cathetometer

from The Century Dictionary.

• noun An instrument for measuring small differences of level between two near points, as, for example, the distance between the levels of the mercury in the cistern and in the tube of a barometer.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An instrument used to measure small vertical distances