Wu Tê-Jen's Mechanism

Mr. Moule proposed the complex arrangement given to the right to satisfy the specification given in the Sung Shih. Keeping the principle of Yen Su, two more axles (wheels B and F/G in the picture) are added, which keep the direction of rotation unaltered, but facilitate changes in (a) rotational speed and (b) pitch. While (a) could improve the accuracy - now more revolutions of the road wheels are available before the small cogwheels disengage again - (b) will certainly take some play out of the train.
Both modifications will increasing friction, thus steadying the pointer on rough terrain.

(click for details)

top view

The function is described as follows:

The gear-wheel (H) fixed to the road-wheel (A) engaged with another gear-wheel (B) immediately above it, and this in turn engaged and disengaged at a right angle with the vertically movable cogwheel (D), seen below its suspension pulley. This in turn moved (when in action) the lower component (F) of the double gear-wheel, the upper part (G) of which moved the central wheel (C).

Source: Needham. Joseph: Science and Civilization in China, Volume 4, Part II, 1965