position and possibly injure the operator the fact that the controller 54 is programmed to initially pressurize the space above the piston before the valve 51 is energized to start the downward movement of the hood thus serves to insure that such a rapid closure will not occur even when the hood has been manually lifted to the open position after the momentary delay the valve 50 is then de energized so that it shifts to its venting position and the valve 51 is moved to its energized position as shown in fig3 b such that the air is directed into the line 46 and into the cylinder 34 below the piston 38 at the same time the charge of air above the piston is vented through the bleed valve 42 at a rate controlled by the setting of the threaded plug 43 preferably the valve 51 is held in the energized position for a short period of time such as about 8 seconds and so that the hood 16 is caused to close to about two thirds of its travel after reaching that point the air line valve 51 is de energized by the controller 54 which allows the hood to continue to close due to gravity with the charge of air above the piston 38 venting at a controlled rate through the bleed off valve 42 this sequence eliminates the possibility that the operator 39 s hand could be caught in the u201c pinch point u201d along the forward edge of the hood and with the forward edge being biased downwardly by pressurized air below the piston as noted above many.