3  definitions  found 
 
  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
 
  Pendulum  \Pen"du*lum\,  n.;  pl  {Pendulums}.  [NL.,  fr  L. 
  pendulus  hanging,  swinging.  See  {Pendulous}.] 
  A  body  so  suspended  from  a  fixed  point  as  to  swing  freely  to 
  and  fro  by  the  alternate  action  of  gravity  and  momentum.  It 
  is  used  to  regulate  the  movements  of  clockwork  and  other 
  machinery. 
 
  Note:  The  time  of  oscillation  of  a  pendulum  is  independent  of 
  the  arc  of  vibration,  provided  this  arc  be  small 
 
  {Ballistic  pendulum}.  See  under  {Ballistic}. 
 
  {Compensation  pendulum},  a  clock  pendulum  in  which  the  effect 
  of  changes  of  temperature  of  the  length  of  the  rod  is  so 
  counteracted,  usually  by  the  opposite  expansion  of 
  differene  metals,  that  the  distance  of  the  center  of 
  oscillation  from  the  center  of  suspension  remains 
  invariable;  as  the  mercurial  compensation  pendulum,  in 
  which  the  expansion  of  the  rod  is  compensated  by  the 
  opposite  expansion  of  mercury  in  a  jar  constituting  the 
  bob;  the  gridiron  pendulum,  in  which  compensation  is 
  effected  by  the  opposite  expansion  of  sets  of  rodsof 
  different  metals. 
 
  {Compound  pendulum},  an  ordinary  pendulum;  --  so  called  as 
  being  made  up  of  different  parts  and  contrasted  with 
  simple  pendulum. 
 
  {Conical}  or  {Revolving},  {pendulum},  a  weight  connected  by  a 
  rod  with  a  fixed  point;  and  revolving  in  a  horizontal 
  cyrcle  about  the  vertical  from  that  point. 
 
  {Pendulum  bob},  the  weight  at  the  lower  end  of  a  pendulum. 
 
  {Pendulum  level},  a  plumb  level.  See  under  {Level}. 
 
  {Pendulum  wheel},  the  balance  of  a  watch. 
 
  {Simple}  or  {Theoretical},  {pendulum},  an  imaginary  pendulum 
  having  no  dimensions  except  length,  and  no  weight  except 
  at  the  center  of  oscillation;  in  other  words  a  material 
  point  suspended  by  an  ideal  line 
 
  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
 
  Hydrometer  \Hy*drom"e*ter\,  n.  [Hydro-,  1  +  -meter:  cf  F. 
  hydrom[`e]tre.] 
  1.  (Physics)  An  instrument  for  determining  the  specific 
  gravities  of  liquids,  and  thence  the  strength  spirituous 
  liquors,  saline  solutions,  etc 
 
  Note:  It  is  usually  made  of  glass  with  a  graduated  stem,  and 
  indicates  the  specific  gravity  of  a  liquid  by  the  depth 
  to  which  it  sinks  in  it  the  zero  of  the  scale  marking 
  the  depth  to  which  it  sinks  in  pure  water.  Extra 
  weights  are  sometimes  used  to  adapt  the  scale  to 
  liquids  of  different  densities. 
 
  2.  An  instrument,  variously  constructed,  used  for  measuring 
  the  velocity  or  discharge  of  water,  as  in  rivers,  from 
  reservoirs,  etc.,  and  called  by  various  specific  names 
  according  to  its  construction  or  use  as  {tachometer}, 
  {rheometer},  {hydrometer},  {pendulum},  etc.;  a  current 
  gauge. 
 
  From  WordNet  r  1.6  [wn]: 
 
  pendulum 
  n  :  an  apparatus  consisting  of  an  object  mounted  so  that  it 
  swings  freely  under  the  influence  of  gravity 

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