The  simplest  case  is  when  0  A  and  0  B  are  parts  of  one  straight
lina  The  required  pitch-line  is  then  an  eccentric  circle,  described
upon  the  straight  line  A  0  B  as  a  diameter.
When  C  A  and  0  B,  as  in  the  figure,  are  not  parts  of  one
straight  line,  the  foUowing  method  may  be  used,  being  an  extension
of  Rule  IV.  of  Article  79,  page  61,  and  having  the  effect  of  giving
a  pitch-line  made  up  of  four  circular  arcs,  whose  radii  deviate  less
from  equality  than  those  of  any  other  combination  of  four  circular
arcs  which  would  answer  the  same  purpose.
From  A  and  B,  perpendicular  to  A  C  and  B  C  respectively,
draw  A  D  and  B  D,  cutting  each  other  in  D.  These  will  be
tangents  to  the  required  pitch-line.  Join  C  D;  bisect  it  in  iE;  and
about  E,  with  the  radius  E  C  =  ED,  describe  a  circle  which  will
traverse  the  four  points  A,  C,  B,  D.  Bisect  the  arc  A  C  B  in  G.
