“Well, it’s this way, as I understand it,” said Rob. “In the first place, the canal is fifty miles long,—forty-one miles through the land and nine miles of channel dredged out in the harbors of Colon and Panama. From Colon to Bah Bohia the route passes for twelve miles through low, swampy ground not much above sea level. Then it cuts into the hills and is practically a more or less shallow ditch as far as a place called Miraflores, nine miles away. The highest point of land that the canal must traverse is Gold Hill, at the famous Culebra, where it is 662 feet above the sea level.
“But right here occurs a ‘saddle’ through which the canal must run. This, at its lowest point, is 312 feet above sea level. Right here is the notorious Culebra Cut, which is an immense excavation nine miles long and, in places, more than three hundred feet deep in solid rock,—think of that!
“Bad as Culebra has been as an obstacle, however, the Chagres River is worse. For 23 miles the canal must follow the valley of this river and cross and recross its bed. The Chagres is an unruly stream. At times it is small, and then again it swells to tremendous size, sweeping all before it and causing great floods. To build the canal the problem was to turn the Chagres into a friend, instead of an enemy, and that, it is believed, has been done in an unique way.
“You must now roughly picture a cross section of the canal route as a flat-topped pyramid. Suppose the top of the pyramid to be hollow and that through that hollow flows the Chagres River. Well, on one side of your cup or hollow is the famous Gatun Dam, in the construction of which 2,250,000 barrels of cement have been used. Below the Gatun Dam is a ‘flight,’ just like a succession of steps of locks. These will be used to lower vessels from the ‘cup’ at the top to the Atlantic level,—or to raise them, as the case may be.
“On the other end of the cup, on the Pacific end that is, will be another flight of locks, the Pedro Miguel and Miraflores locks, which will raise or lower vessels from and to the Pacific. Is that clear? There’s a big cup at the top of our pyramid, and steps, or ‘locks,’ lead down to the levels of the oceans on each side.”
“Oh, it’s as clear as mud,” muttered Tubby, “go on.”
“Now, then, we get to the Chagres and the part it plays,” went on Rob serenely. “That whole ‘cup’ at the top of our pyramid is actually an artificial lake of vast size. As a matter of fact, it will be 165 square miles in area. At Gatun a great dam will hold it in, and at Pedro Miguel the locks will perform the same office. This lake is the valley of Chagres, and the Chagres will be relied on to keep it filled. This immense Gatun Lake, as it is called, is the ‘keystone’ of the canal. Any weakness in the Gatun Dam would ruin the whole project. You can see, of course, why this is so, because the water in that Gatun Lake will be relied upon to fill the locks which will raise vessels up or down.”
“But suppose the Chagres River cuts up ugly, as you said it does sometimes?” asked Merritt.
“Well,” said Rob, “I heard Mr. Mainwaring say that the great lake will be so big that a flood would affect its level no more than a cup of water poured into a bath tub. The river will merely serve to keep the lake filled and supply the water needed to work the locks.”
“That’s a very good description, Master Rob,” said a voice at their elbows.