A.D 1780 Aug 23 N'? 1263. (l SHEET.) 

 PlCKAims Specification. 





/\ 



\P O^^^ 



77u! enrdllid dnvvaif u coicred. 



/ ^ 

 U 



Uavm on Sbme bj Malby &. Soaa . 



London. I'Vuilwl b> Geoboe Edward Eyiie aiidWiLUAM Spottiswooue 

 l^Tnt«^ 10 the (Jiifens most ficcllall Majt-sly 1855 . 



Figure 6. — One of the steam engine "Crank Patents" that 

 hindered James Watt's progress. This patent, granted to 

 James Pickard in 1780, claimed only the arrangement of 

 counterweights, not the crank. The crank pin to which the 

 connecting rod was attached is at Aa. From British Patent 

 1263, August 23, 1780. 



and Watt feared that they might "get into the hands 

 of men more ingenious," who would give Boulton 

 and Watt more competition than Wasbrough and 

 Pickard.'* 



The sun-and-planet arrangement, with gears of 

 equal size, was adopted by Watt for nearly all the 

 rotative engines that he built during the term of the 

 "crank patents." This arrangement had the advan- 

 tage of turning the flywheel through two revolutions 

 during a single cycle of operation of the piston, thus 

 requiring a flywheel only one-fourth the size of the 

 flywheel needed if a simple crank were used. The 

 optional link (jk of fig. 7e) was used in the engines 

 as built. 



From the first, the rotative engines were made 

 double-acting — that is, work was done by steam 

 alternately in each end of the cylinder. The double- 

 acting engine, unlike the single-acting pumping 

 engine, required a piston rod that would push as well 

 as pull. It was in the solution of this problem that 

 Watt's originality and sure judgment were most 

 clearly demonstrated. 



" Muirhead, op. cit. (footnote 3), vol. 3, note on p. 39. 



A rack and sector arrangement (fig. 8) was used on 

 some engines. The first one, according to Watt, 

 "has broke out several teeth of the rack, but works 

 steady." '^ A little later he told a correspondent 

 that his double-acting engine "acts so powerfully that 

 it has broken all its tackling repeatedly. We have 

 now tamed it, however." '^ 



It was about a year later that the straight-line 

 linkage '" was thought out. "I have started a new 



'■'James Watt, March 31, 1783, quoted in Dickinson and 

 Jenkins, op. cit. (footnote 5), p. 140. 



16 Watt to De Luc, April 26, 1783, quoted in Muirhead, 

 op. cit. (footnote 3), vol. 2, p. 174. 



1" Watt's was a four-bar linkage. All four-bar straight-line 

 linkages that have no sliding pairs trace only an approximately 

 straight line. The exact straight-line linkage in a single plane 

 was not known until 1864 (see p. 204). In 1853 Pierre-Frederic 

 Sarrus (1798-1861), a French professor of mathematics at 

 Strasbourg, devised an accordion-like spatial linkage that 

 traced a true straight line. Described but not illustrated 

 (Academie des Sciences, Paris, Comptes rendus, 1853, vol. 36, 

 pp. 1036-1038, 1125), the mechanism was forgotten and 

 twice reinvented; finally, the original invention was redis- 

 covered by an English wiiter in 1905. For chronology, see 

 Florian Cajori, A History of Mathematics, ed. 2, New York, 

 1919, p. 301. 



PAPER 27: KINEMATICS FROM THE TIME OF WATT 

 60US60 — 62 2 



193 



