pinion should have the minimum number of teeth consistent with strength and smooth running, and the rack should have as large a radius as the design and dimensions of the bridge will permit. In the case of vertical-lift bridges operated by ropes wound on drums, a considerable saving in amount of spur gearing can be made by running the operating ropes over pulleys, forming a tackle, whereby a ratio of 2, 3, 4, or even more could be obtained by the roping alone.

Much spur gearing in many cases could be eliminated by the use of worm gearing, which, with the perfection of workmanship now attained, is far more efficient than commonly supposed; and, in general, it may be stated that the efficiency of a well-designed and properly-cut worm-gear will be higher than that of its equivalent in spur gearing. It is believed that worm gearing could be introduced advantageously in many cases, in order to reduce the amount of gearing, or, on the other hand, to increase the speed-ratio and enable smaller and higher-speed motors to be employed. Incidentally, the introduction of worm gearing would eliminate the noise of the high-speed, steel spur-gearing quite commonly employed.

The modern multi-cylinder gasoline engine seems ideal for the operation of movable bridges, as it probably combines the greatest power and economy of operation with the least first cost, weight, and space; but it has not been used extensively on bridges on account of its high speed, 800 to 1200 R.P.M. or more, and the consequent large speed-reduction necessitated— also on account of the requirement for friction clutches to enable the engine to be thrown into and out of gear, because such engines must be started before beginning to move the bridge, and must be run continuously during the cycle of operations. These difficulties can be overcome by the use of a hydraulic reducing-gear attached to the engine shaft and in turn driving a worm gear. By such an arrangement the clutches are eliminated, and a speed reduction of from 40 : 1 to 100 : 1 may be obtained at once and the remaining reduction be made with a short train of gears.

The author believes the ideal apparatus would be two high-speed gasoline engines, each with hydraulic transmission gear and worm gear, the two engines together of sufficient power to operate the bridge at the maximum required speed, and one alone (by means of the hydraulic reducing gear) capable of running it at slower speed. A third engine, smaller than the others, should be used to compress air for air brakes, to work end locks and rail locks, and to circulate cooling water for the transmission gears. The third or smaller engine could be arranged to be thrown into gear and used as an emergency engine, in the event of complete break-down of the main engines, so as to operate the bridge at very slow speed; and it could also be utilized for driving a small dynamo for signal and other lighting. Thus all the functions of the bridge could be performed by means of gas engines, and be made entirely independent of outside sources of power supply. The installation  and  care  of  transmission  lines  and  transformers  would  be  en-tirely  avoided;   and,  obviously,  the  arrangement  proposed  would  be  of