Page:Popular Science Monthly Volume 36.djvu/496

480 are built in this country or in Europe; and in this way, as in many others, the Japanese show that two or three hundred years ago they had advanced to a wonderful degree in the study of applied mechanics and the "strength and resistance of material." The only trouble is, that they stopped advancing for two or three hundred years, and up to ten or twenty years ago were not as far ahead as two hundred years earlier.

In closing, I wish to call your attention for a few moments to some of the differences that exist between the American and English practice of bridge-building, and the causes that have led to these differences. The characteristic difference is in the methods used in joining together the different parts of the bridge. American bridges, as a class, are pin-connected—that is, the different members, when possible, are joined by means of a steel pin passing through holes in the ends of the pieces. These joints are perfectly flexible, and each member is designed to do its own particular work. English bridges, as a class, have "riveted connections"—that is, the members are fastened rigidly together, and each member is designed to act simply as a part of a rigid, inflexible whole.

The causes that have led to this difference are as follow: In the construction of bridges the English engineer started with the flanged girder of cast or rolled iron, or some other form of a stiff beam, and as the bridges increased in size so as to necessitate the framing of a truss, his whole effort was directed toward making that truss as nearly similar to the original flanged or box girder as possible. This led to perfect rigidity at the joints.

The American engineer, on the other hand, had very little or no iron and steel to work with, and of necessity used wood. As the necessary bridges were of considerable span, the only possible solution of the problem was the pinning together of small pieces of wood so as to form a connected series of triangles. The joints in wood could not easily have been made rigid, and it was not desirable that they should be, as the strength of wood is very slight when the strain is applied in any direction other than in the direction of the fibers of the piece, and the use of the pin joint, theoretically at least, insures this line of action. There has been much ingenuity displayed by our engineers, in the years gone by, in the combinations of triangles used in bridge-designing, and in many cases this has led to absurdities. The whole tendency, however, at present in American practice is to extend the use of riveted joints, and in English practice to extend the use of the "pin connections." Both are working in opposite directions, but from opposite sides, and therefore toward the same point.

One great drawback to the more general use of pin connections by English engineers is the immense first cost of the plant