"That you, youngster?" he sang out cheerily, seeing Jim. "I've been down to the office of the Commission doctor, who's fixed this arm for me. The man who saw to it aboard the ship that brought us in hadn't too much time, for there were others who'd been injured by some of those Spaniards who'd been fighting. In consequence I had a bit of pain last night; but I'm easy now. Let's get some breakfast, then you and I'll be off." An hour later found the two down at the point where the dirt trains were already dumping their contents, and just where the huge Gatun dam was to be erected, so, standing on an eminence, Jim was able, with the help of his friend, to follow in a logical manner the plans of the American engineers. For he could look into the long, winding hollow along which at that moment flowed the tributaries of the Chagres River. "It's just as clear as daylight," said Phineas, his face aglow, for anything to do with the Panama Canal warmed him, so great was his enthusiasm. "Away there below us, where you see two rivers coming together to form what is known as the Chagres River, you may take it that the level of the land is just a trifle above that of the sea, and of course the water on this isthmus has found the lowest level possible. It could not get away to the east because of the hill, and west here, where we are, there's another. So that water just flows out between them, the hills themselves forming, as it were, the neck of a bottle. Well, we're just putting a cork into that neck. We're erecting a dam across the valley between these two hills which will be 7700 feet in length, measured across the top, while its base measurement will be 2060 feet." "Enormous!" exclaimed Jim. "But surely such a tremendous mass is hardly necessary?" "What! with 164 miles of water behind it? Young sir, let me tell you that there'll be a clear depth of water of 80 feet all along this end of the lake we're forming. A body of water like that exerts terrific pressure, and to make that dam really secure against a fracture, to make an engineering job of it, as we should say, the dam ought to be constructed of masonry built right into solid rock. But there ain't no rock, more's the pity." "None?" asked Jim. "Then you won't be able to use masonry?" "Right, siree! But we're going to fix the business, and reckon, when the dam's finished, nothing'll move it. Listen here, and jest look away where I'm pointing. There's an army of niggers and European spademen at work along the line the dam's to follow. They're working a trench right across, 40 feet down into the soil. Those engines you can see smoking along there are driving sheet piling of 4-inch timbers 40 feet down below the bottom of that trench. When they have finished the job of piling, the trench'll be filled chuck up with a puddled core of clay that'll act like a sheet anchor." "And so hold the dam in position," suggested Jim.