Am'May,ri9Po9arm'}      Purification  of  Drinking  Water.  223 
hundred  and  twenty  units.  Each  unit  is  essentially  as  follows:  a 
rectangular  concrete  basin  about  eight  feet  deep,  twenty  and  one- 
half  feet  wide  and  sixty  feet  long.  Each  one  of  these  basins  is 
subdivided  into  twelve  small  compartments,  about  ten  feet  square, 
separated  from  each  other  by  partitions  which  form  gutters  about 
one  foot  wide  and  one  foot  deep.  The  compartments  are  arranged 
in  two  parallel  rows  of  six  each.  The  gutters  all  connect  with  the 
long  central  dividing  drain.  At  the  bottom  of  these  basins  per- 
forated pipes  are  arranged  for  carrying  air  and  water  to  the  bottom 
of  the  filtering  bed  of  sand  which  is  two  feet  in  depth.  There  is 
much  more  detail  than  is  here  stated  and  only  sufficient  is  given  to 
convey  understanding  of  the  operation  of  these  filters,  which  is  as 
follows : 
Water  is  run  from  the  storage  basin  with  but  short  time  for 
sedimentation  directly  upon  the  sand.  The  filtration  is  controlled, 
but  permitted  to  proceed  at  a  rapid  rate,  one  hundred  and  twenty 
tanks  delivering  two  hundred  and  forty  million  gallons  in  twenty- 
four  hours.  The  process  continues  until  the  collected  sediment 
causes  such  a  diminution  of  the  flow  that  it  is  no'  longer  efficient, 
then  water  is  shut  off  and  the  bed  allowed  to  drain,  after  which  it 
is  washed  ;  at  the  end  of  eight  minutes  it  is  again  in  commission 
and  restored  to  its  full  activity.  The  washing  is  accomplished  by 
the  attendant  who  stands  with  watch  in  hand  at  an  iron  table  upon 
which  is  arranged  a  maze  of  levers  and  switches.  He  operates  a 
switch,  immediately  the  whirr  of  an  electric  motor  begins,  followed 
by  the  rumble  of  the  direct  connected,  positive  blast,  rotary  air 
pump,  which  the  motor  drives.  The  next  instant  the  sand  in  the 
filter  begins  to  quiver  and  at  once  begins  to  bubble  and  boil  and  a 
lot  of  dirty  water  spills  over  into  the  gutters,  from  which  it  runs 
into  the  sewer.  The  air  is  continued  for  two  minutes ;  a  reversal 
of  the  same  switch  stops  the  air.  A  lever  is  next  moved,  at  once 
clean  water  begins  to  pour  into  the  bottom  of  the  filter,  and  for  one 
minute  a  flood  of  dirty  water  runs  out  at  the  top  into  the  gutters. 
This  ceases  and  again  for  two  minutes  the  air  boils  the  sand  as 
before,  breaking  up  the  mass  into  individual  grains  and  separating 
it  from  the  accumulated  mud.  Finally  the  water  is  again  introduced 
at  the  bottom  and  for  three  minutes  the  mud  is  washed  out  until 
clean  water  overflows.  The  celerity,  thoroughness,  and  economy  of 
this  method  are  but  little  short  of  absolute  perfection ;  in  every 
department  of  this  work  we  see  the  same  idea  carried  out,  the 
