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Langley's specimens were mounted at the end of a revolving arm, so that with the arm revolving, a relative air stream of known velocity could be had. The aerofoil was mounted in such a way that the lift and drag could be measured. In the early experiments of the Wrights, the models were placed in an enclosed channel through which a stream of air was maintained by a fan. The model was attached to a balance system so that the lift and resistance could be measured. This is what is now known as a "wind tunnel," and at present is almost exclusively used in model tests. Several investigators immersed their model aerofoils in running water so that the direction of flow could be visibly observed. While this latter method is of great service in determining disturbances, stream line flow, and general characteristics, it is qualitative rather than quantitative, and cannot be used in obtaining accurate numerical results. A more accurate method of mapping out the direction of flow, eddies, etc., is to introduce smoke into the air stream.