21.  Autographic  Diagrams. — In  various  instruments  used
in  testing,  a  diagram  is  drawn  automatically,  in  which  the  ab-
scissa corresponds  to  the  space  passed  through,  the  ordinate
to  the  force  exerted,  and  the  area  to  the  work  done.  A
familiar  illustration  is  the  steam-engine  indicator-diagram,  in
which  horizontal  distance  corresponds  to  the  stroke  of  the
piston  of  the  engine,  and  vertical  distance  or  ordinates  to  the
pressure  acting  on  the  piston  at  any  point.  The  absolute
amount  of  the  pressures  may  be  determined  by  reference  to  the


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1 6  EXPERIMENTAL   ENGINEERING.  [§

unnecessary  to  attempt  greater  accuracy  of  computation  t
can  be  carried  out  by  a  four-place  table  of  logarithms,  exc
in  cases  where  the  units  of  measurements  are  very  small  \
the  numbers  correspondingly  great.  In  general,  suffici
accuracy  can  be  secured  by  the  use  of  the  pocket  slide-r
the  readings  of  which  are  hardly  as  accurate  as  a  three-pl
table  of  logarithms.  The  slide-rule  will  be  found  of  great  <
venience  in  facilitating  numerical  computations,  and  its  us
earnestly  advised.
