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 Willis 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_).]