In a paper communicated to the Royal Society of Edinburgh, Mr Manley describes a differential densimeter and its successful application in the laboratory to the rapid determination of the relative densities of sea-waters. The paper also contains the suggestion that the instrument would, if suspended in gimbals, be rendered available and convenient for use at sea: thus making it possible for the oceanographer to carry out observations upon the density of sea-water with greater frequency than the usual methods conveniently permit.