having five infinitesimally separated positions 011 a straight line, the deviation and second derivative curves have a character similar to that of linkages based on curvature theory. A symmetrical linkage of the Chebj'chev type is represented in Figs. 9 and 10. A Roberts type linkage is represented in Figs. 11 and 12. These linkages produce fairly inaccurate, but extended, straight-line outputs. Linkages of this group, that are based on the Chebychev minimum deviation theory and those based on

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... curvature theory, will probably give the best results. Figs. 13 and 14 represent a centric-rectangular Evans linkage. This linkage is closely associated with the Scott-Russell exact straighHine slider-crank mechanism and, therefore, gives very good results. There exist two other distinct configurations of the Evans linkage. Based on the results of digital computation, these configurations do not appear to give very satisfactory results. Representative Design Procedure Up to this point, the data have not been reduced to a form which would be most useful to the designer. In this section, the set of linkages having a Ball-double Burmester point will be analj'zed to provide the designer with a technique enabling him to choose the design parameters aa, ab for a specified quality and length of the straight-line output. Previously it was necessary for him to judiciously choose the design parameters in a trial-anderror approach to get a satisfactory straight-line output. Furthermore, it was difficult to assess the quality of the resulting approximation to a straight line.