out regard to the actual amount of current consumed; and, should such a case arise, other forms of power should be carefully considered.

When electricity is purchasable, the consumer is usually confined to the kind of current the producing company will supply, and, therefore, has no choice between alternating and direct current; but whenever such a choice exists, direct current should have the preference, as giving much greater latitude in motor speeds, and, consequently, tending to economy in cost of gearing, and, more important still, as giving a much more flexible and perfect control, especially when shunt-wound fields are used, providing automatic speed-control under positive and negative loadings.

The more usual sizes of movable bridges, requiring motors of from 30 to 100 H.P., when located adjacent to electric plants of reasonable size, may, preferably, be operated directly from the line, if the current be of low tension, or through transformers when it is of high tension; but cases may arise, especially with bridges of very large size, where the power station or the transmission lines cannot stand the starting current or peak load required. In such cases some form of accumulator must be used to extend the current draught over a longer time, so that the motors may be relatively small and within the capacity of the power supply. Electric accumulators (storage batteries) may be used, and the apparatus may be entirely electrical; but such cases, in the author's opinion, offer an excellent opportunity for the use of hydraulic power for the direct movement of the bridge, the primary electric power being employed to produce and store hydraulic pressure in suitable accumulators. This system may be employed also in cases where gasoline engines are desired and where engines of sufficient size to operate the bridge directly would be inconvenient or impracticable; as comparatively small engines could be used to store hydraulic power or compressed air in suitable accumulators.

The conveyance of power from the driving motor or engine to the bridge itself is usually accomplished by gearing, the direct connection to the bridge being by means of gear teeth in the form of strutted racks pivoted to the bridge, or by means of curved segments, forming a gear wheel of large radius bo'ted directly to the bridge girders; and, in the case of vertical-lift bridges, by means of ropes connected to the towers and wound upon drums carried on the moving span.

In all of these cases, as the movement of the span is slow in comparison with the speed of the motor or engine, long trains of spur gearing are necessary; and the motor or engine should be of as slow speed as practicable in order to reduce the amount of gearing to a minimum.

Thus a bascule bridge, operating through about 90° in one and a half minutes, moves at a speed approximately equivalent to 1/6 of a revolution per minute; and to gear this to a motor running at, say, 600 R.P.M. means a total speed reduction of 3600:1. For that reason, the primary reduction directly at the moving span should be as great as practicable, i.e., the main