The  ordinary  process  of  throwing  a  stone  and  bowling  a  cricket-
ball  is  of  much  the  same  nature,  but  the  hand  then  only
describes  a  portion  of  a  circle  of  which  the  shoulder  is  the
centre.  In  all  such  cases,  the  main  difficulty  is  to  judge
correctly  the  proper  moment  at  which  the  ball  or  the  stone  is
to  be  set  free  to  follow  the  direction  in  which  its  inertia  will
keep  it  moving.
130.  In   rapidly-moving   parts   of  machinery  there   is   often
great  danger  of  breakage,  owing  to  the  high  centrifugal  tendency
of  those  portions  which  are  at  a  great  distance  from  the  axis  of
rotation.     Thus,  large  fly-wheels  have  been  known  to  break  to
pieces  when  the  engine  has  from  any  cause  begun  to  move  faster
than  usual.     Similarly,  the  massive  grindstones  used  in  cutlery


54  ELEMENTARY  DYNAMICS.
works  sometimes  break  up  without  any  warning,  and  the  flying
fragments  cause  great  damage  owing  to  their  mass  and  velocity,
like  stones  from  an  immense  sling.*

131.  Wet  mops  are  easily  and  quickly  dried  by  twirling  them
rapidly  round  on  their  handles  as  axes  of  rotation  ;  the  particles
of  water  travel  to  the  extremities  of  the  woollen  strands,  their
centrifugal  tendency  increasing  as  they  get  farther  from  the  centre
(Law  IV.),  and  then  they  fly  off  in  well-marked  tangent  lines.
Mud  is  thrown  off  from  a  quickly -moving  carriage  wheel  in  a
similar  way.
