Abstract: A hydraulic device driven by fluid flowing by the force of gravity provides a rotary driving force suitable for operating a fluid pump or a flywheel. A reciprocating balance beam is pivotally mounted and has a counterweight assembly affixed to one end and a bucket assembly affixed at the other. Upon filling the bucket with fluid, by gravity flow, the counterweight assembly is raised thereby lowering the bucket assembly until it reaches an end limit of travel, whereupon a plunger assembly immediately releases substantially all of the fluid from the bucket within a very short period of time, thereby permitting the counterweight to descend quickly to its initial position. Rods pivotally connected to the balance beam are thereby caused to reciprocate vertically. The rods can drive fluid pumping pistons or can drive a rack and pinion arrangement which cooperates with a ratchet assembly to provide continuous rotary drive input to a flywheel assembly.