222  Purification  of  Drinking  Water.       { Am^™ \$o*rm' 
hinged  to  the  pipe  that  supplies  the  raw  water  after  sedimentation 
to  the  preliminary  filter.  Both  of  the  floating  drums  are  located  at 
the  near  side  of  their  respective  basins  and  their  function  is  to  skim 
the  cleanest  water  from  the  surface  of  the  reservoir.  This  portion 
of  the  water  being  at  the  surface,  carries  the  dissolved  oxygen 
which  is  necessary  in  subsequent  steps  of  the  purification. 
As  pointed  out,  two  principal  considerations  attend  the  purifica- 
tion of  river  water,  namely,  the  removal  of  suspended  matter  and 
the  decomposition  of  soluble  organic  matter.  The  latter  being  the 
most  mysterious  would  seem  the  most  difficult,  yet  as  a  matter  of 
fact  the  reverse  is  true.  Mr.  John  W.  Hill,  formerly  chief  engineer 
in  charge  of  the  work,  personally  told  me  that  the  slow  sand  filters 
could  handle  water  carrying  forty  parts  of  suspended  matter  per 
million  without  difficulty,  but  above  that  turbidity  the  trouble 
increased  with  the  increase  in  turbidity.  This  fact  made  imprac- 
ticable the  accomplishment  of  both  purposes  in  the  same  filter,  upon 
an  economical  basis.  The  necessity  for  preliminary  filtration,  at  a 
rapid  rate,  solely  for  the  removal  of  suspended  matter,  soon  made 
the  preliminary  filter  the  chief  study.  Beginning  with  Roxborough 
where  the  first  type  was  employed,  there  becomes  apparent  under 
inspection  a  steady  advance  in  construction  and  efficiency  until  the 
highest  order  is  found  in  the  last  built  at  Torresdale.  Here  every 
operation  is  carried  out  on  a  gigantic  scale,  and  with  superb  refine- 
ment of  detail,  in  comparison  with  which  the  previous  efforts  are 
palpably  crude.  The  most  notable  example  is  found  in  a  considera- 
tion of  the  two  methods  followed  for  the  removal  of  the  sediment 
when  the  filter  becomes  clogged.  At  Belmont  they  have  rigged  a 
travelling  platform  which  spans  the  bed,  mounted  upon  wheels  that 
run  upon  tracks  secured  at  the  top  of  the  dividing  wall.  Upon 
this  travelling  platform  is  placed  an  electrically  driven  suction 
apparatus  that  lifts  the  deposit  of  mud  from  the  surface  of  the  bed 
and  ejects  it  into  a  sewer.  It  is  possible  with  this  device  and  the 
aid  of  several  men  to  remove  the  deposit  from  one  filter  in  about 
five  hours.  At  Torresdale  the  same  operation  requires  one  man 
just  eight  minutes  by  the  watch.  This  seems  almost  incredible,  yet 
it  is  absolutely  true,  and  it  was  only  brought  into  existence  through 
the  highest  order  of  engineering  skill,  being  the  result  of  evolution, 
experience,  and  observation  gained  in  work  that  had  preceded  it. 
The  preliminary  filter  beds  at  Torresdale  are  radically  different 
from  either  those  at  Roxborough  or  at  Belmont  and  comprise  one 
