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How to Draw a Straight Line 
Cornell university's Reuleaux kinematic model collection includes many linkages; the most popular of these among mathematicians is the PeaucellierLipkin linkage S35. This article is a short introduction (not complete) to the history of the problem of how to change circular motion into straightline motion and vice versa. Some mathematicians formulated this problem as: "How can you draw a straight line?" The peaucellierLipkin linkage was the first precise solution to this problem. When using a compass to draw a circle, we are not starting with a model of a circle; instead we are using a fundamental property of circles that the points on a circle are at a fixed distance from its center, which is Euclid's definition of a circle. Is there a tool (serving the role of a compass) that will draw a straight line? If, in this case, we want to use Euclid's definition: "A straight line is a line which lies evenly with the points on itself" it will not be of much help. One can say, "We can use a straightedge for constructing a straight line!" Well, how do you know that your straightedge is straight? How can you check that something is straight? What does "straight" mean? Think about it! As we can see in some 13thcentury drawings of a sawmill (at right), mechanisms for changing circular motion to straightline motion were in use in the 13thcentury and probably originated much earlier. In 1588 Agostino Ramelli published his book on machines where linkages were widely used. But, of course, there is a vast difference between the linkages of Ramelli and those of James Watt (17361819), a pioneer of the improved steam engine and a highly gifted designer of mechanisms. Watt's partner, machine builder, Matthew Boulton, built engines in his shop "...with as great a difference of accuracy as there is between the blacksmith and the mathematical instrument maker." [Fergusson 1962] It took Watt several years to design a straightline linkage that would change straightline motion into circular motion. He wrote to Boulton: "I have got a glimpse of a method of causing the pistonrod to move up and down perpendicularly, by only fixing it to a piece of iron upon the beam, without chains, or perpendicular guides, or untowardly frictions, archheads, or other pieces of clumsiness…. I have only tried it in a slight model yet, so cannot build upon it, though I think it a very probable thing to succeed, and one of the most ingenious simple pieces of mechanisms I have contrived…". [Fergusson 1962] 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." [Fergusson 1962] "Parallel motion" is a name Watt used for his linkage (see model S24), which was included in an extensive patent of 1784. Watt's linkage was a good solution to the practical problem. But this solution did not satisfy mathematicians who knew that it only traced an approximate straight line. An exact straightline linkage in the plane was not known until 1864. In 1853 PierreFrederic Sarrus (17981861), a French professor of mathematics at Strassbourg, devised an accordionlike spatial linkage that traced exact straight line but it still was not a solution of the planar problem. There were several attempts to solve this problem before Peucellier. Other linkages in this Reuleaux model collection are connected with some of the names of 19th century mathematicians who tried to solve the problem of how to draw a precise straight line. Reuleaux thought that these mechanisms were so important that he designed 39 straight line mechanisms for his collection, including those of Watt, Roberts, Evans, Chebyshev, PeuaucellierLipkin, Cartwright and some of his own design. See all models in the Sseries. The appearance in 1864 of Peaucellier's exact straightline linkage went nearly unnoticed. Charles Nicolas Peaucellier (18321913) was a captain in the French army. He announced his "inversor" linkage in 1864  in the form of a question and without explaining the solution  in a letter to the Nouvelles Annales. Eventually Peaucellier became a general and (as claimed by J.J. Sylvester) was in command of the fortress of Toul. 

For at least 10 years before and 20 years after Peaucellier's final solution of the problem, Professor P.L. Chebyshev, a noted mathematician at the University of St. Petersburg was interested in the matter. Judging by his published works and his reputation abroad, his interest amounted to an obsession. In 1853, after visiting France and England and observing carefully the progress of applied mechanics in those countries, he wrote his first paper on approximate straightline linkages, and over the next 30 years he attacked the problem with new vigor at least a dozen times. Chebyshev noted the departure of Watt's and Evans linkages from a straight line and calculated the deviation as of the fifth degree, or about 0.0008 inch per inch of beam length. He proposed a modification of Watt's linkage to refine the accuracy but concluded that it would "present great practical difficulties."Then he got an idea that if one mechanism would be good, two would be better. So he combined two linkages and got as a result, what is usually called Chebyshev's linkage, in which precision was increased to 13th degree. The steam engine he displayed at the Vienna Exhibition of 1873 employed this linkage. In 1871 Lipmann I. Lipkin (18511875) independently discovered the same straightline linkage as Peaucellier and demonstrated a working model at the World Exhibition in Vienna 1873. After that Peaucellier published details of his discovery with a proof of his solution acknowledging Lipkin's independent discovery. Sylvester claims the French government awarded Peaucellier the "Prix Montyon" (1875) for his invention, whereas Lipkin received a "substantial reward from the Russian government."[Kempe 1877] There is not much we know about Lipkin. Some sources mentioned that he was born in Lithuania and was Chebyshev's student but died before completing his doctoral dissertation. In January 1874 James Joseph Sylvester (18141897) delivered a lecture "Recent Discoveries in Mechanical Conversion of Motion." Sylvester's aim was to bring the PeaucellierLipkin linkage to the notice of the Englishspeaking world.Sylvester learned about this problem from Chebyshev  during a recent visit of the Russian to England. "The perfect parallel motion of Peaucellier looks so simple, " he observed, "and moves so easily that people who see it at work almost universally express astonishment that it waited so long to be discovered." [Fergusson 1962] Later Mr. Prim, "engineer to the Houses" (the Houses of Parliament in London) was pleased to show his adaptation of Peaucellier linkage in his new "blowing engines" for the ventilation and filtration of the Houses. Those engines proved to be exceptionally quiet in their operation. [Kempe 1877] Sylvester recalled his experience with a little mechanical model of the Peaucellier linkage at a dinner meeting of the Philosophical Club of the Royal Society. The Peaucellier model had been greeted by the members with lively expressions of admiration "when it was brought in with the dessert, to be seen by them after dinner, as is the laudable custom among members of that eminent body in making known to each other the latest scientific novelties." [Fergusson 1962] 

And Sylvester would never forget the reaction of his brilliant friend Sir William Thomson (later Lord Kelvin) upon being handed the same model in the Athenaeum Club. After Sir William had operated it for a time, Sylvester reached for the model, but he was rebuffed by the exclamation: "No! I have not had nearly enough of it  it is the most beautiful thing I have ever seen in my life." [Fergusson 1962] In summer of 1876 Alfred Bray Kempe, a barrister who pursued mathematics as a hobby, delivered at London's South Kensington Museum a lecture with the provocative title "How to Draw a Straight Line" which in the next year was published in a small book. In this book you can find pictures of the linkages we have mentioned here. Kempe essentially knew that linkages (rigid bars constrained to a plane and joined at their ends by rivets) are capable of drawing any algebraic curve. Other authors provided more complete proofs during the period 18771902. More about the many connections between linkages and such problems of modern mathematics as algebraic completeness, rigidity, NP completeness can be read in Warren D. Smith paper "Plane mechanisms and the 'downhill principle'". PeaucellierLipkin linkage is also used in computer science to prove theorems about workspace topology in robotics [3]. Some history about linkages and discussion of the PeaucellierLipkin linkage is in: http://www.ams.org/newinmath/cover/linkages1.html. References
