WINDMII,I.S   AND    PUMPS.  369

Pumps. — There  are  four  distinct  types  of  pumps — ,
the  plunger  or  piston  pump,  which  includes  the  wind-
mill, steam,  and  many  devices  of  power  pumps  ;  the
vacuum,  the  rotary,  and  the  centrifugal,  besides  ele-
vators which  raise  water  by  means  of  flights  attached
to  an  endless  chain.  The  plunger  pump,  of  necessity,
moves  the  water  more  slowly,  as  it  only  travels  at  the
speed  of  the  piston.  The  plunger  pump  also  is  de-
signed especially  for  handling  clear  water — grit,  sand,
and  foreign  material  cut  the  pistons  and  barrel  of  the
pump.  While  these  pumps  will  move  the  water
slowly,  they  will  move  it  a  long  distance,  or  against
heavy  pressure  when  properly  designed.  The  pumps
of  next  greatest  capacity  are  the  rotary  pumps.  Of
these  there  are  many  designs.  They  handle  water
much  faster  than  do  plunger  pumps,  but  as  it  is  essen-
tial that  the  working  parts  of  these  pumps  should  fit
closely,  there  is  necessarily  great  fridlion  and  corre-
sponding loss  of  efiiciency,  and  hence  they  are  short-
lived, especially  when  pumping  water  that  is  muddy
or  gritty.  The  pumps  of  greatest  utility  for  low  lifts
are  the  centrifugal  pumps.  These  are  built  with  no
close-fitting  parts  and  no  valves  ;  consequently  there  is
no  fridlion  on  the  parts  of  the  machinery,  and  they  are
not  affected  by  sand,  mud,  or  gritty  water.  Hence,
for  irrigation,  where  the  lift  does  not  exceed  fifty  feet,
centrifugal  pumps  are  recognized  by  all  hydraulic  en-
gineers as  the  most  efficient  and  durable,  the  cheapest
and  best.  The  vacuum  pump  is  an  entirely  different
principle,  having  no  movable  parts,  except  a  small
automatic  shifting-bar  in  the  yoke  to  operate  the
valves.     These  pumps  are  made  with  a  pair  of  cylin-


370


IRRIGATION   FARMING.
