ted here (fig. 9). One of the alternatives is a guided crosshead
(fig. 9, top right).

[Footnote 19: _Ibid._, p. 202.]

[Illustration: Figure 9.--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 of James Watt_ (London,
1854, vol. 3, pls. 21, 22).]

Brilliant as was the conception of this linkage, it was followed up by a
synthesis that is very little short of incredible. In order to make the
linkage attached to the beam of his engines more compact, Watt had
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.[20] Watt combined his straight-line linkage with a
pantograph, one link becoming a member of the pantograph.

[Footnote 20: "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 instead of straight
lines." _Ibid._, p. 71.]

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 could be constructed so that it would not extend
beyond the end of the working beam. This arrangement soon came to be
known as Watt's "parallel motion" (fig. 10).[21] Years later Watt told
his son: "Though I am not over anxious after fame, yet I am more proud
of the parallel motion than of any other mechanical invention I have
ever made."[22]

[Footnote 21: Throughout the 19th century the term "parallel motion" was
used indiscriminately 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). _The 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. in the direction of its length; and contrivances, by which such
alternate rectilinear motions are converted into continuous rotatory
ones, or _vice versa_...." Robert Willi