14 of duct 12 is air permeable in at least certain areas the air permeability can be provided in various ways including but not limited to making pliable wall 14 of a porous material or by perforating an otherwise impervious material for the illustrated example the permeability of wall 14 is provided by a matrix of holes or openings 20 in some cases the openings are about 0 188 inches in diameter and have a center to center spacing of about 0 625 inches the openings may be evenly distributed over an area that is about two inches high and extending along a substantial length of duct 12 however various other hole sizes and distribution patterns are well within the scope of the invention to allow adjustment of the volume and or direction of the air discharging through openings 20 duct 12 includes a movable air deflector 22 deflector 22 is preferably installed inside duct 12 however it is also well within the scope of the invention to install deflector 12 on the exterior of the duct in some cases deflector 22 has an oblong cross sectional area as shown in fig2 and may be of any length e g one foot eight feet twenty feet or longer in the example shown in fig2 deflector 22 has a major thickness 24 and a minor thickness 26 with a plurality of holes 28 extending through the minor thickness 26 in some embodiments major thickness 24 is 1 75 inches minor thickness 26 is about one inch and holes 28 have a diameter of about 0 5 inches with a center to center spacing of about 0 75 inches holes 28 serve as nozzles that direct pressurized air 18 through.