Abstract: An air pollution control device for controlling the velocity of discharge from an industrial chimney or smokestack in order to optimize the plume rise and thereby minimize air pollution comprising a variable pitch truncated cone adapted to be mounted at or near the top of the chimney or stack, said cone being constructed to be expandable and contractible with respect to the axis of the chimney or associated flue or flues and kinematic means manually or automatically operable to control the size of the flow path of the gases through the truncated cone.