From The Collaborative International Dictionary of English v.0.48:

Cathetometer \Cath`e*tom"e*ter\, 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. [Written
   also kathetometer.]
   [1913 Webster]
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