Smeaton wanted to compare the efficiency of different windmill patterns. He didn't have a wind tunnel. Even the cup anemometer had not yet been invented. So Smeaton built the apparatus shown above.
At one end of a counter-balanced arm was a miniature windmill. The arm was attached to a vertical axle with a rope wound around it. A steady pull on the rope rotated the arm at a constant speed, which could be calculated. By pulling at different speeds, the windmill's effectiveness at different wind speeds could be tested. Moving the arm in a circle cancelled out any effect of the real wind. The amount of work done by the windmill was measured by having it lift the weight shown in the center.
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