What  has  been  said  of  the  small  machine  applies  in  general
also  to  medium  sized  engines.  Vertical  machines  here  possess
the  added  advantage  that  it  is  easier  to  dismount  them  by  means
of  overhead  crane  than  is  the  case  with  horizontal  machines.
The  limit  to  a  vertical  engine  comes  in  the  head  room  required.
For  this  reason  all  of  the  very  large  machines,  as  well  as  medium
sized  double-acting  machines  which  require  a  cross-head,  are
horizontal.  Another  reason  that  may  be  cited  is  that  it  is  easier
to  operate  a  medium  sized  or  large  horizontal  engine  than  it  is  a
vertical  because  all  climbing  or  mounting  platforms  is  avoided,
and  the  whole  installation  is  more  completely  under  the  operator's
eye.  Finally,  the  use  of  some  of  the  industrial  power  gases  favors
the  use  of  the  horizontal  machine,  because  any  dust  carried  can
be  much  more  easily  swept  out  of  a  horizontal  than  a  vertical
cylinder   during   regular   operation.
The  double-acting  engine  is  perhaps  not  used  as  widely  as  it
deserves  to  be,  increase  in  power  being  generally  sought  by  multi-
plying the  single-acting  cylinders.  There  is,  however,  to-day  no
reason  why  double-acting  cylinders  are  not  as  reliable  as  the
single-acting.  For  large  machines,  double-acting  cylinders  are
almost  an  economic  necessity,  and  the  clumsy  four-cylinder
double-opposed  large  engine  has  become  thoroughly  obsolete.
The  largest  engines  of  to-day  are  double  or  twin  two-cylinder
tandem  double-acting  engines.
