Abstract: An apparatus having an alternately expanding and contracting chamber comprises at least one pair of cylindrical rollers each having an exterior surface in continuous rolling engagement with the surface of the other roller of the pair, the rollers being rotatable along parallel axes, and two parallel side walls, with each one of the rollers also being in continuous rolling engagement with one of the side walls. A third wall is opposite the rollers and joins the two side walls wherein the chamber is defined between these walls and the pair of rollers, and is enclosed.