is possible, according to the relative position of the normals. If the latter coincide (Fig. 66), the strip becomes a straight line, about points in which, as it forms the boundary line of two fields of turning in opposite directions, both right- and left- handed turning can take place.
If the parallel normals have the same direction (Fig. 67), the between them becomes a field of restraint, the space beyond
1
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c ^
51 X
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a
1
Flu. 64.
Fig. 65.
it being on the one side a field of K H. and on the other of L.H. turning.
Three Points of Kestraint. If a third point of restraint c (Fig. 68) be added to any two others a and &, the tangents at which enclose any angle less than 180, its influence upon the turning depends entirely upon its relative position. If, for in- stance, c be taken upon that part of the figure lying within
ill
Fig. 66.
Fig. 67.
the tangent angle aTb, its normal cuts the other two in the points P and Q; its field of L. H. turning covers the similar field already existing, a V, so that this remains a field of L. H. turning. The E. H. field of c covers the part IP Qd of the similar field already existing, which therefore also remains as before. Of the original field of turning only the small triangle P Q is covered by a pair of dissimilar fields, so that turning about points in
