always available; and, when purchasable at reasonable meter-rate, it is usually the most convenient and economic type of energy to adopt. Formerly, electricity not being available, steam was invariably used, and boilers were installed on or near the bridge. This involved the handling of coal, disposition of ashes, and continuous maintenance of steam pressure; and it was usually found that the fuel expense was nearly constant and almost entirely independent of the number of movements of the bridge. The author knows of bridges where steam is maintained during long periods of inactivity, and during periods of practically closed navigation, thus rendering the operation excessively uneconomic. It is evident that a form of power which may be paid for only as usefully expended is of true economic value. In some large cities of Europe hydraulic pressure or compressed air is purchasable by meter; and bridges erected within reach of such pressure systems can most advantageously be operated thereby, as these types of power give a smooth and perfect control attainable in no other way. In the United States, however, we are confined practically to two sources of power other than steam, viz., electric current and the internal combustion engine; and as electricity is quite generally available, it is most commonly used, but few instances existing where internal combustion engines are employed alone as the primary motive power, though quite often as auxiliaries for emergency use.

With the great improvement reached in the design and construction of the high-speed, multi-cylinder engine of the present day, there seems to be no good reason for its limited employment in the operation of bridges. In many cases the cost of transmission lines, transformers, and other paraphernalia necessary to convey electric current to the bridge site, would exceed the cost of such engines, while the fuel for them is obtainable at almost any cross-roads, consequently true economics demands their more extensive use.

Efficiency, as commonly understood, i.e., ratio of power output to power input, rarely needs consideration in the economics of movable bridges, as the time of motion is generally not much over a minute, or at most two minutes, in one direction,—say on an average three minutes per cycle; and comparatively few bridges are called upon to open more than 10 or 20 times per day on an average throughout the year. Thus the total yearly working time of a fairly-active bridge will ordinarily amount to less than 75 actual days of motion, the power consumed being mainly that required for starting and stopping; and, therefore, efficiency as above defined is of but little importance. True economics demands the selection of power and machinery from the standpoint of as great simplicity and low first cost as is consistent with robustness and durability, and especially with positiveness and ease of control.

In the earlier days of electric supply (and this condition may possibly apply in a few localities today) it was customary to charge a flat monthly or yearly rate for current, based on the peak load of the motors used, with-