. Bulletin. Science. 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 ;
. Bulletin. Science. 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 ; 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 ; 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 197. Please note that these images are extracted from scanned page images that ma
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