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190 several hundred miles of track, so ingeniously arranged that the loaded trains travel down-grade and only empty cars have to be hauled up hill. Much rock is used on the Gatun Dam, and also on the breakwaters at either end of the Canal, but most of the material excavated from the Cut is disposed of by filling up swamps and valleys. Every dirt-train (they would call it that on the Isthmus even if it carried nothing but lumps of rock as big as grand pianos), travels an average distance of ten miles to the dumps and has the right of way over passenger trains, specials, and even mail trains. Only for the President of the United States has the line ever been cleared.

At the dumping-ground, each dirt-train is run out on a trestle, and unloaded in one of two ways. If it is composed of steel dump-cars, they are tipped up either by hand or compressed air. Most of the trains, however, are of big wooden flat-cars, raised on one side, and connected by steel flaps or "aprons," so that a heavy steel wedge, like a snow-plow, can be drawn from one end of the train to the other by a windlass and cable, thus clearing all the cars in a jiffy. (It is great fun to ride on the big wedge when they are "plowing-off.") When the dirt begins to rise above the edge of the trestle, a locomotive pushes up a machine called the "spreader," that smooths it out into a level embankment, and then another machine, the "track-shifter," picks up the ties and rails bodily, and swings them over to the edge of the new ground. Each of these machines does the work of hundreds of laborers.

Two large machine shops, now at Gorgona and