The Collaborative International Dictionary of English v.0.48:

Hydrometry \Hy*drom"e*try\, n. [Cf. F. hydrom[`e]trique.]
   1. The art of determining the specific gravity of liquids,
      and thence the strength of spirituous liquors, saline
      solutions, etc.
      [1913 Webster]

   2. The art or operation of measuring the velocity or
      discharge of running water, as in rivers, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hydrometry
    n 1: the measurement of specific gravity [syn: hydrometry,
         gravimetry]