You searched for: “cathetometer
cathetometer (s) (noun), cathetometers (pl)
An instrument for the accurate measurement of small differences of height; especially, 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: The cathetometer consists of a telescopic leveling apparatus, that slides up or down a perpendicular metallic standard which is very finely graduated.

The telescope in the cathetometer is raised or lowered in order to see the objects or surfaces, and to determine the differences in vertical height which are then shown on the graduated standard.