A large audience, composed mostly of students, was present at the second of the course of lectures given last night at the Jefferson Physical Laboratory. Professor W. S. Chaplin, of the Scientific School, spoke on the subject of "Bridge Building." The lecturer described the materials and explained the principles of bridge construction, illustrating his remarks by means of wooden models. The materials most used in building bridges are wood, iron and steel. Wood, because of its great bulkiness and comparative weakness, has been almost wholly superseded by iron and steel. Steel is better than iron because equal strength can be gained with less weight and bulk and greater durability. It seems probable that steel will be the best material for building bridges in the future. The simplest form of a bridge is a beam supported at the ends. This form is suitable only for very short spans, for the load is transverse and the beam tends to break in the middle of its own weight. Strength is obtained by bracing together two or more systems of trusses. These trusses are made of beams bolted together in triangles, so that the load is distributed into forces acting longitudinally on the beams, either by tension or by compression. To build a bridge of this kind, a scaffolding is necessary to support the materials while being put together. When it is impracticable to build a scaffolding on account of the currents, the bridge is built from each end so as to meet in the middle. The ends are balanced by piers near the shore and anchored firmly on the shore. The most common form of bridge in America is the straight Warren girder bridge. In Europe, the single bow-string bridge, arched on one side, and the double bow-string bridge, arched both top and bottom, are more common. The lecturer concluded by showing photographs of a large number of bridges, including those at Brooklyn, Harlem, Poughkeepsie and Niagara, besides several in Scotland and France.