. Bulletin. La science. Figure 12.-Cartwright's adaptés linéairement mécanisme d'environ 1800, à partir d'Abraham Rees, la Cyclopaedia (Londres, 181 g, "machine à vapeur", pi. 5). éléments simples ; pour les organiser en catégories, et en les comparant ensemble . . ." Farey était d'avis que, bien qu'un esprit comme Watt's pourrait produire de nouvelles idées brillantes, encore le "fonds commun d'idées qui sont parmi les professions actuelles a cette échelle et, de façon générale, s'avérer être d'une meilleure qualité que la moyenne de ces nouvelles idées, qui peut être produite par toute personne de l'exploitation de son propre

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. Bulletin. Science. Figure 12.—Cartwright's geared straight-line mechanism of about 1800, From Abraham Rees, The Cyclopaedia (London, 181 g, "Steam Engine, " pi. 5). simple elements; arranging them into classes, and comparing them together . . ." Farey was of the opinion that while a mind like Watt's could produce brilliant new ideas, still the "common stock of ideas which are current amongst cornmunities and professions, will generally prove to be of a better quality than the average of those new ideas, which can be produced by any individual from the operation of his own mind, without assistance from others." Farey concluded with the observation that "the most useful additions to that common stock, usually proceed from the individuals who are well acquainted with the whole series." '^ To Draw a Straight Line During most of the century after James Watt had produced his parallel motion, the problem of devising a linkage, one point of which would describe a straight line, was one that tickled the fancies of mathematicians, of ingenious mechanics, and of gentlemanly dabblers in ideas. The quest for a straight-line mechanism more accurate than that of Watt far outlasted the pressing practical need for such a device. Large metal planing machines were well known by 1830, and by midcentury crossheads and crosshead guides were used on both sides of the Atlantic in engines with and without working beams. By 1819 John Farey had observed quite accurately that, in England at least, many other schemes had been tried and found wanting and that "no methods have been found so good as the original engine; and we accordingly find, that all the most established and experienced manufacturers make engines which are not altered in any great feature from Mr. Watt's original engine . . ." ^° Two mechanisms for producing a straight line were introduced before the Boulton and Watt monopoly ended in 1800. Perhaps the first was by Edmund Cartwri

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