Page:Popular Science Monthly Volume 62.djvu/562

556 by Sir Samuel Baker more than forty years ago, from mere inspection of the site without surveys. In suggesting a series of dams across the Nile to form reservoirs from Khartoum downwards, he wrote: "The great work might be commenced by a single dam above the first cataract at Aswân, at a spot where the river is walled in by granite hills. By raising the level of the Nile 60 feet, obstructions would be buried in the depths of the river, and sluice-gates and canals would conduct the shipping up and down stream." This single dam, proposed by Sir Samuel Baker forty years ago, is in effect the one which is now on the point of completion. Mr. Willcocks' original design consisted practically of a group of independent dams, curved on plan, and the arrangement of sluices and dimensions of the dam differed considerably from those of the executed work. There is no doubt that the single dam, 1 miles in length, constitutes a more imposing monumental work than a series of detached dams, and that it also offered greater facilities to a contractor for the organization of his work and rapid construction; and, further, the straight dam is better able to resist temperature stresses from extreme heat without cracking. Two dams across the Nile, the old barrage and the Asyût Barrage, have already been described; and it will be hardly necessary to say, therefore, that the Aswân Dam is not a solid wall, but is pierced with sluice openings of sufficient area for the flood discharge of the river, which may amount to 15,000 tons of water per second. There are 180 such openings, mostly 23 feet high by 6 feet 6 inches wide; and where subject to heavy pressure, when being moved, they are of the well-known Stoney roller pattern.

Although the preliminary studies of Mr. Willcocks and the other government engineers occupied some four years, there was neither time nor money to sink shafts in the bed of the river, to ascertain the real character of what was called in the engineer's report 'an extensive outcrop of syenite and quartz diorite clean across the valley of the Nile,' giving 'sound rock everywhere at a very convenient level.' Unfortunately, the rock proved to be unsound in many places to a considerable depth, with schistous micaceous masses of a very friable nature, which necessitated carrying down the foundations of the dam sometimes more than 40 feet deeper than was originally anticipated or provided for in the contract. As the thickness of the dam is nearly 100 feet at the base, this misapprehension as to the character of the rock involved a very large increase in the contract quantity and cost of the granite masonry of the dam. The total length of the dam is about l miles; the maximum height from foundation, about 130 feet; the difference of level of water above and below, 67 feet; and the total weight of masonry over one million tons. Navigation is provided for by a 'ladder' of four locks, each 260 feet long by 32 feet wide.