Some Link Motions — How to Draiv a Straight Line 



31 



head", we see that the height of the "kite" muhiplied by that 

 of the "spear-head" is constant. Let us now instead of amalgamat- 

 ing the long links of the two linkages, amalgamate the short ones. 



Fig. 5 



We then get the linkage of Fig. 6; and if the pivot where the short 

 links meet is fixed, and one of the other free pivots be made to move 

 in the circle of Fig. 3 by the extra link, the other will describe, not 



the straight line PM, but the straight line P'M'. Mr. Kempe says 

 that in this form the motion was applied most successfully to the 

 air engines used to ventilate the Houses of Parliament. 



Again, if to the ordinary Peaucellier cell I add two new links of 

 the same lengths as the long ones, I get the "quadruple" cell of Fig. 

 7, called quadruple because it may be used in four different ways. 

 Here the four points, 0, C, P, 0', lie in a straight line such that C, P, 

 are the poles of a positive cell, whose fulcrum is or 0'; and 0, 0' 

 are the poles of a negative cell whose fulcrum is C or P. In the first 

 case we attach the free end of the extra link to C or P, or 0' beinar 



