266 OUTLINES OF NATURAL PHILOSOPHY. 



359. If the ends of a straight line given in 

 magnitude, describe circles given in position, and 

 having their convexities turned opposite ways, a 

 point may be found in that line, which will de- 

 scribe a curve, having, where it intersects the line 

 of the centres, an arch of contrary flexure, not dif- 

 fering sensibly from a straight line. 



The point to be found divides the given line in the ra- 

 tio of the radii of the circles described by its extre- 

 mities. 



A rectilineal vertical motion is also produced by ano- 

 ther construction. Two of the adjacent angles of a 

 parallelogram, are made to describe concentric circles, 

 so that the side between them passes through their 

 centre, and one of the remaining angles another 

 circle, having its convexity opposed to that of the 

 two former ; then the third angle of the parallelo- 

 gram describes a line that differs insensibly from a 

 straight line. 



PEONY treats of this motion, Arch. Hydraulique, torn. n. 

 1478, &c. ; but has not given a complete theory 

 of it. He says, on the authority of ADAMS, (Geo- 

 metrical and Graphical Essays), that the contri- 

 vance was suggested to Mr WATT by one of the in- 

 struments for describing curves, invented by SOAR- 

 DI. See Nuovi Istrumenti per la descrizione di 

 diverse Curve, &c. del Conte GIAMBATISTA SOAR- 

 DI, Padova, 1752. I have looked into this work 

 with considerable attention, but have found nothing 

 that has an affinity to the motions just described. 



360. In 



