539


a  bridge  or  temporary  platform  over  the  watercourse.  After
noting  the  reading  of  the  dial  he  places  the  meter  in  the  water
at  the  required  depth,  guided  by  a  graduation  on  the  rod,  A\ith
the  clockwork  out  of  gear.      An  assistant  holds  the  meter  in


Fig.  234.
Fig.  234(z.
place.  A  stop-watch  is  commonly  used  for  noting  the  time,
but  an  ordinary  watch  docs  very  well.  At  the  commencement
of  an  even  minute  the  observer  pulls  the  string  to  throw  the
vvhcelwork  in  gear.  After  the  lapse  of  a  suitable  number  of
seconds  he  throws  it  out,  raises  the  meter,  and  notes  the  dial.
540  MEASUREMENT   OE    WATER.
As  indicated  by  dotted  lines  the  instrument  admits  the  attach-
ment of  electric  wires  when  required.
An  Acoustic  Meter. — The  writer,  while  in  the  service  of
the  U.  S.  Engineer  Department,  had  occasion  to  observe
the  working  of  current-meters.  He  became  convinced  that
their  chief  source  of  difficulty  and  error  lay  in  the  means  used
for  conveying  their  indications  to  the  observer.  Especially  is
this  the  fact  when  the  revolving  shaft  operates  a  train  of  wheel-
work  from  which  sediment  cannot  be  excluded.  The  attempt
to  exclude  it  involves  the  use  of  a  stuffing-box,  which  is  in  itself
a  source  of  uncertain  and  varying  resistance.  The  electric
meter,  also,  owes  its  chief  difficulties  to  the  same  cause.  The
necessity  of  being  accompanied  by  attachments  of  wire,  battery,
and  dial  is  a  serious  limitation  on  its  usefulness,  and  the  main-
tenance of  the  current,  unless  in  the  hands  of  trained  elec-
tricians, is  a  source  of  continual  trouble,  vexation,  and  delay.
