it only has been rendered impossible by the addition of the
third restraining point.
If c be so placed that its normal passes through both halves
of the field of turning (Fig. 69), the triangle POQ falls in the
field of restraint of a and b instead of in their field of turning. The parts bO PC and c' Q b' are each covered by a pair of dissimilar fields, and only the parts c P a' and c QOa remain as fields of right- and left-handed turning re- spectively.
If the third point of re- straint be so placed that its normal makes a smaller angle than 180 with those next it, as in Fig. 70, the result is If the normal to c then pass (as shown
in the figure), through the original field of right-handed turning, its
R. H. field entirely covers the L. H. one a b' } and its left-handed
Fig.
widely different.
Fio. 70.
Fig. 69.
one the portion a' P Q b of the right-handed field a' b, so that turning about points in both these areas is prevented. The triangle POQ only is covered by a pair of similar fields, so that in it alone turning, and right-handed turning only, is possible. If c had
