7. A connecting-shaft, carrying bevel gears at each end.
8. A connecting-shaft, working pin-and-roller gears.
9. A connecting-shaft, with universal joints.
10. A row of spur gears to close the gap between the two axles.
11. Cranks and connecting-rods, working double, locomotive-fashion.
12. Friction wheels, with or without spur gears.
13. Pneumatic or hydraulic transmission, with some choice of fluids.
14. A combination of cams and rollers.
15. Balls working in spiral grooves on axles.
This does not exhaust the list of theoretical possibilities, nor mention all the combinations which could be formed. A sufficiently wild inventor could fix up a contrivance, beginning at one axle and ultimately reaching the other, which involved the whole list, and he might then be confident that he had met the long-felt want.