It was safely estimated that there were, in 1888, 208,749 bridges of all kinds, amounting in length to 3,213 miles, in the United States.[4] The wooden bridge and the wooden trestle are purely American products, although they were invented by Leonardo da Vinci in the sixteenth century. From the above statistics it will be seen how much our American railways owe to them, for without them over 150,000 miles could never have been built. The art of building wooden truss-bridges was developed by Burr & Wernwag, two Pennsylvania carpenters, some of whose works are still in use after eighty years of faithful duty (p. 28). A bridge built by Wernwag across the Delaware in 1803 was used as a highway bridge for forty-five years, was then strengthened and used as a railway bridge for twenty-seven years more, and was finally superseded by the present iron bridge in 1875. These old bridge-builders were very particular about the quality of their timber, and never put any into a bridge less than two years old. But when we began to build railways, everything was done in a hurry, and nobody could wait for seasoned timber. This led to the invention of the Howe truss, by the engineer of that name, which had the advantage of being adjustable with screws and nuts, so that the shrinkage could be taken up, and which had its parts connected in such a way that they were able to bear the heavy concentrated weight of locomotives without crushing. This bridge was used on all railways, new and old, from 1840 to about 1870. Had it been free from liability to decay and burn up, we should probably not be building iron and steel bridges now, except for long spans of over 200 feet; and as the table opposite shows, the largest number of our spans are less than 100 feet long. The Howe truss forms an excellent bridge, and is still used in the West on new roads, with the intention of substituting iron trusses after the roads are opened. After 1870, the weights both of locomotives and other rolling stock began to be increased very rapidly. This, together with the development of the manufacture of iron, and especially the invention of rolled beams and of eye-bars, gave a great impetus to the construction of iron bridges. At first cast-iron was used for the compression members, but the development of the rolling-mill soon enabled us to make all parts of rolled iron sections at no greater cost, and rolled iron, being a less uncertain material, has replaced cast-iron entirely. Iron bridges came in direct competition with the less costly Howe truss, and during the first decade of their construction every attempt was made to build them with as few pounds of iron as would meet the strains. [Illustration: Old Burr Wooden Bridge.]