cathetometer

CIDE DICTIONARY

cathetometer, n. [From Gr. vertical height + -meter.].

 An instrument for the accurate measurement of small differences of height; esp. of the differences in the height of the upper surfaces of two columns of mercury or other fluid, or of the same column at different times. It consists of a telescopic leveling apparatus (d), which slides up or down a perpendicular metallic standard very finely graduated (bb). The telescope is raised or depressed in order to sight the objects or surfaces, and the differences in vertical height are thus shown on the graduated standard.  [1913 Webster]

OXFORD DICTIONARY

cathetometer, n. a telescope mounted on a graduated scale along which it can slide, used for accurate measurement of small vertical distances.

Etymology

L cathetus f. Gk kathetos perpendicular line (as CATHETER + -METER)