But perhaps the comparison that will best illustrate the immensity of the task of digging the ditch is that of the big Lidgerwood dirt car, on which so much of the spoil has been hauled away. Each car holds about 20 cubic yards of dirt, and 21 cars make a train. The material removed from the canal would fill a string of these cars reaching about three and a half times around the earth, and it would take a string of Panama Railroad engines reaching almost from New York to Honolulu to move them. Yet all these comparisons have taken account of the excavations only. The construction of the Panama Canal represents much besides digging a ditch, for there were some immense structures to erect. Principal among these, so far as magnitude is concerned, was the Gatun Dam, that big ridge of earth a mile and a half long, half a mile thick at the base, and 105 feet high. It contains some 21,000,000 cubic yards of material, enough to build more than 500 solid shafts like the Washington Monument. Then there was the dam at Pedro Miguel--"Peter Magill," as the irreverent boys of Panama christened it--and another at Miraflores, each of them small in comparison with the great embankment at Gatun, but together containing as much material as 70 solid shafts like our Washington Monument. Besides these structures there still remain the locks and spillways, with their four and a half million cubic yards of concrete and their hundreds and thousands of tons of steel. With all these astonishing comparisons in mind, is it strange that the digging of the Panama Canal is the world's greatest engineering project? Are they not enough to stamp it as the greatest single achievement in human history? Yet even they, pregnant of meaning as they are, fail to reveal the full and true proportions of the work of our illustrious army of canal diggers. They tell nothing of the difficulties which were overcome--difficulties before which the bravest spirit might have quailed.