If the idea did in fact originate with Evans, it is strange that he did not mention it in his patent claims, or in the descriptions that he published of his engines.[33] The practical advantage of the Evans linkage, utilizing as it could a much lighter working beam than the Watt or Freemantle engines, would not escape Oliver Evans, and he was not a man of excessive modesty where his own inventions were concerned.
[ [33] Greville and Dorothy Bathe, Oliver Evans, Philadelphia, 1935, pp. 88, 196, and passim.
Another four-bar straight-line linkage that became well known was attributed to Richard Roberts of Manchester (1789-1864), who around 1820 had built one of the first metal planing machines, which machines helped make the quest for straight-line linkages largely academic. I have not discovered what occasioned the introduction of the Roberts linkage, but it dated from before 1841. Although Roberts patented many complex textile machines, an inspection of all of his patent drawings has failed to provide proof that he was the inventor of the Roberts linkage.[34] The fact that the same linkage is shown in an engraving of 1769 (fig. 18) further confuses the issue.[35]
[ [34] Robert Willis (op. cit. [footnote 2] p. 411) credited Richard Roberts with the linkage. Roberts' 15 British patent drawings exhibit complex applications of cams, levers, guided rods, cords, and so forth, but no straight-line mechanism. In his patent no. 6258 of April 13, 1832, for a steam engine and locomotive carriage, Roberts used Watt's "parallel motion" on a beam driven by a vertical cylinder.
[ [35] This engraving appeared as plate 11 in Pierre Patte's 1769 work (op. cit. footnote 24). Patte stated that the machine depicted in his plate 11 was invented by M. de Voglie and was actually used in 1756.
Figure 17.—Straight-line linkage (before 1841) attributed to Richard Roberts by Robert Willis. From A. B. Kempe, How to Draw a Straight Line (London, 1877, p. 10).
Figure 18.—Machine for sawing off pilings under water, about 1760, designed by De Voglie. The Roberts linkage operates the bar (Q in detailed sketch on left) at the rear of the machine below the operators. The significance of the linkage apparently was not generally recognized. A similar machine depicted in Diderot's Encyclopédie, published several years later, did not employ the straight-line linkage. From Pierre Patte, Memoirs sur les objets plus importants de l'architecture (Paris, 1769, pl. 11).