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These elliptical gears will convert steady rotary motion about one gear axis into unsteady motion about the other gear axis. Both gears rotate about the focii of their respective ellipses. This mechanism also illustrates the concept of rolling centrode curves developed by Franz Reuleaux in his 1876 book. This theory states that the planar motion of two rigid bodies can be represented by the rolling of one body centrode curve on the centrode curve of the other. In this gear mechanism the ellipses themselves are the exact centrodes of each gear.