force it so as to make it carry satisfactorily heavier loads than those for which it was proportioned. In fact, the character of detailing employed previous to the nineties, in which decade the science of bridge design began to be established, was so outrageously unscientific as to lead the author to suggest the axiom that "the best way to repair an old bridge is to throw it into the scrap heap and build a new one."

Quite often bridges are repaired, which, from the standpoint of true economy, should be relegated to the discard. One such case of some importance occurred in the author's practice in the late eighties. It was an old Post Truss bridge across the Missouri River at Fort Leavenworth, Kansas, which had been seriously injured by the burning of a large portion of the wooden floor. The total cost of the repairs was somewhat in excess of one hundred thousand dollars—an amount greater than the sum total of all the subsequent incomes from both railway and highway traffic. In extenuation of his action in repairing this structure, the author might mention the fact that he was not consulted about the economics of the case or the advisability of repairing, but was simply given the job of engineering the reconstruction of the damaged structure. However, he is not sure that, at that stage of his career and in those days of primitive bridge design and construction, his judgment was far enough developed to enable him to come to a truly economic decision, had the problem of the economics of the case been submitted to him.

The method of solving such a problem is to estimate upon a liberal basis the probable cost of the repairs, and upon a conservative basis the probable duration of life of the repaired structure, also the probable costs of an entirely new bridge both at the date of consideration and at the expiration of the said life. If the latter cost, plus the cost of the repairs with compound interest thereon up to the time of the renewal, plus the net cost of removal of old structure (i.e., cost of the work less scrap value) is smaller than the net cost of immediate removal, plus the cost of a new structure, built immediately, plus compound interest on the sum of these two costs at the assumed later date, plus the small value of the deterioration of the new structure in the interval between the said two dates, then the repairs will be warranted.

It is evident that the correct determination of the answer to any such economic question demands wide experience, sound judgment, correct vision, and a practical acquaintance with the theory of economics.

In the old days a large portion of the work of bridge examination and repairing fell to the lot of the consulting engineers; but such now is far from being the case, because it is only for very large and important structures, or those having movable spans of a complicated character, that the independent specialists are retained on repairs and reconstruction. Such work is ordinarily done by the bridge engineers regularly employed by the railroads, the states, and the municipalities; and these men have become exceedingly expert therein.