398  Intelligence  and  Miscellaneous  Articles. 
General  Hannyngton  has  shown  that  the  machine  of  De  Colmar  can 
be  employed  in  the  construction  of  astronomical  and  nautical  Tables. 
Looking  at  the  fact  that  calculating  machines  have  been  long  felt 
to  be  a  desideratum,  and  the  barren  results  which  have  unfortunately 
followed  the  labours  of  most  of  the  ingenious  pioneers  in  the 
achievement  of  their  perfection,  I  cannot  help  thinking  either  that 
sufficient  confidence  cannot  be  obtained  in  favour  of  machine-calcu- 
lation, or  that  they  are  not  placed  before  us  with  the  recommendations 
they  deserve. 
Tamworth  House, 
Mitcham  Common. 
ON  A  NEW  METHOD  OF  MEASURING  THE  VELOCITY  OF  ROTATION. 
BY  PROF.  A.  E.  DOLBEAR. 
While  experimenting  with  the  gyroscope,  I  have  often  wished  to 
know  its  velocity,  but  knew  of  no  way  to  determine  it  when  it  was 
set  in  motion  in  the  usual  way  with  a  string.  I  have  lately  found  a 
simple  and  exact  way  of  doing  this  ;  and  a  description  of  the  plan 
may  be  of  interest  to  others,  as  it  can  be  used  to  measure  the  velo- 
city of  wheels  of  every  size  and  every  possible  speed  without  incon- 
venience and  without  expense. 
If  a  short  piece  of  wire  be  soldered  to  the  end  of  one  branch  of  a 
common  tuning-fork,  one  end  of  the  wire  projecting  a  little  on  one 
side,  and  the  fork  made  to  vibrate  at  the  same  time  that  the  point  of 
the  wire  is  drawn  over  a  piece  of  smoked  glass,  an  undulating  line 
is  made  ;  and  if  the  rate  of  vibration  of  the  fork  is  known,  the  velo- 
city of  the  moving  hand  can  be  found  by  counting  the  number  of 
undulations  in  an  inch  on  the  glass.  This  has  been  used  to  deter- 
mine linear  velocity ;  but  it  can  also  be  applied  to  rotary  with  great 
precision.  I  have  a  large  fork  with  branches  8  inches  in  length  that 
vibrates  171  times  in  a  second  as  determined  by  a  resonant  tube  ;  it 
has  copper  wire  fastened  to  one  branch  and  projecting  about  one- 
fourth  of  an  inch, — made  for  showing  to  a  class  the  waving  line 
upon  smoked  glass.  A  brass  disk,  4  inches  in  diameter,  mounted 
gyroscopically,  was  smoked  upon  one  side  and  set  revolving  by 
a  string.  The  vibrating  fork  was  then  brought  to  it  in  such  a 
manner  that  its  vibrations  were  along  the  radius  of  the  disk,  but  as 
soon  as  it  touched  it  bounded  away,  and  nothing  could  be  deter- 
mined with  it.  A  cone  of  india-rubber,  about  a  quarter  of  an  inch 
in  height,  was  then  fastened  with  sealing-wax  near  one  end  of  the 
branch,  and  this  tried  as  before,  with  entirely  satisfactory  results,  the 
markings  being  well  defined  and  unmistakable. 
When  the  number  of  vibrations  the  fork  makes  a  second  is  known 
and  the  number  of  undulations  made  once  round  the  disk,  the  former 
number  divided  by  the  latter  will  give  the  rate  per  second  of  velocity. 
Thus,  if  the  fork  made  100  vibrations  in  a  second  and  the  disk  turned 
round  but  once  in  the  same  time,  it  is  evident  that  there  would  be 
a  hundred  undulations  on  it ;  and  if  the  disk  turned  fifty  times 
a  second,  there  would  be  but  two  undulations  for  each  revolution. 
