The locks are built in an excavation at the east end of the dam, in rock, and will lift vessels from the Atlantic level to the level of the Lake of Gatun. The spillway is a concrete-lined opening cut through a hill of rock near the center of the dam. When supplied with suitable gates, it will regulate the level of the lake.
The dam proper is about 9,000 feet long over all, measured on its crest, including locks and spill way, but for only five hundred feet of this great distance will it be subjected to great pressure. During this space there is, or will be, a weight of about eighty-five feet depth against the barrier. For only about half its length will the head of water on the dam be over fifty feet.
It will be seen from the above description that the point of attack on the dam would naturally be where the pressure is greatest, also at the locks, which would make a mighty channel for the flood of water, and which would be difficult to repair. The spillway, too, if enlarged by explosives, would make a nasty hole to build up.
Now another point which Ned had considered when he looked over the crude drawings he had discovered. Hard rock underlies the dam near the surface of the ground except for about one-fifth of its entire length. Here the rock dips down to a minimum depth below sea-level of from 195 feet in the depression east of the spillway to 255 feet in that west of the spillway. Here, of course, would be another point of attack by one designing permanent mischief.
These depressions or valleys have been slowly filled during past ages. Measured from sea-level down, the first 80 feet consists of sand and clay; the next 100 feet or so is stiff blue clay, while the last 20 to 60 feet is a conglomerate, composed of sand, shells and stone. It will be readily seen that great damage might be done by a raging torrent boring into the sand and clay of the first strata.
Now, the outer walls of rock are 1,200 feet apart, the interval being filled with spoil from the canal and lock excavations. The south “toe,” as it is called, has a height of 60 feet, while the north or down-stream “toe” is 30 feet high. Spoil from the excavations will be dumped outside the “toes” until the dam is 2,000 feet in width at the bottom. The top of the dam is, or will be, 30 feet above water level and have a width of 100 feet. The channel of the spillway is 300 feet wide.
Ned had figured it out that one attacking the dam would naturally seek to enlarge the locks and the spillway and also to burrow in under the bulk of the dam where the sand and clay had been washed in below sea-level by countless years of flood and storm. The locks and spillway, enlarged, would require years of active work for repair; the sand and clay, if subjected to high explosives, would cause the crest of the dam to drop in on the north side and so enfeeble the entire structure, requiring the gigantic work of constructing new foundations.
Therefore, when Ned saw the four men moving toward the spillway, saw them part and seek the vulnerable points which have been described above, he knew that the time he had been waiting for had come. The treacherous rascals were there to do their wicked work that night—to carry out plans long formed and well considered—and they were opposed only by the inexperienced patrol leader from New York and his three chums, Frank Shaw, Glen Howard, and Peter Fenton. It will be remembered that Jimmie McGraw, Jack Bosworth, and Harry Stevens were at the old stone house on the road to Las Cruces from Gamboa, and that George Tolford had accompanied Tony to the Chester camp.
On reaching Gatun the boys had slipped out of the lights of the station and descended immediately to the bottom of the cut. They were at once accosted by a foreman, but the explanation Ned gave seemed more than sufficient, for Dan Welch, the man in charge of a group of workers on the locks, at once summoned his assistant to the job and remained with the boys.
“I have heard about you, Ned Nestor,” Welch said; “in fact, about half the men in the workings at Gatun have heard of you.”