Abstract: A water-driven monitor is provided as a stand-by pre-aimed fire fighting device installed at a fixed location for fire protection of a structure or equipment, such as aircraft, in a hanger structure. The monitor is oscillated by providing a diversion of a minor flow of the water or water/foam to be discharged from the monitor, to reciprocate an annular piston in an annular cylinder in up and down directions. A pair of concentric barrel cams having helical cam tracks therein and a cam ring with attached cam rollers interconnect a fixed base assembly containing the piston and cylinder and a water or water/foam discharge assembly, such that linear piston motion includes oscillatory motion which is transferred to the discharge assembly. A four-way spool valve controls fluid flow to opposite sides of a piston head and a toggle action shuttle actuates the valve spool to reverse the piston movement and the direction of discharge tube oscillation.