controlled valve 64 a pressure switch 65 which closes on being subjected to hydraulic pressure is connected to pipe 17 a low pressure hydraulic accumulator 66 is connected to the pipe joining valves 61 and 62 the control box 19 contains an assembly of electronic components as indicated in fig6 principally a 555 timer module 67 and a transistor 68 referring to fig1 and 6 the system operates as follows when hydraulic fluid under pressure is not being fed through pipe 17 the switch 65 is open the solenoid valve 64 connects the pilot gallery to the pipe 18 and the rams 4 are connected through valves 61 and 62 to pipe 18 the pistons in rams 4 are in the lowered position and weight 1 is at rest on the head of the chisel 3 its rest position thereby positioning magnet 14 adjacent detector 14a which then sends a signal to the control box 19 to close a trigger switch to energise the control circuit switch 65 is actuated by the pressure in pipe 17 initially causing the transistor 68 to conduct and actuate the solenoid in valve 64 valve 61 closes valve 60 opens and valve 63 conducts hydraulic fluid to maintain a set maximum pressure in pipe 17 the fluid under pressure in pipe 17 passes through valve 60 into feed pipes 15 to the rams 4 this causes the rams to raise weight 1 when the magnet 13 reaches detector 13a as the weight rises detector 13a generates a signal resetting the 555 timer module 67 in control box 19 thereby de energizing the solenoid in valve 64 this closes valve 60 and opens valves 61 and 63 cutting off the feed of high pressure fluid to the rams and connecting the rams to.