detector means 42 situated in the wedge shaped space pressure detector means 42 senses the pressure in the space s and generates a signal indicative thereof which is received by an adjustment or control unit 43 which in turn sends a signal to the regulating device 41 in this manner the pressure level in the space s can be maintained at a desired level through the adjustment of the position of the blow box 21 the pressure indicative signal received by the control unit 43 may be compared to a set point value input into the control unit 43 in a conventional manner in addition to or in lieu of the positional adjustment described above the pressure level in the wedge shape space s can be adjusted through the adjustment of the pressure level of the air supplied to the blow box 21 referring to fig3 a control valve 45 is positioned in the line between the source of compressed air 46 and the blow box 21 for adjusting the pressure level of the air supplied to the blow box the control valve 45 receives a signal from the control unit 43 to adjust the pressure level of the air supplied to the blow box by adjusting the pressure level of the supplied air the speed of the air jets a 1 discharged from the blow box 21 can be.