referring to fig1 there is shown in diagrammatic side view an office type chair 1 . it comprises a seat 3 mounted on a roller frame 5 by means of a telescopic height adjuster 7 . the seat 3 is adjustable so as to be pivotally adjustable so as to adjust between a generally horizontal position and a forward tilting position . the seat 1 also has a back rest 9 which is adjustable for rake . control of the three types of adjustment is carried out by a three lever arrangement 11 of which the first lever 13 provides for seat portion tilt , the second lever 15 provides for seat height and the third lever 17 provides for back rake . the present chair is provided with a locking arrangement in accordance with the invention in connection with the first lever 13 although it will be understood that the invention could equally be applied to any of the levers or to all of them . the lever 13 is selected in the present situation as this is believed to be the operation which is most difficult to perform adequately of the three . fig2 shows a perspective view of the three lever mechanism 11 of which only the mechanism of the lever 13 will be considered . it is to be observed that neither the connection between the mechanism and the feature being controlled , usually a bowden cable or the like , nor the spring for biasing the lever 13 are shown for the sake of clarity . as can be seen , the lever 13 is connected to pivot about an axle 21 with which it moves and which carries a rotary cam element 23 , also tied to the movement of the lever 13 . the cam element 23 has a cam surface 25 provide with a step 27 which co - operates with a spring urged detent 29 as will be described hereafter . the cam element 23 also has a circumferentially extending slot 31 in which is located a slider 33 freely slidable therein . the slider 33 has a projection 35 which also interacts with the detent 29 . the operation of the mechanism will now be describe with reference to fig3 to 7 : fig3 shows the lever 13 in the locked position . in this position , the cam element 23 is positioned with its step 27 above the detent 29 so that the detent rides on the high part of the cam 23 . the lever 13 is retained in this position by a biasing spring ( not shown ). the slider 33 is located in the clockwise end of the slot 31 . in order , to unlock the seat tilting mechanism , the lever 13 is raised through the position shown in fig4 where it will be seen that the cam element 23 has rotated clockwise and the projection 35 of the slider 33 has engaged the detent 29 and is starting to move anticlockwise in the slot 31 . in fig5 the lever has reached its unlocking position . in this position , the detent 29 has passed over the step 27 so as to engage behind it , thus preventing a return of the lever 13 to its locking position . at this time also , the slider 33 has been pushed back to the anticlockwise end of the slot 31 . having attained this position , the lever 13 can be released and the position of the chair seat 3 can be adjusted . once the desired adjustment of the seat 3 has been achieved , the lever 13 is released to return to its locking position as shown in fig3 . to achieve this , the lever 13 is raised still further , again rotating the cam element 23 clockwise . the projection 35 of the slider 33 remains in contact with the detent 29 but the slider 33 cannot be moved any further anticlockwise . instead , it is moved clockwise by the end of the slot 31 , forcing the detent 29 outwards away from the cam element 23 , releasing it and permitting anticlockwise movement of the lever 13 under the action of its spring to its locking position shown in fig6 . in passing to this position , the engagement of the projection 35 of the slider on the detent 29 causes the slider 33 to remain stationary and thus be repositioned at the clockwise end of the slot 31 ( fig7 ). at this point , the slider 33 is again entrained by the slot 31 to move anticlockwise and the projection 35 disengages from the detent 29 , allowing the detent 29 to reengage the surface 25 of the cam element 23 . fig8 to 14 show a second embodiment of the invention . fig8 shows a perspective view of the elements of a second locking mechanism in accordance with the invention . here only the parts actually involved in the locking are shown . thus the mechanism basically comprises three parts viz : a cam 51 pivoted about a pivot bore 41 , a spring urged pin assembly 43 pivoted about a bore 45 and a pivoted slider arrangement 47 pivoted about a bore 49 . the lever 13 ( fig9 to 14 ) is constrained to pivot about the same axis as the cam 51 and to move therewith . the cam has an outer surface having a detent recess 81 thereon . it also has a recessed portion 46 in which are located a cam island 67 with upper and lower edges 65 and 85 respectively . at the right hand end of the cam island 67 is a hook recess 83 . the base portion of the cam recess also has a hook recess 77 . the cam , recess has a left hand side wall 57 and a right hand side wall 75 . the cam also has a bore 48 to receive the end of a bowden cable 79 as can best be seen in fig9 to 14 . this bowden cable 79 acts to provide locking and unlocking of the chair feature to be adjusted in known manner . the spring urged pin assembly comprises a wire element having a spring part 91 . two arms 93 and 95 ending in right angle bends to form two pins 53 and 55 which co - operate with the various parts of the cam recess 46 as will be described hereafter . the pivoted slider arrangement 47 comprises an arm 97 pivoted about a bore 49 and urged against the surface of the cam 51 by spring means ( not shown ). it carries a stop 69 adjacent to the bore 49 and a slider 59 at its free end . the operation of the mechanism will now be describe with reference to fig9 to 14 : fig9 shows the lever 13 in the locked position . in this position , the cam element 51 is in its most clockwise position with both pins 53 and 55 against the left hand wall 57 thereof and the slider 59 is sliding on the high part 61 of the cam element 51 . the cam element 51 from is then in its most clockwise position . the lever 13 is retained in this position by a biasing spring ( not shown ). in order to unlock the seat tilting mechanism , the lever 13 is raised through the position shown in fig1 where it will be seen that the cam element 51 has rotated anticlockwise thereby and the second pin 55 has slid rightwards along the surface 63 on the lower surface of the cam recess 46 and caused the upper pin 53 to be raised upwards and passed forward into the recess 83 in the cam island . the lower pin 55 has moved past the end of the surface 63 . in this position , the slider 59 of the slider arrangement 47 will have dropped into the detent recess 81 and will prevent any significant return movement of the lever 13 . release of the lever 13 will cause it to move clockwise causing the lower pin 55 to enter the hook recess 77 and prevent further clockwise movement , locking the mechanism in this position as shown in fig1 . the upper pin 53 will also drop out of the recess 83 , leaving it free . at the same time , the slider 59 will also be brought up against the shoulder of the detent recess 81 so as to assist the prevention of clockwise movement of the cam 61 with the lever 13 in this position , adjustment of the chair can take place . to move the lever 13 back to its locking position once adjustment of the chair has taken place , the lever 13 is again raised slightly moving the lower pin 55 out of the hook recess 77 and , as a result , moving the upper pin 53 along the underside of the cam island 67 so as to clear it . further upward movement of the lever 13 moves the lower pin 55 along the bottom surface of the cam recess 46 raising the upper pin 53 around the end of the cam island 67 so as to engage the upper surface thereof ( fig1 ). at the same time , the slider 59 moves out of the detent recess 81 . release of the lever 13 at this point will cause it to move down under the action of its spring and rotate the cam 61 clockwise . the pin 53 moves along the upper surface of the cam island 67 engaging the stop 69 and raising the slider 59 into a position ( fig1 ) in which it is clear of the detent recess 81 . the released lever 13 continues to move downwards , further rotating the cam 61 clockwise . the upper pin 53 moves along the upper surface of the cam island , still maintaining the stop 69 in its raised position so that the slider 59 again engages the outer surface of the cam 61 beyond the detent recess 81 as shown in fig1 . finally , the lever returns to its starting position as shown in fig9 with the two pins 53 and 55 engaging the leftward wall 57 of the cam recess 46 , thus preventing any further clockwise movement of the cam 61 and thus also the lever 13 . it will be appreciated that various modifications or additions may be made to the above described embodiment without departing from the scope of the invention . for example , the operation of the mechanism could be reversed , with the lever operation being downwards instead of upwards . in the second embodiment , the pins 53 and 55 could be mounted on a pivotally arranged biased plate . also , it will be appreciated that the slider arrangement 47 is not essential to the operation of the mechanism and merely provides additional strength . it can therefore be omitted in some circumstances . furthermore , while the mechanism has been described for use with an office type chair , it will be understood that the mechanism could be applied in other situations where a locking mechanism of this type would be useful .