Velocities  of  Cannon  Balls,  See.  57 
■which  was  fattened  to  a ftrong  iron  ftem  on  the  hack 
part  of  it  by  fcrew-bolts,  having  a thick  iron  axis  at  the 
top,  whofe  ends  were  turned  truly  cylindrical,  to  roll 
pretty  freely  in  fockets  made  to  receive  them ; the  whole 
being  fupported  by  a four-legged  ftand  of  very  ftrong 
timber,  which  -was  firmly  fixed  in  the  ground,  a is  the 
face  of  the  cube  into  which  the  bails  were  fired;  by 
means  of  the  blow  it  is  made  to  fwing  round  the  axis 
bc,  and  the  chord  of  the  arch  thereby  deferibed  is  mea- 
fured  by  the  tape  def  fattened  to  the  bottom  of  the  wood 
at  D,  and  Aiding  with  fome  flight  friction  through  a 
little  machine  of  brafs,  fixed  at  e for  that  purpofe,  the 
tape  being  marked  with  inches  and  tenths,  for  the  more 
eafily  meafuring  of  the  chord  or  part  of  it  drawn  through 
by  the  pendulum.  The  whole  length  of  this  pendulum, 
from,  the  middle  of  the  axis  to  the  ribband  at  d,  was 
1 9 2 j inches.  The  weight  and  the  other  djmenfions  were 
taken  each  day  when  the  experiments  were  made,  and 
then  regiftered;  and  the  manner  of  difeovering  the 
places  of  the  centers  of  gravity  and  ofcillation  was  as 
follows : 
To  find  the  center  of  ofcillation,  the  pendulum  was 
hung  up,  and  made  to  vibrate  in  fmall  arcs,  and  the  time 
of  making  two  or  three  hundred  vibrations  was  obferved 
by  a half-fecond  pendulum.  Having  thus  obtained  the 
Vol„  LXVIII.  I time 
