connection  with  the  selection  of  this  machinery.  There  is  no  defi-
nite law  by  which  the  selection  of  machinery  for  any  plant  can
be  reduced  to  an  exact  science,  and  several  combinations  of  ma-
chinery are  possible  in  almost  any  plant  and  will  give  reasonable
satisfaction.
In  the  above  discussion  only  units  of  a  uniform  capacity  have
been  considered  and  it  is  usually  desirable,  other  things  being  equal,
to  have  similar  machines  so  that  a  minimum  number  of  repairs
and  duplicate  parts  may  be  kept  in  stock.  On  the  other  hand,  if  a
long,  low  night  load  is  probable,  it  may  be  desirable  to  install  one
or  more  units  of  a  capacity  suitable  to  carry  such  load  efficiently.
264.  Possibilities  in  Prime  Movers. — A  third  matter  for  the
careful  consideration  of  the  designing  engineer  is  the  possibility
of  a  prime  mover  that  is  to  be  used  for  operating  the  machines  in
question.  If  a  steam  or  gas  engine  is  to  be  used  as  the  motive
power,  there  is  a  wide  range  of  selection  in  speed,  capacity,  and
economy  of  such  machinery,  and,  as  a  general  rule,  the  prime  mover
may  be  selected  to  conform  to  the  generator  or  other  machine  that
is  to  be  operated  thereby.  In  the  selection  of  "v^^ter  wheels  for
prime  movers  the  conditions  are  radically  diflferent  and  the  selection
of  the  size  and  capacity  of  the  units  to  be  operated  is  often  modi-
fied or  controlled  by  the  waterwheels  and  the  conditions  under
which  they  will  be  obliged  to  operate.
