Draw  A  Y,  making  the„angle  CAY  equal  to  the  angle
which  the  screw  fnaK$^^  a  plane  perpendicular  to  its
axi&  Draw  C  Y  mlChdicular  to  A  C,  cutting  A  Y  in
Y,  and  Y  Z  perpwiilyar  to  A  Y,  cutting  A  C  produced
in  Z.  A  Z  ^1  be  the  required  radius  of  curvature.  Its
length  may  also  be  found  by  calculation,  as  follows : —


\         curcumferenceV


65.  xf«nMi  FUch.  {A.  M.,  472.)— By  the  normal  pitch  pjg,  31,
of  a  linear  screw  is  to  be  understood  the  distance  &om
one  coil  to  the  next,  measured,  not  parallel  to  the  axis,  but  along
the  shortest  line  on  the  cnrlindrical  surface  between  the  two  coils ;
that  is  to  say,  along  another  helix  or  linear  screw  which  cuts  all
the  coils  of  the  original  linear  screw  at  right  angles.  The  normal
jntch.  is  to  be  determined  &om  the  development  of  the  screw,  as
follows : — In  fig.  30,  from  c  let  fall  c  n  perpendicular  to  C  «;  en  will
be  the  normal  pitch.  The  straight  line  c  n  is  part  of  the  develop-
ment of  the  normal  hdix,  as  it  may  be  called.  When  produced,  it
cuts  E/,  the  development  of  the  next  coil,  in  0,  and  n  0  =  0  n  is  dso
the  normal  pitch.  By  finding  the  intersections,  such  as  p,  of  the
development  of  the  normal  helix  with  the  series  of  straight  lines
paralld  to  the  axis,  any  number  of  points,  such  as  P,  in  the  pro-
jection of  the  normal  helix,  0  N  O  P,  may  be  found  if  required.
