It  will  be  noticed  that  the  area  MFF'B,  enclosed  between  the  base
and  the  curve  and  shown  shade-lined,  is  the  diagram  of  work  done
upon  the  crank-pin  during  a  half-revolution  of  the  crank-shaft,  the
ordinates  representing  the  turning  effort  in  pounds,  and  the  base
representing  the  distance  through  which  the  effort  has  been  exerted  ;
and  this  area  is  exactly  equal  (neglecting  friction)  to  the  work  done
upon  the  piston  during  a  single  stroke,  as  given  by  the  indicator
diagram.  If  S  =  stroke  of  piston,  and  P  =  mean  pressure  on  piston,

then  S  X  -  =  path  of  crank-pin  during  one  stroke  of  piston,  and  MK,

o
mean  tangential  pressure  on  crank-pin,  =  P  x  -


=  (pxJ)x(Sx|)


The  line  KL  (Fig.  265)  has  been  called  the  line  of  mean  tangential
pressure,  but  it  is  also  the  line  of  resistance  due  to  the  load  (assuming


the  load  constant) ;  for  unless  the  mean  effort  and  the  mean  resistance
are  equal,  there  must  be  a  change  of  speed  of  revolution  of  the  engine,
the  speed  increasing  or  decreasing  as  the  effort  is  greater  or  less  than
the  resistance.
