Cayley was interested in reducing the 'direct resistance' (drag) of the spars in his gliders and developed the first instrument for testing streamlining. He cut the flat section of the spar to half the frontal area of the wider, curved part, to test the theory of the time that they would have the same drag.

- Click the finger to test the comparitive drag of each side of the spar

- drag the small plate onto different positions on the spar and repeat until the drag on each side of the spar is balanced