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Transactions Tennessee Academy of Science 



gines. This apparatus does not give "parallel motion," but ap- 

 proximate "rectilinear motion." 



The simplest form of it is shown in Fig. 1. Here there are three 

 links, AD, BC, and CD. A and B are fixed points, the radial links 

 AD and BC are equal, and the point P is taken at the mid-point of 

 the traversing link CD. The distance between the pivots, C and D, 

 is such that when the radial links are parallel the line joining C and 

 D is perpendicular to the radial bars. Then the curve described by 



the tracing point P is, if the apparatus does not differ much from 

 its mean position, approximately a straight line. The reason of this 

 is that the pivots C and D describe arcs of circles which are turned 

 in opposite directions, and thus the point midway between them 

 tends to curve neither the one way nor the other, and thus moves in 

 a straight line. But this line is only approximately a straight line 

 for a short distance, and it will be seen that if the linkage is moved 

 much from its normal position, the Point P really describes a curve 

 shaped like the figure 8. 



In 1861., to omit mention of other attempts, eighty years after 

 Watt's discovery, the problem was first solved by M. Peaucellier, an 

 officer of Engineers in the French artny. Pcaucellier's apparatus is 

 shown in Fi". 2. It has seven links. 



