the manner of determining the concentration of solids in suspension. Letting r represent the suspension density, rl the density of liquid, and rs the particle density, all in grams per liter, we have the equation, r = rl + (c/1000)(1 - rl/rs). Although the buoyant force on a hydrometer is determined directly by the suspension density, r, hydrometer scales can be calibrated in terms of c for particular values of rl and rs. The large size of hydrometer bulb necessary to give adequate sensitivity reduces the depth discrimination of the instrument, but this limitation can be overcome by a simple correction (Day 1965).