The bridge over the Illinois River at La Salle, on the Illinois Central Railroad, shows the style of bridge technically called a “deck” bridge, in which the train is on the top.  This bridge consists of eighteen spans of one hundred and sixty feet each, and cost one hundred and eighty thousand dollars.  The bridge over the Kennebec River, on the line of the Maine Central Railroad, at Augusta, Maine, is another instance of a “through” bridge.  It cost seventy-five thousand dollars, has five spans of one hundred and eighty-five feet each, and was built to replace a wooden deck bridge which was carried away by a freshet.

[Illustration:  Bridge at Augusta, Maine.]

The bridge on the Portland and Ogdensburg Railroad which crosses the Saco River is a very general type of a through railway bridge.  It consists of two spans of one hundred and eighty-five feet each, and cost twenty thousand dollars.  The New River bridge in West Virginia consists of two spans of two hundred and fifty feet each, and two others of seventy-five feet each.  Its cost was about seventy thousand dollars.

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.

[Illustration:  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.

[Illustration:  Phoenix works.]