Page:Popular Science Monthly Volume 61.djvu/274

268 After leaving Bohio-Lake, at Obispo, a flight of two locks is found at Pedro Miguel, about 7.9 miles from the former or 21.5 miles from Bohio. These locks have a total ordinary maximum lift of 60 ft., divided into two lifts of 30 ft. each. The fifth and last lock on the route is at Miraflores. The average elevation of water between Pedro Miguel and Miraflores is 30 ft. above mean sea-level. Inasmuch as the range of tide between high and low in Panama Bay is about 20 ft. the maximum lift at Miraflores is 40 ft. and the minimum about 20. The twin locks at Miraflores bring the canal surface down to the Pacific Ocean level, the distance from those locks to the six-fathom curve in Panama Bay being 8.45 miles. There are therefore five locks on the Panama route, all arranged on the twin plan, and, as on the Nicaragua route, all are founded on rock.

Near Obispo a pair of guard gates are arranged 'so that if it should become necessary to draw off the water from the summit cut the level of Lake Bohio would not be affected.'

An unprecedented concentration of heavy cutting is found between Obispo and Pedro Miguel. This is practically one cut although the northwesterly end toward Obispo is called the Emperador while the deepest part at the other end, about three miles from Pedro Miguel, is the great Culebra cut with a maximum depth on the center line of the canal of 286 ft. On page 93 of the Isthmian Canal Commission's report is the following reference to the material in this cut:

There is a little very hard rock at the eastern end of this section, and the western two miles are in ordinary materials. The remainder consists of a hard indurated clay, with some softer material at the top and some strata and dikes of hard rock. In fixing the price it has been rated as soft rock, but it must be given slopes equivalent to those in earth. This cut has been estimated on the basis of a bottom width of 150 feet, with side slopes of one on one.

When the old Panama Canal Company began its excavation in this cut considerable difficulty was experienced by the slipping of the material outside of the limits of the cut into the excavation, and the marks of that action can be seen plainly at the present time. This experience has given an impression that much of the material in this cut is unstable, but that impression is erroneous. The clay which slipped in the early days of the work was not drained, and like wet clay in numerous places in this country it slipped clown into the excavation. This material is now drained and is perfectly stable. There is no reason to anticipate any future difficulty if reasonable conditions of drainage are maintained. The high faces of the cut will probably weather to some extent, although experience with such clay faces on the isthmus, indicates that the amount of such action will be small. As a matter of fact the material in which the Culebra cut is made is stable and will give no sensible difficulty in maintenance.