An apparatus has been built to enable the specific gravity of the water in the sump of the N.E.L. Hydraulic Machinery building to be read directly, with an accuracy of 1 in 50 000. A watertight, hollow, stainless-steel sphere, 15 in. in diameter, loaded with shot so that it just sinks in water, hangs from a semi-automatic balance in a Perspex tank through which sump water is circulated. Tables, based on a previous calibration of the apparatus with distilled water, enable balance readings to be converted directly to specific gravity values for any given temperature from 13.0 to 21.0° C. A cheap form of the apparatus for use in hydraulics laboratories is suggested. The possibility is discussed of increasing the accuracy to better than 1 part per million for research on the density of liquids.