The Lyman Viaduct, on the Connecticut Air-line Railway, at East Hampton, Connecticut, is one hundred and thirty-five feet high and eleven thousand feet long.
These specimens will show the general character of the iron bridges erected in this country. When iron was first used in constructions of this kind, cast iron was employed, but its brittleness and unreliability have led to its rejection for the main portions of bridges. Experience has also led the best iron bridge-builders of America to quite generally employ girders with parallel top and bottom members, vertical posts (except at the ends, where they are made inclined toward the centre of the span), and tie-rods inclined at nearly forty-five degrees. This form takes the least material for the required strength.
SACO BRIDGE.
The safety of a bridge depends quite as much upon the design and proportions of its details and connections as upon its general shape. The strain which will compress or extend the ties, chords and other parts can be calculated with mathematical exactness. But the strains coming upon the connections are very often indeterminate, and no mathematical formula has yet been found for them. They are like the strains which come upon the wheels, axles and moving parts of carriages, cars and machinery. Yet experience and judgment have led the best builders to a singular uniformity in their treatment of these parts. Each bridge has been an experiment, the lessons of which have been studied and turned to the best effect.
PHOENIX WORKS.
There is no doubt that iron bridges can be made perfectly safe. Their margin is greater than that of the boiler, the axles or the rail. To make them safe, European governments depend upon rigid rules, and careful inspection to see that they are carried out. In this country government inspection is not relied on with such certainty, and the spirit of our institutions leads us to depend more upon the action of self-interest and the inherent trustworthiness of mankind when indulged with freedom of action. Though at times this confidence may seem vain, and "rings" in industrial pursuits, as in politics, appear to corrupt the honesty which forms the very foundation of freedom, yet their influence is but temporary, and as soon as the best public sentiment becomes convinced of the need for their removal their influence is destroyed. Such evils are necessary incidents of our transitional movement toward an industrial, social and political organization in which the best intelligence and the most trustworthy honesty shall control these interests for the best advantage of society at large. In the mean time, the best security for the safety of iron bridges is to be found in the self-interest of the railway corporations, who certainly do not desire to waste their money or to render themselves liable to damages from the breaking of their bridges, and who consequently will employ for such constructions those whose reputation has been fairly earned, and whose character is such that reliance can be placed in the honesty of their work. Experience has given the world the knowledge needed to build bridges of iron which shall in all possible contingencies be safe, and there is no excuse for a penny-wise and pound-foolish policy when it leads to disaster.