If the normals to the first two points of restraint be parallel and have similar directions, the addition of a third point may, as Fig. 74 shows, convert a part of each of the fields of turning into a field of restraint; or it can, if its normal be parallel and opposite to the first two, entirely neutralize one of the fields of turning, and reduce the other to a strip of limited breadth
(Fig. 75): or, lastly, if its normal
11111. 11 ,1
be placed between the other two,
and opposite to them in direc- tion, the whole of the original fields of turning will be covered by fields of dissimilar name, so that turning will be entirely prevented.
Four and Five Points of Ke- straint. In cases where turning cannot be prevented by three points
of restraint, and we have seen that this is the rule, the object can be attained by the addition of a fourth point, if it be so placed that its field of turning covers those of the other points dissimilarly (that is to say; a right- over a left- and a left- over a right-handed field.) If for instance a fourth point d be added to the three shown in Fig. 70, in such a way that
Fig. 74.
Fig. 75.
its noimal passes to one side of the field of turning OP Q, so as to cover it with a field of dissimilar name (Fig. 77), no turning can take place. In a case like Fig. 72 a fourth point of restraint d, covering with its L H. field the remaining field of R. H. turning converts it, as shown in Fig. 78, into a field of restraint. In the cases shown in Figs. 74 and 75 this can also be done.
