amount of hydraulic fluid in the hydraulic circuit at a substantially constant level inter alia to prevent the free pistons in the accumulators from hitting the bottoms of the accumulators also as shown in fig7 and 8 the system can be simplified even further by omitting the high pressure accumulator and the corresponding valve in systems comprising a gas spring 10 the system can be operated merely by means of the valve 7 b between the hydraulic cylinder 3 and the low pressure accumulator 6 when this valve 7 b is closed the pressure line 14 of the pump 11 communicates with the cylinder space beneath the piston 4 and the piston 4 and impact weight 2 are lifted by the hydraulic medium against the action of the gas spring 10 when the valve 7 b is open the hydraulic medium is withdrawn from beneath the piston 4 by the underpressure in the return accumulator and the suction line of the pump 11 i e the hydraulic medium is circulated through the system by the pump and the impact weight is accelerated by the gas spring if the system is at a sufficient depth e g at depths greater than 500 meters preferably greater than 1000 meters the gas spring can also be omitted by establishing fluid communication between the cylinder space above the piston and the surroundings e g by a hydraulic cylinder that is open at one end in hydraulically operated systems shown in fig8 in a first position of the valve in this example a 3 2 valve 7 the low pressure accumulator 6 and the suction line of the pump 11 communicate with the cylinder space beneath the piston 4 but the cylinder space above the piston 4 communicates with the pressure.