Catch-pins,  x,  are  provided,  to  prevent  the  beam  descend-
ing too  far  should  the  engine  make  too  long  a  stroke  ;  two
wooden  springs,  yy,  receive  the  blow.  The  great  beam  is
carried  on  sectors,  zz,  to  diminish  losses  by  friction.
The  boilers  of  Newcomen's  earlier  engines  were  made  of
copper  where  in  contact  with  the  products  of  combustion,
and  their  upper  parts  were  of  lead.  Subsequently-  sheet-
iron  was  substituted.  The  steam-space  in  the  boiler  was
made  of  8  or  10  times  the  capacity  of  the  cylinder  of  the
engine.  Even  in  Smeaton's  time,  a  chimney-damper  was
not  used,  and  the  supply  of  steam  was  consequently  very
variable.  In  the  earlier  engines,  the
cylinder  was  placed  on  the  boiler  ;
afterward,  they  were  placed  sepa-
rately, and  supported  on  a  founda-
tion of  masonry.  The  injection  or
"  jack-head  "  cistern  was  placed  from
12  to  30  feet  above  the  engine,  the
velocity  due  the  greater  altitude
being  found  to  give  the  most  perfect
distribution  of  the  water  and  the
promptest  condensation.
Smeaton  covered  the  lower  side
of  his  steam-pistons  with  wooden
plank  about  2±  inches  thick,  in  order
that  it  should  absorb  and  waste  less
heat  than  when  the  iron  was  directly

exposed  to  the  steam.  Mr.  Beighton  was  the  first  to  use  the
water  of  condensation  for  feeding  the  boiler,  taking  it  di-
rectly from  the  eduction-pipe,  or  the  "hot-well."  Where
only  a  sufficient  amount  of  pure  water  could  be  obtained  for-


Fie.  22  ew

Engine,  lies.
68       THE  STEAM-ENGINE   AS  A  TRAIN   OF  MECHANISM.
feeding  the  boiler,  and  the  injection-water  was  "  hard,"  Mr.
