Page:The Kinematics of Machinery.djvu/251

 FORCE-CLOSURE IN PRIMITIVE MACHINES.

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hand stretching the cord by force-closure. In the blow-tube the prism or sliding- pair is used for guiding the projectile in a form already very complete, in the musket it is smoothly bored out, in modern ordnance the bullet and barrel are formed accurately into a twisting-pair, the force-closure of the former being at last completely done away with.

The Chinese scoop-wheel, which we above described 3 carries the stamp of antiquity on it in its use everywhere of force- closure. The driving element itself is force-closed both in the bed of the stream into which the wheel dips, and in the basket-work paddles which it drives before it; force-closed in the bamboo scoops which carry it upwards, and in the channels which direct its

FIG. 173.

course over the land ; the shaft of the -wheel, too, is force-closed in its angular bearings. To what an extent the mere struggle against or counteracting of disturbing motions has lain at the base of the invention of kinematic closure, we may see for instance in the very ancient Noria, a Spanish water-wheel. 40 Its shaft lies upon somewhat inclined frames without any notching whatever being made to receive it (Fig. 173). The tendency of each journal, as it turns forward, is to roll further down the frame, and this is prevented by small projecting pieces. We know that this and similar bearing-arrangements have by degrees become transformed into carefully fitted turning-pairs ; that the water has been en- closed in a channel, then in guides surrounding the wheel and so on, and has thus more and more become used with pair-closure. Still there can be no doubt that in water- wheels generally there are still left distinct traces of the former use of force-closure in all their parts.