70 that is biased by a spring 72 against a sealing ring 74 so that the interior chamber 61 of the cylinder 60 is normally sealed with sufficient biasing force that there is no leakage of water from the chamber 61 under the action of normal water pressure of the water source the cylinder 60 is provided with coaxial extension 76 with an interior bore 77 that extends beyond the valve 70 and includes an outlet port 78 a source of air pressure is connected to communicate with and provide air to the bore 77 at 80 as illustrated air introduced at 80 flows through the bore 77 and exits at the outlet port 78 which port is connected to one of the water spray conduits 30 30 39 fig2 air for control of the air cylinder and piston 68 is introduced at two ports 82 and 84 a suitable conventional control valve 86 receives air under pressure from a suitable source as illustrated and selectively applies the compressed air to one or the other of the ports 82 and 84 as conventionally directed by a signal supplied electrically or hydraulically at control line 88 it will be appreciated that introduction of air through the port 82 will cause the piston 64 to be moved to its extended position while the directing of air through port 84 will cause the piston 64 to be retracted in this latter connection an adjustable stop 90 is provided with a threaded barrel 92 to adjustably control the extent of retraction of the piston 64 and thus control the size of.