WHITWQRTH'S QUICK RETURN MOTION.
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not concern us here, nor does the spur-wheel. In the body of the latter, however, we can recognise the coupler, the two elements of which are represented by the open cylinder 2 and the pin 3. The latter fits into and carries the block c, which in its turn moves m the open prism of the slide d.
If the pin 2 be further expanded until it includes also 3, we obtain the arrangement shown in Fig. 235. In this case we have made, as we are always at liberty to do, the element of the cylinder pair 2 which belongs to the crank a as an open figure. The
coupler b becomes an eccentric disc which swings about the full cylinder 3 of the block c, while it remains always in contact with the open disc 2 of the crank a.
Instead of placing 3 within 2 we may allow 2 to fall within 3, as in Fig. 236. The coupler 6 is again an eccentric disc ; but it now oscillates in a ring forming part of the block c, while the crank pin drives it by internal contact. The reader whose eye is not yet accustomed to detect the abstract form of such me- chanisms as these behind their constructive outline, and to whom therefore they may be somewhat difficult to understand, will find
x 2
