than  half  of  its  stroke  to  the  right.  A  further  movement  of  the
valve  in  the  direction  of  the  arrow  will  close  the  left-hand  port  to
the  admission  of  steam,  that  is,  cut-off  takes  plaice.  Expansion
will  now  commence  in  the  head-end  of  the  cylinder,  and  the  force
of  the  expanding  steam  will  cause  the  piston  to  continue  its
movement  to  the  right.
Compression.  As  the  position  of  the  piston  in  Fig.  17  is
reached,  the  valve  will  have  moved  to  cover  the  right-hand  port,
as  shown  in  the  illustration.  The  unexhausted  steam  trapped  m
the  crank  end  of  the  cylinder  will  be  compressed  by  the  expansive
force  of  the  steam  previously  admitted  to  the  head^end  of  the
cylinder  acting  upon  the  piston  and  moving  it  to  the  right.  Ex-
pansion, theoretically,  should  cease  immediately  at  the  point  of
release,  but  in  practice  there  is  a  gradual,  although  rapid,  de-


THE  LOCOMOTIVE  UP  TO  DATE


55


crease  in  pressure  on  the  piston,  causing  a  slight  prolongation  of
expansion.
Release.  Because  the  valve  shown  in  these  illustrations  is  line-
and-line  with  the  steam  ports,  that  is,  it  has  no  exhaust  lap  or
clearance,  the  point  of  the  commencement  of  compression  in  the
crank-end  corresponds  to  the  point  of  release  in  the  head-end.
Fig.    17.
If  the  valve  had  exhaust  lap,  release  would  follow  compression ;
if  it  had  clearance,  release  would  take  place  before  compression
began.  This  is  readily  understood  by  referring  to  Fig.  17.  Ex-
haust will  continue  from  the  point  of  release  shown  in  Fig.  17
at  the  left-hand  port,  until  the  point  of  <:ompression  on  the  return
stroke  of  the  piston.
Admission — Crank-end.     Compression  will  continue  from  the
point  shown  in  Fig.  17  to  that  shown  in  Fig.  18,  where  the  valve


Fig.    18.
