The Collaborative International Dictionary of English v.0.48:

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

The Collaborative International Dictionary of English v.0.48:

Pendulum \Pen"du*lum\, n.; pl. Pendulums. [NL., fr. L.
   pendulus hanging, swinging. See Pendulous.]
   A body so suspended from a fixed point as to swing freely to
   and fro by the alternate action of gravity and momentum. It
   is used to regulate the movements of clockwork and other
   machinery.
   [1913 Webster]

   Note: The time of oscillation of a pendulum is independent of
         the arc of vibration, provided this arc be small.
         [1913 Webster]

   Ballistic pendulum. See under Ballistic.

   Compensation pendulum, a clock pendulum in which the effect
      of changes of temperature of the length of the rod is so
      counteracted, usually by the opposite expansion of
      differene metals, that the distance of the center of
      oscillation from the center of suspension remains
      invariable; as, the mercurial compensation pendulum, in
      which the expansion of the rod is compensated by the
      opposite expansion of mercury in a jar constituting the
      bob; the gridiron pendulum, in which compensation is
      effected by the opposite expansion of sets of rods of
      different metals.

   Compound pendulum, an ordinary pendulum; -- so called, as
      being made up of different parts, and contrasted with
      simple pendulum.

   Conical pendulum or Revolving pendulum, a weight
      connected by a rod with a fixed point; and revolving in a
      horizontal circle about the vertical from that point.

   Pendulum bob, the weight at the lower end of a pendulum.

   Pendulum level, a plumb level. See under Level.

   Pendulum wheel, the balance of a watch.

   Simple pendulum or Theoretical pendulum, an imaginary
      pendulum having no dimensions except length, and no weight
      except at the center of oscillation; in other words, a
      material point suspended by an ideal line.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pendulum
    n 1: an apparatus consisting of an object mounted so that it
         swings freely under the influence of gravity

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "pendulum":
   Indian file, array, articulation, bank, buzz, catena, catenation,
   chain, chain reaction, chaining, concatenation, connection,
   consecution, continuum, course, cycle, descent, drone,
   endless belt, endless round, file, filiation, gamut, gradation,
   hum, line, lineage, metronome, monotone, nexus, oscillator,
   periodicity, plenum, powder train, progression, queue, range, rank,
   recurrence, reticulation, rocker, rocking chair, rocking stone,
   rotation, round, routine, row, run, scale, seesaw, sequence,
   series, shuttle, shuttlecock, single file, spectrum, string,
   succession, swath, swing, teeter, teeter-totter, teeterboard,
   teetery-bender, thread, tier, train, vibrator, windrow