Page:Popular Science Monthly Volume 32.djvu/467

Rh unbroken, and with the vegetation undisturbed." Again, "The effect of the move of the soil was very curious; at one point the bottom of the deflection cut had risen 3 metres where the greasy clay had forced itself underneath, and at another I saw a surveying picket which had moved out of line 2 metres, where there was no break in the top soil." To my mind, the deep cut of the Culebra is the great problem of the enterprise; already its side threatens to bar the way.

Leaving this puzzle to the contractors, and looking toward Panama, the first third of the last division descends gradually to 4 metres level at Pedro Miguel; the middle portion forms a plane from 5 to 8 feet above the sea, and over the last 6 kilometres flow the waters of the Pacific to an average depth of 5 metres, and reaching the maximum as Naos is approached.

At Colon the highest tides do not exceed 58 centimetres, or 23 inches, while at Panama high tides reach 4 metres, or 13 feet, and spring tides even G metres, or 20 feet. That vessels may pass at all stages of tide, the depth of the canal from Naos to the present crossing of the Rio Grande beyond Corozal—9·4 kilometres—will be 9 metres at lowest ebb.

The company has consulted the French Academy of Sciences concerning the probable effect of this difference of tide-level on the canal, and has been told that a lock or tidal gate will not be needed. The director-general does not oppose this view, but thinks that an answer to this question at present must be based on theory—that, as excavations progress, the effect must be watched; and that upon the knowledge obtained the decision must rest. Plans for a tidal gate have been prepared, and, if needed, it will be placed at the Boca, near Panama.

The line of the canal is crossed twenty-eight times by the Chagres between Gamboa and Colon, and thirteen times by the Rio Grande between Culebra and Panama. To avoid the dangers of current and overflow that would exist if these streams entered the canal, deflections are excavated to carry them to the sea in beds on each side of the canal. The deflections of the Upper Chagres will drain the Gamboa basin and the water-shed north of the canal, and will discharge their waters into the Boca Grande, east of Colon; those of the Lower Chagres will transport to the present mouth of the Chagres, the tributaries now entering the river on its left bank, the most important of which are the Obispo, the Arena, and the Trinidad. The Rio Grande will be deflected entirely to the right of the canal, and will enter the sea at La Boca. The total length of the deflections will be 64 kilometres. Some of them will be 40 metres wide and 3 deep; others, 30 metres wide and 5 deep. Erosion will increase these dimensions. Lying in valleys where feasible, the soil is alluvial, and easily excavated. It has occasionally happened that a subterranean stream has undermined the banks, and caused a break. The engineers claim that so far the repairs are effective, and I see no cause to disagree with them.