Realizing that the failure of their gliders to match calculated performance was the result of errors in the experimental data published by their predecessors, the Wrights constructed a small wind tunnel with which to gather their own information on the behaviour in an airstream of model wings of various shapes and sizes. The brilliance of the Wright brothers, their ability to visualize the behaviour of a machine that had yet to be constructed, was seldom more apparent than in the design of their wind-tunnel balances, the instruments mounted inside the tunnel that actually measured the forces operating on the model wings. During the fall and early winter of 1901 the Wrights tested between 100 and 200 wing designs in their wind tunnel, gathering information on the relative efficiencies of various airfoils and determining the effect of different wing shapes, tip designs, and gap sizes between the two wings of a biplane