and locked position respectively turning to fig5 the control arm 26 may be moved in an unlocking direction u until a snap ring 46 provided on one end of the control arm 26 contacts the side of the main bracket 22 the control arm 26 is in an unlocked position whenever the enlarged bushing 30 ceases to block the seat plate side wall 21 a referencing fig6 the control arm 26 may be moved in a locking direction l until snap ring 46 contacts the side wall 21 b of the seat plate 20 as aforementioned in the locking position the spring latch 38 is engaged by the notch 42 in the control arm 26 fig2 as will be apparent to a person skilled in the art retainers other than the snap ring 46 may be used for the purpose of limiting the axial sliding of the control arm 26 such as a cotter pin a hitch pin a woodruff key or a lynch pin in operation a user may rotate the control arm 26 to depress the gas cylinder actuator in order to change the height of the chair when the user of the chair 10 slides the control arm 26 from the unlocked position to the locked position the spring latch 38 pops into engagement with the arm notch 42 thus latching the control arm 26 in place in a releasable manner the spring constant of the.