When compressed air is admitted through passage 1 and valve 2 to the left end of cylinder 3, pistons 4, rigidly attached together, are moved to the right. At this, rack a turns segment gear 5 and its shaft A on which blade 6 is rigidly mounted. This also turns blade 7, linked to blade 6, about fixed axis B. Air from the right end of cylinder 3 is exhausted through passages 8 and 10 and valve 11 to the atmosphere. As piston 4 travels to the right, sleeve 12 shifts the shank of link 13 which is attached to crosspiece 14, joining valve members 2 and 11. This moves the valve members to the right. Now, compressed air admitted through passage 1 flows through nipple 9 and passage 8 to the right end of cylinder 3, swinging blades 6 and 7 in the opposite direction. Air from the left end of the cylinder is exhausted through valve 11 to the atmosphere.