James Watt (1736-1819), improver of the steam engine, was a highly gifted designer of mechanisms, although his background included no formal study of mechanisms. Indeed, the study of mechanisms, without immediate regard to the machines in which they were used, was not introduced until after Watt's important work had been completed, while the actual design of mechanisms had been going on for several centuries before the time of Watt. Mechanisms that employed screws, cams, and gears were certainly in use by the beginning of the Christian era. While I am not aware of unequivocal evidence of the existence of four-bar linkages before the 16th century, their widespread application by that time indicates that they probably originated much earlier. A tantalizing 13th-century sketch of an up-and-down sawmill (fig. 1) suggests, but does not prove, that the four-bar linkage was then in use. Leonardo da Vinci (1452-1519) delineated, if he did not build, a crank and slider mechanism, also for a sawmill (fig. 2). In the 16th century may be found the conversion of rotary to reciprocating motion (strictly speaking, an oscillation through a small arc of a large circle) and vice versa by use of linkages of rigid members (figs. 3 and 4), although the conversion of rotary to reciprocating motion was at that time more frequently accomplished by cams and intermittent gearing. Nevertheless, the idea of linkages was a firmly established part of the repertory of the machine builder before 1600. In fact one might have wondered in 1588, when Agostino Ramelli published his book on machines,[1] whether linkages had not indeed reached their ultimate stage of development. To illustrate my point, I have selected the plate of Ramelli that most appeals to me (fig. 5), although the book exhibits more than 200 other machines of comparable complexity and ingenuity. [Footnote 1: Agostino Ramelli, _Le Diverse et Artificiose Machine_, Paris, 1588.] [Illustration: Figure 1.--Up-and-down sawmill of the 13th century. The guide mechanism at lower left, attached to the saw blade, appears to be a 4-bar linkage. After Robert Willis, trans. and ed., _Facsimile of the Sketch-Book of Wilars de Honecort_ (London, 1859, pl. 43).] [Illustration: Figure 2.--Slider-crank mechanism of Leonardo da Vinci (1452-1519), redrawn from his manuscript notebooks. A frame saw is depicted at the lower end of the guides. From Theodor Beck, _Beiträge zur Geschichte des Maschinenbaues_ (Berlin, 1899, p. 323).] [Illustration: Figure 3.--Blowing engine by Vanuccio Biringuccio, about 1540, showing conversion of motion of the waterwheel shaft from rotation to oscillation. From Theodor Beck, _Beiträge zur Geschichte des Maschinenbaues_ (Berlin, 1899, p. 120).]