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:

Rheometer \Rhe*om"e*ter\, n. [Gr. "rei^n to flow + -meter.]
   [Written also reometer.]
   1. (Physics) An instrument for measuring currents, especially
      the force or intensity of electrical currents; a
      galvanometer.
      [1913 Webster]

   2. (Physiol.) An instrument for measuring the velocity of the
      blood current in the arteries.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rheometer
    n 1: an instrument for measuring the flow of liquids (especially
         arterial blood)