to mass-produce complex machines with moving parts (guns) using a system that entailed the use of rough-forged parts, with a milling machine that milled the parts to near-correct size, and that were then "filed to gage by hand with the aid of filing jigs." Historians differ over the question of whether Hall or North made the crucial improvement. Merrit Roe Smith believes that it was done by Hall. Muir demonstrates the close personal ties and professional alliances between Simeon North and neighboring mechanics mass-producing wooden clocks to argue that the process for manufacturing guns with interchangeable parts was most probably devised by North in emulation of the successful methods used in mass-producing clocks. It may not be possible to resolve the question with absolute certainty unless documents now unknown should surface in the future. Late 19th and early 20th centuries: dissemination throughout manufacturing Skilled engineers and machinists, many with armory experience, spread interchangeable manufacturing techniques to other American industries including clockmakers and sewing machine manufacturers Wilcox and Gibbs and Wheeler and Wilson, who used interchangeable parts before 1860. Late to adopt the interchangeable system were Singer Corporation sewing machine (1870s), reaper manufacturer McCormick Harvesting Machine Company (1870s–1880s) and several large steam engine manufacturers such as Corliss (mid-1880s) as well as locomotive makers. Typewriters followed some years later. Then large scale production of bicycles in the 1880s began to use the interchangeable system. During these decades, true interchangeability grew from a scarce and difficult achievement into an everyday capability throughout the manufacturing industries. In the 1950s and 1960s, historians of technology broadened the world's understanding of the history of the development. Few people outside that academic discipline knew much about the topic until as recently as the 1980s and 1990s, when the academic knowledge began finding wider audiences. As recently as the 1960s, when Alfred P. Sloan published his famous memoir and management treatise, My Years with General Motors, even the longtime president and chair of the largest manufacturing enterprise that had ever existed knew very little about the history of the development, other than to say that [Henry M. Leland was], I believe, one of those mainly responsible for bringing the technique of interchangeable parts into automobile manufacturing. […] It has been called to my attention that Eli Whitney, long before, had started the development of interchangeable parts in connection with the manufacture of guns, a fact which suggests a