The capability of the dam to maintain the waters of the lake at a sufficient level in the dry season depends upon their not finding a ready way either through the dam itself or below it. The construction of the dam is believed to guarantee its own practical impermeability. Not only is there a puddled core, but the mud, sand, and rocks of which the principal mass will be composed will be laid down in the manner best calculated to secure compactness. With regard to underground flow, there is an underlying bed of indurated clay which is regarded as sufficiently impervious, and wherever the puddled core and piling are imbedded in that clay it may, I think, be assumed with some confidence that the leakage will be unimportant. On referring to the section (map), however, it will be seen that there are in the valley two old river gorges, which to a depth of 200 and 260 feet are filled only with gravel, sand; sand, shells, and wood; clayey sand, and so forth. These gorges, measured on the section shown in the figure, have widths of about 1,200 and 500 feet respectively at the depth to which the sheet piling goes, and extend about 120 and 180 feet below. How much water may escape by these gorges it is difficult to say. This leads us to the next division of our subject. _On the Supply of Water Available for the Needs of the High-level Canal._ The construction of the Suez Canal was a work of excavation pure and simple. The construction of any kind of canal across the Isthmus of Panama involves another task, second only in importance to the primary work of excavation, viz., that of regulating the rivers. In the case of a sea-level canal the problem would have been how to get rid of their waters, particularly in the rainy season. In the actual case of an 85-foot-level canal, the regulation of the rivers, particularly of the Chagres, presents two aspects, viz.:-- (1) In the wet season, disposing of the surplus waters. (2) In the dry season, conserving water supplied by the rains so as to meet the waste caused (_a_) by locking, (_b_) by evaporation, (_c_) by percolation. The arrangements for taming the torrents of the Chagres and its tributaries have already been described. They are, briefly, the construction of the Gatun dam and its spillway. Turning to the other aspect of the problem, I have to answer the question, What is the guarantee that there will be sufficient water in the dry season? [Illustration: IN THE CUT, WIDTH 500 FEET.] [Illustration: IN THE CUT, LOOKING SOUTH TOWARDS CULEBRA.]