The  plumb-bobs  are  sometimes  hung  down  hatchways,  or  they
may  be  suspended  from  standards  on  deck,  so  that  all  observers  will
be  under  the  eye  of  the  one  responsible  for  the  test,  who  can  at  the
same  time  personally  verify  the  location  of  the  weights.  On  a  large
ship  one  responsible  observer  can  pass  from  place  to  place  without
vitiating  results.  To  avoid  swinging  of  the  plumb-bobs  in  the  wind
they  may  be  boxed  in,  leaving  only  a  place  for  the  observation  of  the
motion  of  the  line  of  the  plumb-bob  over  the  divided  horizontal  scale.
The  following  precautions  must  be  taken  to  insure  satisfactory
results:

1.  The  ship  must  be  in  quiet  water;  a  basin  or  a  dock  is  to  be
preferred.
2.  There  should  be  but  little  wind,  and  the  ship  should  be  placed
with  the  head  (or  stem)  to  the  wind.  This  condition  is  often  un-
avoidably violated.
3.  All  lines  from  the  ship  to  the  shore  should  be  cast  off  or  made
slack.    A  head  or  a  stem  line  to  prevent  drifting  may  be  allowed.
4.  The  hold  should  be  pumped  dr}',  and  all  loose  objects  and  ma-
terials that  are  liable  to  shift  should  be  removed  or  secured;  the
observers  and  the  laborers  for  moving  the  weights  must  be  in  ap-
pointed positions  when  observations  are  taken.  Persons  not  needed
for  the  test  should  be  sent  ashore.  Failure  to  follow  this  condition
scrupulously  is  more  liable  to  give  unsatisfactory  results  than  any
other  circumstance.
ADDING  AND  MOVmG   JVEIGHTS,


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5.  Th€  draught  of  the  ship  must  be  measured  at  the  how  and
stem,  to  afford  data  for  the  adcuktions  of  the  locations  of  the  centre
of  buoyancy  and  the  metacentre.
