largely depends upon the production of that simple and important element in railroading, its rail.[1] [1] Mr. Dudley’s rails, and those of other designers, are fully illustrated and discussed in “Railway Track and Track Work,” by E. E. Russell Tratman. Second edition. New York, Engineering News Publishing Co. [Illustration: Dudley rails.] [Illustration: Steel cross-ties and rails.--Carnegie Steel Co., Pittsburg.] In Ninth Street, Pittsburg, the rails of the traction line are for some distance carried on steel ties similar in form, as here shown. CHAPTER III FORM--_Continued_. BRIDGES Roofs and small bridges may be built much alike . . . The queen-post truss, adapted for bridges in the sixteenth century, was neglected for two hundred years and more . . . A truss bridge replaces the Victoria Tubular Bridge . . . Cantilever spans at Niagara and Quebec . . . Suspension bridges at New York . . . The bowstring design is an arch disguised . . . Why bridges are built with a slight upward curve . . . How bridges are fastened together in America and England. [Illustration: King-post truss. AK, king-post.] Roofs and Bridges Much Alike. Rails are girders used by themselves: girders are often combined in trusses; of these much the largest and most important are employed for bridges. There is now under construction near Quebec a cantilever bridge whose channel span of 1,800 feet will be the longest in the world. See page 29. It will take us a little while to understand how so bold a flight as this was ever dared. We will begin with a glance at a truss of the simplest sort, such as we may find beneath the roof of an old-fashioned barn. A pair of rafters, AB and AC, are inclined to each other at an obtuse angle, and are fastened to the horizontal beam, BC, at B and C. Their apex, A, is joined to BC by the king-post, AK, which binds the three strongly and firmly. This whole structure makes up a triangle, and so does each of its halves, ABK and AKC. No other shape built of straight pieces will keep its form under strain. Take in proof say four pieces of lath and unite them with a freely turning pin at each corner to make the frame, ABCD; it is easily distorted by a slight pull or push; but insert cross-pieces, AC and BD so as to divide the square into triangles, and at once the frame resists any strain not severe enough to break the wood or crush its fastenings. As the roof presses down the frame ABC, its sides, AB and AC, tend to slide away at their lower ends, B and C, but this is prevented by the horizontal beam, BC, which while it holds them in place is itself so stretched as to be held level and straight. This calling into play of tension constitutes the chief merit of the truss, and enables it in roofs and bridges to span breadths impossible to simple beams bending downward under compressive strains. Not only in houses, but in ships, the truss has great value; it