THE  LOCOMOTIVE  UP  TO  DATE  60J

The  pencil  mechanism  is  designed  to  afford  sufficient  strength
and  steadiness  of  movement,  with  the  utmost  lightness;  there-
by eliminating  as  far  as  possible  the  effect  of  momentum,  which
is  especially  troublesome  in  high  speed  work.  Its  fundamental
kinematic-principle  is  that  of  the  pantograph.  The  fulcrum  of
the  mechanism  as  a  whole,  the  point  attached  to  the  piston-rod,
and  the  pencil  point  are  always  in  a  straight  line.  This  gives
to  the  pencil  point  a  movement  exactly  parallel  with  that  of
the  piston.  The  mechanism  is  theoretically  correct  as  well  as
mechanically  accurate ;  the  result  is,  therefore,  mathematical  pre-
cision in  the  pencil  movement,  not  merely  an  approximation.
The  movement  of  the  spring  throughout  its  range  bears  a  con-
stant ratio  to  the  force  applied;  and  the  amount  of- the  move-
ment of  the  piston  is  multiplied  six  times  at  the  pencil  point.
The  pencil  lever,  links,  and  pins  are  all  made  of  hardened  steel ;
the  latter — slightly  tapering — are  ground  and  lapped  to  fit  ac-
curately, without  perceptible   friction  or  lost  motion.
Springs.  In  order  to  obtain  a  correct  diagram,  the  height
of  the  pencil  of  the  indicator  must  exactly  represent  in  pounds

per  square  inch  the  pressure  on  the  piston  of  the  steam  engine  at

•

every  point  of  the  stroke ;  and  the  velocity  of  the  surface  of  the
drum  must  bear  at  every-  instant  a  constant  ratio  to  the  velocity
of  the  piston.  These  two  essential  conditions  have  been  attained
at  a  great  degree  of  exactness  in  the  Crosby  indicator,  by  a
very  ingenious  construction  and  nice  adaptation  of  both  its
piston  and  drum  springs.
