The carriage of a ruling engine is driven by a hydraulic system servo-controlled by an optical interferometer, so that the position of the carriage is determined in terms of the wavelength of light. The arrangement is free from the errors of the conventional lead-screw system and is far cheaper to construct.
W. R. Horsfield, "Ruling Engine with Hydraulic Drive," Appl. Opt. 4, 189-193 (1965)