no earth needs to be removed in order to get the requisite depth of channel. In the Culebra Cut the excavation for the sea level canal would have to be carried a clear 80 feet, most of it through hard material, deeper than that for the lock canal.

The principal feature of this plan is the great dam at Gatun which is to be thrown across the channel of the Chagres River. The waters thus intercepted in their course to the sea will collect in the valley basin and form the huge artificial lake. The lake will also furnish water for the lockages and for power, while affording practically open navigation through the greater part of its length.

The dam is to be an enormous solid structure, calculated to withstand the forces of nature and, as the report states, one that "could only be destroyed by making excavations which would require a large force working for a long time." The crest of this dam, as planned, stands at 135 feet elevation, or 50 feet above the summit level of the lake; at the top it is 100 feet in width, and at water level, 374 feet; at its base it is 2,625 feet, or one-half mile, in thickness. These dimensions have been somewhat changed.

This dam will have a face, reinforced with riprap, extending a distance of one and a half miles and connecting two hills. A rock wall along its foot will parallel the face. These rock structures, together with the flanking hills, will form a kind of box. Specially selected material, consisting of clay and sand carried in water, will be pumped into this enclosure, and when the water has drained off, there will be left a compact and hard mass, impervious to seepage. Near the centre of the face a spillway, with sluice gates, will permit of the regulation of the stand of water in the lake.

It is confidently expected that this massive structure, being, as it were, welded into the earth at bottom and along its sides, will withstand any earthquake to the force of which it is likely to be subjected. It will be much stronger than the dams at San Leandro and Pilarcitos, connected respectively with the waterworks of Oakland and San Francisco, which are at present the largest in existence. The latter was not in any degree injured by the great earthquake of San Francisco, although it lay within the zone of disturbance. "This huge mass will exert a pressure upon its foundation of one ton to the square foot for every

twenty feet of its height. Its great weight will be an element of safety, provided the foundation is not susceptible to percolation. The Gatun Dam is the key to the plan, and its importance has made its site the centre of the critical investigations to which the plan has been subjected continuously since its inception. Borings innumerable have been made during the past four years, and it is safe to say that our engineers are as familiar with the underlying strata as they are with the surface of the ground. The fund of applicable data has been enlarged by the construction of experimental dams, by soil analysis, by water tests, and by geological examinations. In short, the dam and lock sites at Gatun have been explored exhaustively, and from every possible point of contact. The results show conclusively: (1) That the foundations are suitable, and perfectly safe for the construction of a stable and watertight earth dam of such material as is available and near at hand. (2) That the concrete spillway and concrete locks will rest upon rock foundations of the most satisfactory description.

991

This dam has been made the object of the

1 From an article by the Author in the Review of Reviews, April, 1909.

severest criticism by the advocates of a sea level canal. The wildest misstatements have been disseminated regarding it and an attempt has been made to create the impression among the public that it is experimental in its nature and that the engineers in charge of the work have comparatively little knowledge of the ground upon which the dam is to stand. A trivial slide, such as engineers always look for in fills before the material has finally settled, was eagerly seized upon as a basis for a sensational report that a serious cave-in had occurred. One of the newspapers printed an absurd story to the effect that a subterranean lake had been disclosed. The matter was made so much of by a large proportion of the press that public confidence was seriously impaired and the President determined to appoint a special commission to investigate. The body was composed of engineers whose experience was particularly great in the matter of dams. After ten days of investigation on the spot, the Commission reported that the dam, its plan, site, and the material that was to be used in its construction, were all that could be desired.

The water supply is, of course, a matter of the utmost importance. The subject was

treated by General Abbott, an unquestionable authority, in a separate paper which was embodied in the report. The records of the flow of the Chagres and its tributaries during fifteen years showed 1,250 feet per second in the driest seasons. On a conservative estimate, 2,577 feet per second can be depended on during the entire three months of least rainfall.

"To determine the number of lockages which this quantity of water will provide for, the following provisions and assumptions have been made: Intermediate gates are to be provided for at Pedro Miguel and Sosa, so as to give a chamber length of 600 feet (the full length of the lock being 900 feet), and it is assumed that the intermediate gates will be used for eighttenths of the lockages. . . . It is further assumed that all ships passing in one direction will use one set of locks and all ships passing in the other, another set. (All the locks on the Canal are to be in duplicate.) On this assumption the same quantity of water is used whether a ship passes through a single lock or through two or three in flight. The lift to the normal level at Pedro Miguel is 30 feet and at Gatun 28.50 feet per lock. The quantity of water required per lockage at Pedro Miguel, on the as