Page:The Kinematics of Machinery.djvu/229

Rh rubbing, stirring, or grinding which stands in close connection to polishing, smoothing, or forming by friction.

How long a time elapsed before the to-and-fro turning of the twirling-stick became a continuous rotation is mere matter of conjecture. In any case its length must have been such as to leave far behind it our historic period. It is certain that undershot water-wheels, which may well be the first representatives of machines containing continuous rotation, appear in very early times: they imply, however, the existence of some degree of culture. Where they were used for irrigation at least, they presuppose a settled tribe building on the ground. We have traditions of their use in Mesopotamia in a form corresponding to that still to be found there, a wooden wheel with clay buckets. The scoop-wheels of the oldest kind still used in China, of which one is represented in Fig. 162, have a wooden axle, but all their other parts are of bamboo and basketwork, no metal being used; the shaft rests in V-shaped wooden bearings. They have a diameter of from 6 to 12 metres, and discharge the water raised by the bamboo scoops or corfs into a wide trough from which a channel conducts it to the land. Geiger, who does not mention these wheels, gives an earlier date to the praying wheels still used