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Page:Popular Science Monthly Volume 32.djvu/465

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in opening the Mindi cut is due to the wish to complete the excavation of rocks, and thus render submarine blasting unnecessary. Of the eleven dredges at work in this division, four are European, and vary from 60 to 180 horse-power; the remaining seven are American, of 240 horse-power, their maximum capacity of excavation being 6,000 cubic metres per day. But repairs to machinery, rains, stoppage during the extreme heat of the day and at night, and other delays that can not be remedied, have reduced the daily yield to 3,000 cubic metres.

The level of the remaining bed of the first division is from 4 to 10 metres above the sea. Part of a hill near Bohio remains at the elevations of 20 and 28 metres.

The Chagres is both the upper and the lower limit of the second division, and crosses it seventeen times in its length of 17 kilometres. The average level is 12 metres above the sea, except in a single hill of 25 metres height and in a sudden rise to the same elevation at the end of the division. The excavators are moved on railway tracks by an engine of 8 to 10 horse-power, and empty their buckets into cars on adjacent rails. Every facility for dredging is presented by the Chagres River, the depth of which is such that dredges can be put at each crossing.

Just within the third division is the Gamboa hill, where occurred the great explosion of 1886 in honor of M. Ferdinand de Lesseps. The charge was 8,250 pounds of dynamite and powder, and blew out 30,000 cubic metres of material. Farther on is the Corrosita, still 45 metres above the sea. But the most remarkable feature of the division is the great barrage at Gamboa. Its central line will cross the Chagres between the Cerro Obispo and the Cerro Santa Cruz. Its length at the base will be 300 metres, its height 35 metres, and, with a reveted slope of four to one, it will contain 10,000,000 cubic metres of rock and clay. No excavation is needed for the foundation; a bridge is now building across the valley, and from it trains will discharge into the valley below their loads of rock and earth excavated from the Corrosita and neighboring sections. The pressure of water in the basin will seal the dam by forcing the clay into the interstices of the rock mass, and by deposits brought down by the river. The capacity of the basin will be one billion cubic metres, or double the accumulation of waters during the worst rainy season. Nature has furnished the other walls of this reservoir in the ridges on each side of the Chagres, and in the natural ascent of the valley toward Cruces. The outlet of the basin will be a derivation of the Chagres around the hills of Barucco and Carga-Plata to the bend north of the forty-fourth kilometre; the outflow will depend upon the height of the water in the basin, but it will never be such that, when coupled even with the drainage of the remaining water-shed of the Upper Chagres, floods can occur in the lower course of the river.