any of several instruments for rapidly determining distances, directions, and differences of elevation.
Origin of tachymeter
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M. GAETANO CAÏRO has invented an instrument, to which he has given the name of Tachymeter (rapid measurer). Its object is to give the area of plane surfaces bounded by any outline whatever, without the necessity of any arithmetical operation. ["Magazine of Popular Science and Journal of the Useful Arts," Volume 2, 1836]
Online Etymology Dictionary, © 2010 Douglas Harper