Due to a mis-communication between me and Mr Editor, the paragraph headed Principles of operation has been a bit scrambled. My apologies to anyone who is mystified. It should read

Imagine for a moment Hackworth gear with a telescopic vibrating lever gh such that when the crank had rotated 90 or 270 degrees (die block fully up or downdown) gh was shortened. The die block would then rise and fall equally. If we take this setup, decouple the bottom of the lever from the return crank and fix the die block on the slide rotate axis j, the bottom of the vibrating lever gh would then move in a straight horizontal line.

We obviously cannot have a telescopic vibrating lever, but there are several straight line linkages to try. Joy's valve gear uses the Scott-Russell linkage formed by members fe, dge and gh. The layout of the gear is shown in fig. 32.

Can a mod please correct the spelling mistake in the title, should be Joy not Joiy