BRIDGES OF TIMBER, THEN STONE, THEN STEEL--THE STARUCCA VIADUCT--THE FIRST IRON BRIDGE IN THE U. S.--STEEL BRIDGES--ENGINEERING TRIUMPHS--DIFFERENT TYPES OF RAILROAD BRIDGE--THE DECK SPAN AND THE TRUSS SPAN--SUSPENSION BRIDGES--CANTILEVER BRIDGES--REACHING THE SOLID ROCK WITH CAISSONS--THE WORK OF "SAND-HOGS"--THE CANTILEVER OVER THE PEND OREILLE RIVER--VARIETY OF PROBLEMS IN BRIDGE-BUILDING--POINTS IN FAVOR OF THE STONE BRIDGE--BRIDGES OVER THE KEYS OF FLORIDA. When the habitations of man first began to multiply upon the banks of the water courses, the profession of the bridge-builder was born. The first bridge was probably a felled tree spanning some modest brook. But from that first bridge came a magnificent development. Bridge-building became an art and a science. Men wrought gigantic structures in stone, long-arched viaducts, with which they defied time. Then for two thousand years the profession of the bridge-builder stood absolutely still. With the coming of the iron and steel age it moved forward again. The development of a fibre of great strength and without the dead weight of granite gave engineers new possibilities. They began in simple fashion, and then they developed once again, with marvellous strides. Steel, the dead thing with a living muscle, could span waterways from which stone shrank. Steel redrew the maps of nations. Proud rivers at which the paths of man had halted, were conquered for the first time. Routes of traffic of every sort were simplified; the railroad made new progress; and economic saving of millions of dollars was made to this gray old world. The earliest of the very distinguished list of American bridge-builders erected great timber structures for the highroads and the post-roads. Some of them went back many centuries and came to the stone bridge, in many ways the most wonderful of all the artifices by which man conquers the obstructive power of a running stream. But the building of stone bridges took time and money, and time and money were little known factors in a new land that had begun to expand rapidly. So at first the railroad followed the course of the highroad and the post-road, and took the timber bridge unto itself. In some cases it actually fastened itself upon the highroad bridge, as at Trenton, N. J., where a faithful wooden structure built by Theodore Burr in 1803 was strengthened and widened in 1848 to take the first through railroad route from New York. It continued its heavy dual work until 1875 when it was superseded by a steel bridge. A dozen years ago the railroad tracks were moved from that structure to a magnificent and permanent stone-arch built near-by. Thus the railroad crossing the Delaware at Trenton has, in this way, typified step by step every stage of the development of American bridge-building.