s in his _Principles of
Mechanism_ (London, 1841, p. 399) described parallel motion as "a term
somewhat awkwardly applied to a combination of jointed rods, the purpose
of which is to cause a point to describe a straight line...." A. B.
Kempe in _How to Draw a Straight Line_ (London, 1877, p. 49) wrote: "I
have been more than once asked to get rid of the objectionable term
'parallel motion.' I do not know how it came to be employed, and it
certainly does not express what is intended. The expression, however,
has now become crystallised, and I for one cannot undertake to find a
solvent."]

[Footnote 22: Muirhead, _op. cit._ (footnote 3), vol. 3, note on p. 89.]

[Illustration: Figure 10.--Watt's "parallel motion." Engine's working
beam is pivoted at _A_. Pivot _F_ is attached to the engine frame. From
Dyonysius Lardner, _The Steam Engine_ (Philadelphia, 1852), pl. 5
(American ed. 5 from London ed. 5).]

The Watt four-bar linkage was employed 75 years after its inception by
the American Charles B. Richards when, in 1861, he designed his first
high-speed engine indicator (fig. 11). Introduced into England the
following year, the Richards Indicator was an immediate success, and
many thousands were sold over the next 20 or 30 years.[23]

[Footnote 23: Charles T. Porter, _Engineering Reminiscences_, New York,
1908, pp. 58-59, 90.]

[Illustration: Figure 11.--Richards high-speed engine indicator of 1861,
showing application of the Watt straight-line linkage. (_USNM 307515_;
_Smithsonian photo 46570_).]

In considering the order of synthetic ability required to design the
straight-line linkage and to combine it with a pantograph, it should be
kept in mind that this was the first one of a long line of such
mechanisms.[24] Once the idea was abroad, it was only to be expected
that many variations and alternative solutions should appear. One
wonders, however, what direction the subsequent work would have taken
if Watt had not so clearly pointed the way.

[Footnote 24: At least one earlier straight-line linkage, an arrangement
later ascribed to Richard Roberts, had been depicted before Watt's
patent (Pierre Patte, _Memoirs sur les objets les plus importants de
l'architecture_, Paris, 1769, p. 229 and pl. 11). However, this linkage
(reproduced here in figure 18) had no detectable influence on Watt or on
subsequent practice.]

In 1827 John Farey, in his exhaustive study of the steam engine, wrote
perhaps the best contemporary