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Page:Popular Science Monthly Volume 32.djvu/466

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THE POPULAR SCIENCE MONTHLY.

In the last days of November the water usually rises to eight metres above the ordinary level. In the valley of the Chagres the annual rainfall is 3 metres. The average discharge of the river during the wet season is 134 cubic metres per second, and 666 during the floods. In the exceptional rise of 1879 it reached 1,930 cubic metres, but it must be remembered that such discharges seldom last more than forty-eight hours; for, as Lieutenant Kimball states in his valuable report just published, "The floods are of short duration, showing that they result from large local rainfall, and not from extensive watershed." When the barrage and derivations are completed, I believe that the problem of the Chagres will be solved.

Another interesting feature of this division is the two aqueducts that will be built near Emperador to carry the waters from the mountain valleys on the northern and eastern side across the canal into the Obispo River. Their elevation will be the present levels of these sites, and vessels will pass under them. The bed of the cuttings at the end of the division have now a level of 55 metres.

The original elevation of the Culebra in the plane of the axis of the canal was 108 metres; the cuttings have reduced it to 78 metres. The width of the cut at the summit is 300 metres, the slope of the sides being forty-five degrees. But a serious question at this point lies in the accumulation of material by wash, land-slides, and fissures. Last year 78,000 cubic metres of earth fell into the canal. The hill on the right side of the cut is formed of dolerite and sand, and no wash or slip can occur from it. But on the left side I found strata of clay covered with a mixture of alluvium, sand, and conglomerate. During the wet season this deposit becomes saturated, and the increased weight, coupled with the dip of the strata, causes it to slip over the smooth surface of the clay into the canal. The clay in turn contracts during the dry season, fissures result, and hence another source of land-slides; and the natural wash of torrential rains is a third cause of deposit in the bed of the cut.

But a far more serious problem apparently is the annual movement of this side toward the axis of the canal. It varies from 12 to 18 inches, and the contractors acknowledge that its remedy may require heavy expenditure for increased slope, if nothing more. As yet, however, this can not be regarded as an actual danger. The removal of so much material from the Culebra must affect the position of the center of gravity of the mass, and it may be that this movement results from a settling to the new conditions. This is the more hopeful view, and a reasonable one, but there is greater cause to fear that this is a movement of the whole hill-side, and not an earth-slide from the higher portions of the bank. The clay of Culebra is of the same bed as the "greasy" or slipping clay of the adjoining section of Paraiso. Referring to the latter. Lieutenant Kimball says its movement "in some places carries one bank almost intact across the cut with the top sur-