Whether spans are long or short, engineers are fairly well agreed as to the best proportions for girders and panels. They consider that a girder should have about one-twelfth to one-tenth as much depth as span; and that the weight of a web should be about equal to that of its flanges. They usually give panels twice as much depth as length, with a tendency to increase the proportion of depth to length, in order to minimize the deflections and oscillations which shorten the life of a structure. For definite lengths of span, particular types of construction are preferred; usually for lengths of from 20 to 125 feet, plate girders are chosen; for spans of 125 to 150 feet riveted lattice trusses are built; for spans of 150 to 600 feet pin-connected trusses are employed. Here we reach the economical limit of a length for simple trusses; beyond 600 feet the engineer is obliged to have recourse either to a cantilever or a suspension bridge. [Illustration: Part of lattice girder bridge, showing rivets.] Whatever the breadth of the stream or the chasm over which he is to build a roadway, each case must be studied in the light of its special circumstances. There must be due regard to business as well as to engineering considerations; the designer will bear in mind that types of parts customarily turned out at great steel works are procurable in less time, and at less cost, than novel types requiring to be manufactured to order. Then, in speed of construction, he will remember that a pin-connected bridge can be built much faster than a riveted structure. Furthermore, every part must be vastly stronger than ordinary duty requires. Tempests and floods may suddenly arise; at any instant a derailment or a collision may create a strain of the utmost severity; and even under ordinary circumstances it must not be forgotten that train loads grow constantly heavier because economy lies that way. [Illustration: Upper shelf, unladen, has upward curve or camber. Lower similar shelf is straightened by its load.] One detail of bridge design is worth a moment’s attention. When a book-shelf is a thin board, quite straight as manufactured, it sags in the middle when fully burdened. This downward dip may be avoided by making the shelf at first with a slight curve which brings the middle a little higher than the ends. In bridge building a like curve, or camber, is given to each span so that when fully loaded it will be level or nearly so. In a span of 500 feet it is found that a rise of half a foot at the centre is sufficient. In suspension bridges, for the sake of strengthening the structure, the camber far exceeds this ratio. [Illustration: Pin connecting parts of a bridge.]