The subject of economics in engineering may properly be termed a new one, in spite of the facts that for several decades the leading specialists, in a more or less desultory way, have given the matter some attention in their designing, and that there have been several small treatises written upon the mathematical theory of engineering economics, notably Prof. J. C. L. Fish's excellent little book which bears that title, and which every progressive engineer ought to read. Besides bridgework, the only engineering specialty that has received any attention worth mentioning in respect to the important question of economics is railroading—and that field still needs a vast amount of investigation. It is a branch of engineering which, like bridgework, readily lends itself to economic study; and it is to be hoped that ere many years the economics of railroading in all its branches will be thoroughly solved by a number of America's most able railroad men, and that they will give to the engineering profession the benefit of their experience and investigations.

As far as relates to engineering, the term "Economics" has been thus defined by the author in the "Glossary of Terms" given in "Bridge Engineering"—"The science of obtaining a desired result with the ultimate minimum expenditure of effort, money, or material." It is upon the basis of that definition that this treatise is predicated.

When determining, from the standpoint of economy, which one is the best of a number of proposed constructions or machines, there should be computed for each case the four following quantities, and their sums:

A. The annual expense for operation.

B. The average annual cost of repairs.

C. The average annual cost of renewals.

D. The annual interest on the money invested.

That one for which the sum of these items is least is the most economic of all the proposed constructions or machines; but this statement is truly correct only when the costs of operation, repairs, and renewals are averaged over a long term of years; or else for a comparatively short period of time, when the conditions in respect to wear and deterioration at the end of that period are practically the same for all cases.

The principal economic investigation which occurs in engineering practice is that of determining the financial excellence of a proposed enter-

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