THE FRENCH DESIGN.–Fig. 27 is an enlarged section of the original design of the new French Co. This design has no core wall, but at the up-stream toe a concrete wall was to be built across the river between the two lines of sheet-piling. At the down-stream toe a large amount of riprap was to be placed to prevent destruction of the dam during construction. In this case it would be necessary to construct a temporary dam above and also to use the excavation for the locks as a flood spillway. This method would involve considerable risk to the work, on account of the large volume of flood waters it might be necessary to take care of during construction. ISTHMIAN CANAL COMMISSION.–The dam proposed by the Isthmian Canal Commission is shown by Fig. 28. This was designed to be an absolutely water-tight closure of the geological valley, by using a masonry core wall carried down to bed rock. The maximum depth being 129 ft., it was planned to rest the concrete wall on a series of pneumatic caissons reaching to rock. The spaces between the caissons would be closed and made water-tight. Both slopes of the earth embankment were to have horizontal benches and be revetted with loose rock. MR. MORISON’S DESIGN.–To appreciate fully the object and aim of the third design, Fig. 29, which may be called a new type, although similar in many respects to the North Dike of the Wachusett reservoir already illustrated and described, it should be stated that the equalized flow of the Chagres River is put at 1,000 cu. ft. per sec. Of this quantity it is estimated that 500 cu. ft. would be needed for lockage and 200 cu. ft. for evaporation. This leaves 300 cu. ft. per sec. available for seepage and other losses or to be wasted. [Illustration] [Illustration] [Illustration: FIGS. 27 TO 29.–DESIGNS FOR THE BOHIO DAM, PANAMA CANAL.] It will thus be seen that a scarcity of water is not in this instance a condition demanding an absolutely water-tight dam. The amount of seepage permissible without endangering the stability of the structure is the real point now to be discussed. The third design, which was proposed by Mr. Morison, is shown by Fig. 29. The topography and configuration of this dam site is not unlike that of the San Leandro Dam, California, soon to be described, while the general design is similar, as has been remarked, to the North Dike of the Wachusett Reservoir. This third design contemplates a compound structure, formed by two rock-fill dams situated about 2,120 ft. apart, with the intervening space filled with loose rock, earth and other available material. Immediately below the upper and higher rock-fill dam, it is proposed to place across the canyon a puddle wall 50 ft. in width, resting over two lines of sheet-piling 30 ft. apart. This piling would probably not reach farther than 50 ft. below tidewater, the solid rock floor being about 100 ft. deeper.