be in the line of true economy to be somewhat extravagant in the cost of superstructure so as to avoid all possibility of disaster during erection.

In this connection it should be noted that it will sometimes be more economic to build a light structure at the outset and later remove it and replace it by a heavier one, rather than to build the heavier one at first, provided that the light structure will serve the purpose adequately for a number of years, and that the replacement can be done without too serious an interruption of traffic. For instance, it might be cheaper to build a $50,000 bridge now and replace it in fifteen years by a bridge costing $100,000 than to build the $100,000 structure at first; for the compound interest on the $50,000 additional investment for fifteen years at six per-cent amounts to $70,000. It will rarely pay to anticipate traffic requirements by more than twenty or thirty years, in case that thus anticipating them involves a large additional expenditure.

In respect to the economics of design and construction of bridges, the following general suggestions are pertinent:

Anticipating the Future

In all engineering work of both designing and construction, true economy necessitates a thorough consideration of future requirements and possible eventualities, also a provision for meeting the same. For instance,in designing a structure one should consider possible future additions of loading and how to accommodate them; and in construction one should anticipate delays, floods, storms, and other possible difficulties, and should prepare his programme so as to meet them effectively and without any unnecessary expenditure of time, labor, or money. Foresight of this kind is an important element of success in the career of every engineer.

First Cost

It must not be forgotten that under the item "First Cost" must be considered all items of expense, of every kind whatsoever, that may be incurred before the structure is ready for operation. It should include not merely the ordinary estimated cost of construction, or the amount of money to be paid to the contractor, but also promotion expenses, discount due to sale of bonds below par, commissions or other considerations for services rendered, interest on money during construction, right-of-way, rents and minor incidental expenses during construction, administration, legal expenses, engineering, and a proper contingency allowance.

Compensating Factors in Economic Comparisons and Frequent
Wide Range of Economic Limits

In the making of economic studies, assumed variations in proportions or types will nearly always increase the cost of certain factors and reduce that