| http://www.w3.org/ns/prov#value | - As illustrated, surface 101, that contacted by the wind on the power stroke, is of roughened material, e.g., burlap, coarse cloth, plastic or metal screening or cut foamed synthetic organic polymeric plastic whereas the opposite side 103, which faces the wind on the return stroke, is smooth, e.g., of synthetic organic polymeric plastic film or sheet, such as polyethylene, polypropylene, polyvi
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