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.