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The Collaborative International Dictionary of English v.0.48 (gcide)

Hydrometer \Hy*drom"e*ter\, n. [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.
[1913 Webster]

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.
[1913 Webster]

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. Hydrometric


Tachometer \Ta*chom"e*ter\, n. [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:
[1913 Webster]
(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.
[1913 Webster]
(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.
[1913 Webster]
(c) (Physiol.) An instrument for measuring the velocity of
the blood; a haematachometer.
[1913 Webster]