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LXXVTL The Parallel Motion of Sarrut and some Allied 

 Mechanisms. Bij G. T. Bennett, Emmanuel College, 

 Cambridge*. 



§ 1. A NY student of mechanism who has gained his 

 JLTL knowledge of the subject from the current text- 

 books might be pardoned for supposing that the cardinal 

 problem of obtaining exact rectilinear motion by the use of 

 link work was first solved in the year 1864 by Peaucellier. 

 Most of the works which deal with the subject, after describing 

 the approximate ,; parallel motions ,J of Watt and others, place 

 first in their account of the exact rectilinear motions now known 

 a discussion of the inverting mechanism of Peaucellier. The 

 Encyclopaedia JBritannica (vol.xxii. p. 512), more explicit, and 

 expressing certainly the common view, states f that " it was for 

 long believed that the production of an exact straight-line 

 motion was impossible until the problem was solved by the 

 invention of the Peaucellier cell." When this discovery had be- 

 come known, the subject of linkworks attracted much fruitful 

 attention ; and other solutions of the problem, some requiring 

 a smaller number of pieces,, were soon found. These mecha- 

 nisms, however, gave rectilinear motion only to a single 

 point : (or, more strictly, to all points of a line, in one of the 

 pieces, parallel to the hinge-lines ; or else to all points of a 

 hinge-line connecting two pieces). There remained yet to 

 be solved the more complete problem, embracing and includ- 

 ing the other, of giving rectilinear translational motion to a 

 whole piece. This also has since been contrived in several 

 ways ; all, however, more or less complicated. It seems 

 therefore very desirable to point out what appears to have 

 escaped notice or to have been entirely forgotten : namely, 

 that this last problem itself was solved exceedingly simply 

 by Sarrut in the year 1853, eleven years before Peaucellier's 

 announcement of his inverting cell. 



§ 2. Sarrut's description of his mechanism appears in the 

 Comptes Rendus of the Academic des Sciences for 1853 

 (vol. xxxvi. p. 1036) ; and later (p. 1125) is given the report 

 of Poncelet upon Sarrut's mechanism. The Academy agreed 

 11 de lui accorder son entiere approbation et de decider que 

 le court memoire qui en contient Pexplication sera insere dans 

 le liecueil des Savants Strangers, avec la description du 

 inodele que l'auteur y a joint/'' But it seems never to have 

 appeared. 



The mechanism may be thus briefly described : — A moving 



* Communicated by the Author. 



f Quotinjr Kempe's "How to draw a straight line.'' 



