Abstract: An elongated cylindrical body is provided of a greater length than its diameter and including a cylindrical side wall. The body is open at its opposite ends and journaled for rotation about an axis generally coinciding with the center axis of the body. The body includes a plurality of circumferentially spaced longitudinally extending slots similarly slightly inclined relative to radial planes of the body passing through the slots. One set of corresponding edge portions of the cylindrical side wall defining corresponding longitudinal edges of the slots include integral outwardly projecting substantially rectangular vanes extending along the slots and the vanes comprise outwardly deflected integral positions of the cylindrical side wall of the body. Further, in a modified form of the invention, the other set of corresponding longitudinal edges of the slots include inwardly projecting substantially rectangular vanes extending along the slots.