The hearing nerves of vertebrates have their endings on the basilar membrane. The total length of this membrane in man is 35 mm. Its width varies continuously from 0.04 to 0.5 mm., and accordingly the stiffness of the membrane decreases over its length one hundred fold. The whole membrane is imbedded in fluid, and, when it is set in vibration, waves travel from the stiff part of the membrane toward the softer part. The properties of these waves are of major interest.