From The Collaborative International Dictionary of English v.0.44 [gcide]:
Hydrometer \Hy*drom"e*ter\, noun [Hydro-, 1 + -meter: cf. F. hydrom['e]tre.]
1. (Physics) An instrument for determining the specific gravities of liquids, and thence the strength spirituous liquors, saline solutions, etc.
Note: It is usually made of glass with a graduated stem, and indicates the specific gravity of a liquid by the depth to which it sinks in it, the zero of the scale marking the depth to which it sinks in pure water. Extra weights are sometimes used to adapt the scale to liquids of different densities.
2. An instrument, variously constructed, used for measuring the velocity or discharge of water, as in rivers, from reservoirs, etc., and called by various specific names according to its construction or use, as {tachometer}, {rheometer}, {hydrometer}, {pendulum}, etc.; a current gauge.
From The Collaborative International Dictionary of English v.0.44 [gcide]:
Tachometer \Ta*chom"e*ter\, noun [Gr. ta'chos swiftness, speed (fr. tachy's quick) + -meter: cf. F. tachom['e]tre.] An instrument for measuring the velocity, or indicating changes in the velocity, of a moving body or substance. Specifically: (a) An instrument for measuring the velocity of running water in a river or canal, consisting of a wheel with inclined vanes, which is turned by the current. The rotations of the wheel are recorded by clockwork. (b) An instrument for showing at any moment the speed of a revolving shaft, consisting of a delicate revolving conical pendulum which is driven by the shaft, and the action of which by change of speed moves a pointer which indicates the speed on a graduated dial. (c) (Physiol.) An instrument for measuring the velocity of the blood; a h[ae]matachometer.
From WordNet (r) 2.0 [wn]:
noun
1: measuring instrument for indicating speed of rotation [syn: {tach}]
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