as possible, the rubbish of the quarry and screenings of lime being flung in occasionally to assist the consolidation of the materials. The form of the outer slope, below low water line, has been effected by the action of the sea, and is ascertained to be at from 3 to 4 feet of base to 1 of perpendicular altitude. From low water upward, the work has been set artificially and inclined at 5 to 1. The inner slope next the land is nearly 2 ft. base to 1 altitude. The foreshore shown at D, which is from 30 to 70 ft. wide at different parts of the work, rises from the toe of the slope, to a height of 5 ft. above low water at its outer extremity, and serves to break the waves before they reach the main work; thus diminishing their force, and, at the same time, preventing the recoil of the wave from undermining the base of the slope.

68. The several sluices, gates, &c., constructed for the Ancholme drainage, being of the best description, may be briefly described as applicable for similar works in future. The sluice at Ferraby consists of three openings, each 18 ft. wide, with cills 8 ft. below that of the old sluice, and from 2 to 3 ft. below low water of spring tides in the Humber. The lock is 20 ft. wide in the clear, and 80 ft. long between the gates, giving a clear water-way of 74 ft., with an additional fall of 8 ft. The masonry is of best Yorkshire stone; and the foundations, which are in alluvial silt and clay, are upon piles 12 in. diameter, of beech, elm, and fir, from 24 to 28 ft. in length, and fitted with wrought iron hoops and shoes. When the piles were driven and the heads levelled, the earth was excavated to a depth of 2 ft. below them, and the spaces filled with blocks of chalk rammed soundly in, and grouted with lime and sand. Cap-cills

The whole was

Baltic fir-plank, Inverted arches crown, are built

of Memel fir, elm, or beech, 12 in. square, were fitted on the pile-heads and firmly spiked down, the intermediate spaces being afterwards filled with solid brickwork, set and grouted with best Roman cement. then covered with a 3-in. flooring of bedded in lime, pozzolana, and sand. of solid stonework, 18 in. deep at the upon this platform, and the work carried upon them. Two sluice gates were provided for each opening in the sluice, with draw-doors fitted in a water-tight groove by means of pinions of wrought iron, which work in screws connected with vertical rods. These draw-doors are for regulating the navigation level (which is 13 ft. 8 in. above the cill), and to preserve a depth of 8 ft. 9 in. at Brigg, which is 9 miles distant, and 6 ft. 6 in. at Haarlem Hill lock, 18 miles distant. The gates are self-acting, being shut by the tide, and opened by the head of fresh water as soon as the tide falls below the level of the inside water. Four pairs of lock-gates were provided for the lock; two pairs pointing to the sea, and of sufficient height to exclude the highest tides: the other two pairs, pointing to the land, are high enough to control the navigation of the level. These gates were wholly constructed of the best English oak, well fitted together with wrought iron straps and bolts. The lock is filled and emptied through side culverts in the masonry, provided with cast iron sluices, sliding upon brass faces, and worked with pinions and screws of wrought iron. The works also included several bridges of various spans and forms of

construction.

69. In the application of catch-water drains it is preferable to discharge their contents at a higher point of the river, or main receiving channel, than that at which the low land drains are emptied. This principle

was very successfully adopted by the late Mr. Rennie, in the drainage of the East, West, and Wildmore Fens, bordering on the river Witham, and comprising about 75,000 acres. The drain for the high land waters was made to discharge into the Witham at a distance of three miles above the discharge of the low land waters.

70. The drainage of a low fenny district being arranged as far as the judicious selection of separate channels for the high and low lands, and provision made, with sluices, &c., for their communication with each other and with the river at pleasure, it remains to consider the state in which this river must be maintained in order togive efficiency to the internal system of drains by which the district is traversed. For this purpose it is evidently necessary that the channel be adequate in dimensions and suitable in form to maintain an active and sufficient current through it, and these conditions require a direct course and proper fall for the channel. If the direction be tortuous, the projecting banks will be washed into the bed and impede the flow of the current, and if the bed be on a dead level, or have an inadequate inclination, the flow will be sluggish, and lend no assistance to the discharge. Besides these conditions, it is necessary that the outfall of the river into the sea be of ample dimensions and unencumbered with shoals, bars, or other solid accumulations. These arise from the depositions of alluvial matter, which is liable to be brought in by the tides from the neighbouring coast, and also brought down with the drain-water from the interior country. This matter remains suspended in the water until the velocity is diminished, which generally occurs at the entrance to the river, owing jointly to the reduced inclination of the river bed near the sea, and the resistance suffered from the wind and waves, and it is

then deposited, and by continual augmentation forms a fatal obstruction to the efficiency of the current. To determine the precise fall or inclination required for the bed of the channel, many experiments have been tried, but it will evidently be, to a considerable extent, controlled by the obstructions which may exist to the discharge of the waters. If the outfall be unimpeded, 4 or 5 in. per mile will be sufficient fall, but if obstructions exist, in the form of old bridges, sinuosities, &c., from 12 to 18 in. per mile will be found requisite.

71. Among the notable works of this kind which have been executed in this country, we may mention those for improving the rivers Ouse and Nene. The chief defect in the former existed above the town of Lynn, where the river turned almost at right angles to its general course, and in a length of 5 miles formed a semicircle of only 2 miles in diameter. The channel was, moreover, so irregular in width and encumbered with shifting sands, that the tidal and drainage waters were unable to force a passage, and disastrous inundations were the results. In the year 1724 this evil was understood, and a proposition made by Bridgeman for improving the river by making a direct cut which should intercept the bend here described. Succeeding engineers concurred in this recommendation; but it was not until the year 1817 that an act was obtained for executing this important work, which was named the Eau Brink Cut, and confided to the late Mr. Rennie. The works were finished on the 19th July, 1821, and have proved highly successful, lowering the low water line in the river several feet, and completing the drainage of more than 300,000 acres of land. A work of similar character was executed in the year 1829, by Telford and Rennie, at the outfall of the river Nene,

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