Figure g. — Watt's mechanisms for guiding the 

 upper end of the piston rod of a double-acting 

 engine (British Patent 1432, April 28, 1784). 

 Top left, straight-line linkage; top right, crosshead 

 and guide arrangement; lower left, piston rod A 

 is guided by sectors D and E, suspended by 

 flexible cords. From James P. Muirhead, The 

 Origin and Progress of the Mechanical Inventions oj 

 James M'att (London, 1854, vol. 3, pis. 21, 22). 



plumbed his experience for ideas; his experience had 

 yielded up the work done much earlier on a drafting- 

 machine that made use of a pantograph.-" Watt 

 combined his straight-line linkage with a pantograph, 

 one link becoming a member of the pantograph. 



The length of each oscillating link of the straight- 

 line linkage was thus reduced to one-fourth instead 

 of one-half the beam length, and the entire mechanism 



-° "It has only one fault," he had told a friend on December 

 24, 1773, after describing the drafting machine to him, "which 

 is, that it will not do, because it describes conic sections in- 

 stead of straight lines." Ibid., p. 71. 



could be constructed so that it would not extend 

 beyond the end of the working beam. This arrange- 

 ment soon came to be known as Watt's "parallel 

 motion" (fig. 10).^' Years later Watt told his 

 son: "Though I am not over anxious after fame, yet 



-1 Throughout the 19th century the term "parallel motion" 

 was used indisciiminately to refer to any straight-line linkage. 

 I have not discovered the origin of the term. Watt did not 

 use it in his patent specification, and I have not found it in 

 his writings or elsewhere before 1808 (see footnote 22). T/ie 

 Cyclopaedia (Abraham Rees, ed., London, 1819, vol. 26) 

 defined parallel motion as "a term used among practical 

 mechanics to denote the rectilinear motion of a piston-rod, &c. 



PAPER 27: KINEMATICS FROM THE TIME OF WATT 



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