The Pressure of Sound: Relation between Pressure and Energy Density.—An argument is given, following a method used by Larmor, to show that a certain general type of radiation will exert a pressure. The pressure of small sound waves is found to agree with this result, but for finite waves the conditions for the application of the argument are not satisfied. These finite waves do exert a pressure which depends upon the relation between pressure and density, the pressure being zero in a certain important case. This theory has been developed by Lord Rayleigh. It however appears that any actual aerial wave does exert a pressure not zero. The pressure on an absorbing sphere is a second order effect in the product (ak), and is therefore not considered in the usual treatment of spherical obstacles. Waves of energy density of 0.5 ergs/cm.^3 or greater apparently must be treated as finite.