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extremity of the rod AF. JFSZ is a fixed guide at right angles 
to AD upon which the extremity F of AF slides. 
E is the middle point of AF, and ZS is a rod equal in 
length to £AF, and moveable round pins at ES. 
The motion of F may be made as small as we please by 
increasing the length of AP. 
Hence we may approximate as closely as we like to a case 
of linkwork where the friction is entirely reduced to that round 
pivots. 
For example, if @ were attached to a piston-rod whose 
travel was 10”, the travel of A would be 2”, and the travel of 
F would be less than 54”, if the rod AF equalled AC. 
We might moreover do away with the sliding of F by 
making it the extremity of a third rod, and so on. 
Looking back at fig. I. we see that C would describe a 
straight line very nearly if KD instead of a straight line were 
the arc of a very large circle; and this reasonmg may have 
originally have suggested to Watt his parallel motion. 
