a system for conversion of hydrostatic pressure variations of sea waves into useful energy is disclosed . the system is comprised of a sealed casing with a membrane - like cover . the cover operates piston under the pressure of the waves . the piston compresses air into vessel , from which it is discharged to drive turbine and produce electric energy . the piston is recoiled back upwards by second piston , reciprocable within cylinder 58 . the effective volume of the cylinder 58 is variable for counter - pressure adjustment purposes .