Abstract: An instrument for providing an altitude representation in accordance with ambient atmospheric pressure has a mechanism that is manually-operable to effect adjustment to the setting of a datum against which the representation is provided. A mechanical feedback is applied to the mechanism in accordance with the datum setting such that the setting of the datum depends both upon the manual operation of the mechanism and the feedback applied to the mechanism as a result of that manual operation. The mechanism may include a worm gear that is rotatable in accordance with manual operation of the mechanism in which case the feedback is introduced into the mechanism by longitudinally displacing the worm gear.