280  Dr.  A.  J.  Mayer's  Acoustical  Experiments 
beats ;  or  they  may  constrain  each  other  into  a  common  forced 
oscillation;  and  thus  both  will  give  the  same  number  of  vibrations, 
yet  may  be  removed  from  equality  when  separately  sounded. 
The  process  I  have  adopted  is  as  follows  : — Three  forks  are  taken 
that  are  supposed  to  make  the  same  number  of  vibrations  in  a 
given  time.  They  are  supported  on  india-rubber  tubing  and 
are  thus  insulated.  One  of  the  forks  is  now  loaded,  so  that  it 
gives  two  or  three  beats  in  a  second  with  one  of  the  other  two 
that  are  to  be  brought  into  exact  unison.  The  interval  of  time 
occupied  by  twenty  or  thirty  of  these  beats  is  accurately  mea- 
sured by  means  of  a  chronograph  (one  of  CasehVs  registering 
stop-watches  does  well).  The  interval  occupied  by  the  same 
number  of  beats  given  by  the  second  fork  is  now  ascertained; 
and  if  it  differs  from  that  given  by  the  first,  the  quicker  vibrating 
fork  is  made  to  give  the  same  number  of  beats  as  the  slower  by 
loading  it  with  wax.  When  the  forks  have  thus  been  carefully 
adjusted,  I  have  had  no  difficulty  in  projecting  the  ball  in  exp.  1 
at  a  distance  of  60  feet ;  and  I  believe  that  it  could  have  been 
accomplished  at  a  distance  of  100  feet. 
The  ball  of  cork  should  be  spherical,  so  that  it  will  always  just 
touch  the  fork,  no  matter  how  it  may  rotate  around  its  suspend- 
ing thread,  which  latter  should  consist  of  only  one  or  two  fibres 
of  unspun  silk.  The  cork  is  rendered  as  smooth  as  possible,  and 
is  then  varnished.  This  is  important ;  for  the  varnish  gives  a  firm 
coating  to  the  ball  without  sensibly  increasing  its  weight,  and  is 
especially  useful  in  covering  the  minute  asperities  or  elastic  pro- 
jections on  its  surface,  which  otherwise  would  act  as  "  buffers  " 
to  the  impacts  of  the  fork  and  deaden  its  projectile  effects. 
The  above-stated  conditions  having  been  attained,  no  physicist 
will  have  any  difficulty  in  repeating  these  experiments. 
A  machine  has  been  devised  by  which  a  uniform  motion  of 
translation  can  be  given  to  the  forks ;  and  with  this  I  propose 
making  a  quantitative  investigation  of  the  phenomena,  using  an 
apparatus  essentially  the  same  in  its  action  as  the  one  here 
described. 
We  may  substitute  for  the  suspended  cork  ball  a  light  plane 
mirror  held  between  two  stretched  vertical  fibres  while  one  of  its 
edges  just  touches  the  fork.  The  motions  of  a  beam  of  light 
reflected  from  the  mirror  to  a  screen  indicate  most  beautifully 
the  vibrations  of  the  fork.  This  ingenious  and  most  delicate 
arrangement  for  indicating  vibrations  is  due  to  Professor  O.  N. 
Rood,  of  Columbia  College,  New  York,  who  first  used  it  in  a 
public  lecture  delivered  in  New  York  city  on  the  28th  of  last  De- 
cember. We  have,  however,  in  our  special  work  found  the  image 
of  the  projected  ball  more  convenient  and  sufficiently  delicate  for 
our  experiments. 
