Physical Society, April 17.—E. G. Richardson: Edge tones. If a fluid leaves an orifice as a jet and strikes an edge, two vortex streets are formed on each side of the wake and maintain the jet in pendulation at a definite frequency. The tones so produced are examined from theoretical and experimental points of view, and relations connecting the frequency with (a) velocity, (b) distance from orifice to edge, (c) width of orifice, (d) form of the orifice, are tested. All the features of the phenomena can be explained in terms of the hydrodynamics of a viscous fluid, without compressibility being postulated in addition.