Cathetometer

[Sarah]

An instrument for the accurate measurement of small differencesof height; esp. of the differences in the height of the uppersurfaces of two columns of mercury or other fluid, or of the samecolumn at different times. It consists of a telescopic levelingapparatus (d), which slides up or down a perpendicular metallicstandard very finely graduated (bb). The telescope is raised ordepressed in order to sight the objects or surfaces, and thedifferences in vertical height are thus shown on the graduatedstandard. [Written also kathetometer.]