Editing Hydrometer (section)

==Use in soil analysis==
A hydrometer analysis is the process by which fine-grained soils, [[silt]]s and [[clay]]s, are graded. Hydrometer analysis is performed if the grain sizes are too small for [[sieve analysis]]. The basis for this test is [[Stokes' Law]] for falling spheres in a viscous fluid in which the terminal velocity of fall depends on the grain diameter and the densities of the grain in suspension and of the fluid. The grain diameter thus can be calculated from a knowledge of the distance and time of fall. The hydrometer also determines the specific gravity (or density) of the suspension, and this enables the fraction of particles of a certain equivalent particle diameter to be calculated.<ref>Fakhry A. Assaad, Philip Elmer LaMoreaux, Travis H. Hughes (ed.), '' Field Methods for Geologists and Hydrogeologists'', Springer Science & Business Media, 2004 {{ISBN|3540408827}}, p.299</ref>