500 KTNEMA TICS OF MA CHINER Y.
affected by the fact that the number of such place- changing machines is comparatively very small. The difference between the two classes is intrinsically important, for it removes an apparent want of congruity between prime-movers and other machines, and will further serve to explain some remarkable analogies.
Let us turn now, lastly, to the employment of living agents, that is, of the muscular power of men and animals, to drive machines, looking first at the use of human muscles. The common statement here has been that the receptor is some portion of the machine having a form and motion suitable for receiving the action of the driving body, the hand or arm or foot of the worker. In a common grindstone, for instance, the treadle is considered the receptor, the foot or leg of the workman the motor. Although this is certainly what appears on the surface to be the case, the real constitution of the machine must be stated differently. If the crank-pin were made sufficiently long, the grindstone might be turned by the hand of the workman ; or if the workman held one end of a cord attached to the crank-pin he could easily, by periodic pulls or jerks, keep the stone in motion.* The treadle is not therefore an indispensable part of the machine. All three methods of driving have, however, this in common, that the body of the worker becomes kinematically chained with the machine. Under certain circumstances this chaining may be very complex, in the case before us, however, it admits of tolerably exact deter- mination. In the first place the crank a with the 'coupler b, the treadle c, and the frame d (Fig. 356) form a lever-crank (") d . In this mechanism c is the driving link, so that its special formula runs (C" 4 ')iK The workman places his foot on c'., the prolongation of c } aud (supposing the centre 1' of his hip-joint not to move) his leg forms with the treadle c' three links, the crank, coupler, and lever, of another lever-crank (C" 4 ') d , to the frame d' of which the hip of the worker belongs. His knee-joint forms the pair 2' and his ankle joint the pair 3'. The joint 4 is common to both the trains of this compound mechanism, as is also the frame d d' which carries the fixed elements of the pairs 1, 4 and 1'. The worker exerts muscular force at I 7 and 2', and indeed at 3' also,
- As the Kalmuck priest drives the prayer- wheel for example, or the Japanese
peasant the reel upon which her silk is wound. J?.
