Page:Popular Science Monthly Volume 62.djvu/560

554 area of about 300,000 acres under such irrigation, by throwing more water at a higher level into the great Ibrahimiyah Canal, whose intake is immediately above the Barrage (Fig. 2).

The piers and arches are founded upon a platform of masonry 87 feet wide and 10 feet thick, protected up and down by a continuous and impermeable line of cast-iron grooved and tongued sheet piling, with cemented joints. This piling extends into the sand bed of the river to a depth of 23 feet below the upper surface of the floor, and thus cuts off the water and prevents the undermining action which caused so much trouble and expense in the case of the old barrage. The height of the roadway above the floor is 41 feet, and the length of the piers up and down stream 51 feet. The river bed is protected against erosion for a width of 67 feet up stream by stone pitching, with clay

puddle underneath to check infiltration, and down stream for a similar width by stone pitching, with an inverted filter-bed underneath, so that any springs which may arise from the head of water above the sluices shall not carry sand with them from underneath the pitching.

It is easy enough to construct dams and barrages on paper, but wherever water is concerned the real difficulty and interest is in the practical execution of the works, for water never sleeps, but clay and night is stealthily seeking to defeat your plans. On the Nile the conditions are very special, and in some respects advantageous. There is only one flood in the year, and within small limits the time of its occurrence can be foretold, and arrangements made accordingly. It would have been impossible to have carried out the Nile works on the system adopted had the river been subject to frequent floods. The working season for below-water work on the Nile lies practically between