on  the  Translation  of  a  Vibrating  Body.  279 
off".     Although  when  I  did   so  I  was   only  about  a  foot  from 
fork  1,  yet  the  cork  ball  remained  at  rest  agaiust  its  prong. 
Exp.  3. — Again  I  walked  towards  1  as  in  experiment  2,  but 
I  did  not  remove  the  fork  from  its  case  after  it  was  placed  on  it. 
The  ball  remained  at  rest  until  the  moment  I  suddenly  stopped 
walking;  but  at  that  instant  the  ball  flew  from  the  fork,  while 
an  assistant,  whose  ear  was  close  to  the  case  of  fork  1,  while  his 
eye  was  directed  to  the  screen,  found  that  at  the  instant  I  stopped 
walking  the  fork  No.  1  sounded,  while  the  ball  jumped  from  its 
prong. 
Exps.  4  and  5. — These  experiments  were  exactly  like  2  and  3, 
except  that  I  walked  away  from  fork  1  instead  of  approaching 
it.     The  results  were  the  same  as  in  exps.  2  and  3. 
Exp.  6. — Fork  No.  3,  giving  254  vibrations  per  second,  was 
sounded  as  in  exp.  1.  It  had  no  effect  in  moving  the  ball.  I 
now  screwed  the  fork  off  its  case,  and,  standing  about  thirty  feet 
from  fork  1,  with  my  arm  I  swung  the  case  towards  fork  1,  and 
while  it  was  approaching  it  I  placed  fork  No.  3  on  its  case ;  the 
proper  velocity  (from  8  to  9  feet  per  second)  having  been  ob- 
tained, the  ball  was  suddenly  projected  from  fork  1.  On  greatly 
increasing  or  decreasing  the  above  velocity  of  the  moving  case, 
the  vibration  of  fork  3  produced  no  effect  on  fork  1. 
Exp.  7. — Fork  No.  4,  which  gave  two  vibrations  a  second 
more  than  No.  1,  was  substituted  in  exp.  6,  but  was  placed  on 
its  swinging  case  when  this  was  receding  from  fork  1.  Theeffect 
of  this  motion  and  of  varying  velocities  was  the  same  as  in  exp.  6. 
Exp.  8. — I  placed  fork  3  before  the  lantern,  and  swung 
fork  1  as  in  exp.  7 ;  the  effects  were  the  same  as  described  in 
exp.  7. 
Exp.  9. — I  now  placed  fork  4  before  the  lantern  and  moved 
fork  1  as  fork  3  was  moved  in  exp.  6.  The  effects  on  the  ball 
were  the  same  as  in  exp.  6. 
These  are  the  simple  means  I  have  arrived  at  to  show  the 
change  of  wave-length  produced  by  the  translation  of  the  vibra- 
ting body.  By  analogy  they  clearly  unfold  that  exquisite  modern 
method  of  determining  the  motions  of  a  heavenly  body  by  varia- 
tions in  the  refrangibility  of  the  rays  which  it  emits — motions 
often  impossible  even  to  detect  by  any  other  means.  I  there- 
fore deem  it  proper  that  I  should  proceed  to  state  the  delicate 
conditions  on  which  depend  the  perfection  of  experiments  which 
so  satisfactorily  elucidate  the  nature  of  those  grand  and  refined 
problems  offered  to  spectral  observation,  while  they  afford  an 
experimental  proof  of  the  important  theorem  that  Doppler  esta- 
blished in  1841. 
It  is  first  of  all  essential  that  forks  1  and  2  should  really  be 
in  unison.     Two  forks  sounded  together  may  give  no  perceptible 
