the  head  of  each  lateral,  so  that  the  sewage  is  well
diluted  by  the  time  it  reaches  the  final  outlet,  very
little  solid  matter  remaining.  The  sewage  might  just
as  well  be  delivered  into  open  ditches  from  the  siphon
catchment,  and  these  could  serve  as  head  ditches  at  the
land  to  be  irrigated,  provided,  of  course,  the  grade
would  be  sufficient.  In  winter  the  surplus  sewage
might  be  condudled  to  various  reservoirs,  where  it
could  be  stored  or  allowed  to  seep  away  as  desired.
In  selecfting  ground  for  a  sewage  farm  account
must  be  taken  of  the  relative  elevation  of  the  land,  and
of  the  town,  manufacfturing  establishment,  or  residence
from  which  the  material  comes.  Whenever  possible,
as  a  matter  of  economy,  the  farm  should  be  seledled


396  IRRIGATION   FARMING.
with  reference  to  the  sewage  reaching  it  by  gravity.
If,  however,  the  location  does  not  admit  of  such  pro-
cedure, pumps  may  be  utilized,  although  this  fre-
quently will  entail  considerable  additional  expense  in
first  cost  of  plant  as  well  as  in  the  annual  outlay  for
operation  and  maintenance.  In  some  cases,  where
land  can  be  reached  by  gravity  by  going  considerable
distance  or  can  be  covered  by  pumping  within  a  short
distance,  carefully  prepared  estimates,  taking  into
account  all  the  elements  of  first  cost,  as  well  as  the
annual  cost  of  maintenance  and  operation,  may  show
that  it  is  cheaper  to  deliver  the  sewage  a  long  distance
by  gravity  than  a  shorter  distance  by  pumping.
