Hydrometric pendulum

a species of hydrometer consisting of a hollow ball of ivory or metal suspended by a treated from the center of a graduated quadrant, and held in a stream to measure the velocity of the water by the inclination given to the thread; a kind of current gauge.

See also: Hydrometric

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.