permanent magnet assemblies 45 and 46 to effect closure of the gate valve 11 in operation to close the gate valve 11 the pneumatic actuator 34 is actuated in a manner as previously described with regard to the embodiment of fig1 and 2 to cause the slide block 75 to slide longitudinally of the central cylinder 36 and guide rods 37 and 38 toward the closed position as shown in fig3 and 4 a stop 98 is disposed at the end of the housing 12 projecting inwardly thereof when the carrier plate 61 engages the stop 98 the compression springs 77 are compressed and the yokes 78 and 79 continue to advance while actuating the rollnut assemblies 84 to produce rotation of the permanent magnets 54 within the permanent magnet assemblies 45 and 46 thereby producing an increasing magnetic closure force between the gate plate 25 and the valve seat 14 this magnetic force ultimately produces compression and thus gas tight sealing of the compression ring 32 against the valve seat 15 when it is desired to open the gate valve 11 the pneumatic forces are reversed in the pneumatic actuator 34 to cause the slide block 75 to be retracted on the guide rods 37 38 and central cylinder 36 when the slide block 75 has been retracted sufficiently to engage the stops 76 on the rods 69 and 71 further retractive movement of the slide block 75 is coupled to the carrier plate 61 for.