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KINEMATICS OF MACHINERY.

upon the rollers was very great. The change from the twirling cord, wound several times round the drums, to our own endless band may well have occurred through this mechanism. The two pulleys of Fig. 167 are two whirling rods with their cords connected to each other. Gradually the number of turns on each pulley has been diminished to -one, or indeed, to a half turn, as in Fig. 166. At the same time the broad flat band is substituted for the small round cord. At last by some chance the cord has been used uncrossed. We cannot forget how firmly the tightening pulley has been held to, even to our own day ; to this hour it is a favourite example in many mechanical text-books. Or lastly, that my readers may realise how extremely gradual the change of form has been, let them ' remember what difficulty many people in our own day have in un- derstanding that in driving by means of a wire rope, the rope has merely to be laid upon the surface of the pulley, the application of the cord of the whirling-drill being here carried to its extreme limit. 1 '2 3 4 . 5

FIG. 168.

A very important employment of rotary motion, one which we may well regard as an early step in the direction of continuous turning, is that for twisting the thread in spinning. In primitive times this operation may have required two persons; later on it was found possible for one man to accomplish the task by twining the fibres into thread by rolling them with the palm of the hand on the thigh, as is even now done by certain Indian nations. From this it would appear that the introduction of the spindle, and espe- cially of the hand-spindle, belongs to a much later time, coming