In tracing the history of the arch it is interesting to note the influence which the choice of building material available at any period has had on the shape, dimensions and general appearance of the arch, and it affords food for thought that, in certain respects, the pendulum has swung back to the solid type of arch with which the first beginning was made. The only materials at the disposal of the first arch builders were masonry or brickwork, and the only form of arch possible with brick or stone joined with mortar is a solid arch of such dimensions that, the thickness at any point is sufficient to keep the stress line within the middle third, the material being unable to take tensile stresses. This necessary condition naturally makes the dead load of these arches a considerable factor and imposes a limit on the magnitude of the span it is possible to use; for small spans this form of arch still holds its own, as evidenced by the every day use of brick or masonry arches for bridges or other constructional purposes. The solid arch held its sway undisputed until the end of the 18th century when Cast Iron made its appearance as bridge material. Some of the engineers dealing with solid arches at that time went so far as to design spans of 200 ft. to 500 ft. (Peronnet), and a bridge of his design over the Seine of 300 ft,. span would have been executed but for a competitive scheme in Cast Iron. The comparatively light and elegant appearance of arches made in this material, combined with the greater latitude in span dimensions, due to the reduction of dead weight, gave rise to an extensive development in bridge consbruction with Cast Iron arches, of the type known so well to most engineers, with ribs and lattice work of a very pleasing description. Both here and in France there are quite a number of these arches still in existence as evidence of the flourishing period of Cast Iron; the St. Louis bridge in Paris with fixed arches of 210 ft. span and 19 ft. rise was built in 1862 when the more modern material Wrot Iron had already begun to make its presence felt within the field of bridge building. While Cast Iron is an excellent compression material and is able to stand tension to a small degree, the introduction of Wrot Iron and Steel in rolled sections gave the bridge builder a material equally good in compression and tension, and therefore ideal for constructional purposes; the requirements in regard to the stress line no longer imposed any limits as to shape, while the lightness of the structure made it possible to increase the span to dimensions undreamed of with Masonry or Cast Iron. The advent of another material that will stand both tension and compression, reinforced Concrete, to a certain extenturned the development towards the solid arch, and gave an impulse to the revival of this type of arch which has set its deep and indelible mark on the bridge building of the present time.