four spirally grooved drums connected by a train of gears to an electric motor. Each of these drums controls the operating ropes for one corner of the span; one pair of operating ropes, leading from the top of the drum, passes over a deflecting sheave at the corner of the span, thence downward, and connects near the bottom of the tower; and a corresponding pair of operating ropes leading from the bottom of the drum passes under a deflecting sheave at the corner of the span, thence upward, and connects near the top of the tower. All of the drums are similarly connected so that when they are operated in one direction the ropes leading to the tops of the towers are wound on the drums, and those leading to the bottoms of the towers are payed ofl', lifting the span by the force exerted upon the corner sheaves. The machinery for operation is thus exceedingly simple, consisting of one transverse shaft extending across the span; one pair of reduction gears in a frame inside of the house, and the back gears upon the motor. An indicator and limit switch is provided by which the current is automatically cut ofi' and solenoid brakes applied to the motors at near the upper and near the lower limit of the run of the span. For ordinary operation there is a hand band-brake. which is equipped with a push button switch, so that when the span is under control the operator can hold off the emergency brake, and bring the span to rest in the control of the hand brake.
To provide for emergency operation, instead of the usual hand operating capstan, the span is equipped with a 12-horsepower gasoline engine connected through a speed reducer to the machinery, so that in case of failure of electric current the operator will be able to move the span at slow speed without having to seek outside assistance. There is a manually operated lock at each end of the span in connection with a centering device so that the span may be locked down. The solenoid brakes in connection with the down haul ropes also serve as a lock. Bussbar trolleys are attached to one tower and a sliding contact shoe on the lift span supplies the current to the motors during operation. The operating ropes are attached at the tops of the towers to small drums with worm gear connections so that adjustment for the stretch of the operating ropes can readily be made. When the span is at its upper limit the counterweights are two feet above the floor level and a special arrangement of the trolley wires under the counterweight is made. This consists of swinging frames upon which the trolley wires are fastened, and which are pushed down by the counterweights and
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