Figure 20.—Chebyshev's combination (about 1867) of Watt's and Evans' linkages to reduce errors inherent in each. Points C, C', and C" are fixed; A is the tracing point. From Oeuvres de P. L. Tchebychef (St. Petersburg, 1907, vol. 2, p. 93).
Figure 21.—Left: Chebyshev straight-line linkage, 1867; from A. B. Kempe, How to Draw a Straight Line (London, 1877, p. 11). Right: Chebyshev-Evans combination, 1867; from Oeuvres de P. L. Tchebychef (St. Petersburg, 1907, vol. 2, p. 94). Points C, C', and C" are fixed. A is the tracing point.
There is a persistent rumor that Professor Chebyshev sought to demonstrate the impossibility of constructing any linkage, regardless of the number of links, that would generate a straight line; but I have found only a dubious statement in the Grande Encyclopédie[40] of the late 19th century and a report of a conversation with the Russian by an Englishman, James Sylvester, to the effect that Chebyshev had "succeeded in proving the nonexistence of a five-bar link-work capable of producing a perfect parallel motion...."[41] Regardless of what tradition may have to say about what Chebyshev said, it is of course well known that Captain Peaucellier was the man who finally synthesized the exact straight-line mechanism that bears his name.
[ [40] La Grande Encyclopédie, Paris, 1886 ("Peaucellier").
[ [41] James Sylvester, "Recent Discoveries in Mechanical Conversion of Motion," Notices of the Proceedings of the Royal Institution of Great Britain, 1873-1875, vol. 7, p. 181. The fixed link was not counted by Sylvester; in modern parlance this would be a six-link mechanism.
Figure 22.—Peaucellier exact straight-line linkage, 1873. From A. B. Kempe, How to Draw a Straight Line (London, 1877, p. 12).
Figure 23.—Model of the Peaucellier "Compas Composé," deposited in Conservatoire National des Arts et Métiers, Paris, 1875. Photo courtesy of the Conservatoire.