Page:Popular Science Monthly Volume 62.djvu/565

Rh of the Dam foundation in the still waters above the rubble dams, and pumps were fixed to lay dry the bed of the river. This was the most exciting time in the whole stage of the operations, for no one could predict whether it would be possible to dry the bed, or whether the water would not pour through the fissured rock in altogether overwhelming volumes. Twenty-four 12-inch centrifugal pumps were provided to deal if necessary with one small channel; but happily the sandbags and gravel and sand embankments staunched the fissures in the rock and interstices between the great boulders covering the bottom of this channel, and a couple of 12-inch pumps sufficed. The open rubble dam itself, strange to say, checked the flow sufficiently to cause a difference of nearly 10 feet in the level of the water above and below;

but when the up-stream sandbag dam was constructed the difference was 20 feet, so that the down-stream sandbag dam was a very small one compared with the other.

The masonry of the dam is of local granite, set in British Portland cement mortar. The interior is of rubble, set by hand, with about 40 per cent, of the bulk in cement mortar, four sand to one of cement. All the face-work is of coursed rock-faced ashlar, except the sluice linings, which are finely dressed. This was steam-crane and Italian masons' work. There was a great pressure at times to get a section completed before the inevitable rise of the Nile, and as much as 3,600 tons of masonry were executed per day, chiefly at one point in the