Reuleaux stated a theorem, attributed to others, that the planar motion of any two bodies can be realized by the pure rolling of two bodies. Willis (1841) in fact showed that two spiral arms can counter-rotate in pure rolling without slip about parallel axes. When the two rotating spirals are contained in a solid cavity, this principle can be used to either produce a fluid pump or an engine. (F.C.Moon,2003)

Application: Water meter; invented by Payton.
This two-toothed form of chamber-wheel train found much use in England as a water meter after being exhibited at the Paris exhibition of 1867, and it can also be used as a water motor. (h. Roehl, c. 1950)

Francis Moon 2003-05-29