MACHINERY FOR TURNING THE SPAN AND LIFTING
THE ENDS OF SAME

POWER.

When a draw-span is to be opened frequently, some kind of mechanical power must be used. The kind of power best adapted to any particular span depends upon a number of conditions, more especially the location of the bridge.

A gasoline-engine is an economic and convenient form of power for small spans which do not require more than twelve (12) or fifteen (15) horse-power to operate.

Duplicate electric motors, where direct connections can be made with electric-light or street-railway power-plants, are very efficient, convenient, and reliable; but in no case is it safe to depend upon storage-batteries for power. The use of electric motive power is therefore confined to bridges located in or near towns or cities.

Where over twelve (12) or fifteen (15) horse-power is required for operating the spans, and where electrical connections cannot be made, the steam-engine is the best form of power to use, except possibly in some special cases where water-power can be had conveniently.

Except in the case of short, light drawbridges, whenever mechanical power is employed it is necessary to apply the same to the rack by two pinions located diametrically opposite each other. If with this arrangement the tooth-pressure be still too high, it will be necessary to replace each pinion by a pair of pinions located as close together as practicable. With pinions located far apart some kind of an equalizer must be employed to divide the work equally between them, on account of the unavoidable, slight irregularities in the toothspacing of the entire rack. When electrical power is adopted, the equalizing may be done by means of electrical connections between the duplicate motors;

but with any other power a mechanical equalizer between the two radial shafts must be employed.  There will be no equalizing needed between the