17 the locking position of the locking members 16 and 17 for the case of the longitudinal adjustment is shown in fig7 in which an engaging member 20 engages in a receiving opening 28 in the embodiment shown in the drawings in the closed position of the locking device the first and second locking members 16 and 17 are pressed against each other whereby the two locking members become frictionally connected with each other in addition to a formlocking connection that results from engagement of the engaging member s 20 in the receiving hole s 28 the frictional connection is amplified by the pressure applied by the engaging members 20 which do not engage in the receiving holes 28 against the outer surface of the second locking member 17 though frictional engagement of the first and second locking members 16 and 17 in the closed position of the locking device is preferred it is also contemplated and possible that the first locking member 16 would not be pressed against the second locking member 17 in the closed position of the locking device rather the first locking member 16 in the later case would be spaced from the second locking member 17 by a small distance smaller.