162 
Proceedings of the Royal Society 
ing as the centres are on opposite sides, or on one side of the con- 
necting rod, moves in a straight line. The simple explanation of 
the underlying principle is that the curvature in one direction is 
modified by the curvature in an opposite direction, due to the 
motion of the other bar. In both the original and the modified 
form of this three-bar motion there is a divergence from the straight 
line, which though inappreciable for small angles, becomes some- 
what more apparent for larger ones. 
The determination of the parallel point is given in the formula 
QF : FP : : AP : BQ 
In the figures 1 and 2, A.P and BQ are the arms; PQ the con- 
Fig. 1. Fig. 2. 
necting link. Let the centres A and B be chosen so that the line 
mM shall bisect the versed sines of BQ and AP; the throw QK 
being equal to PT. 
