396  Mr.  Mond  and  Dr.  Wilderman  on  a 
Our  adjustable  fly  is  an  improved  type  of  Breguet  governor. 
Its  purpose  is  a  double  one  :  first,  to  counteract  all  small 
irregularities  o£  speed ;  secondly,  to  adjust  the  absolute  value 
of  the  time  of  each  revolution  when  different  pairs  of  wheels 
are  used  or  when  the  springs  in  the  natural  course  of  time 
vary  in  elasticity.  As  to  the  first  purpose,  it  should  be 
remarked  that  only  when  the  irregularities  in  the  movement 
of  the  clock  independent  of  the  governor  are  very  small,  is 
the  governor  effective  ;  when  the  variations  are  too  great  the 
governor  serves  no  purpose  and  is  a  decoration  only.  The 
adjustment  of  the  absolute  values  of  the  speed  of  each  revo- 
lution is  usually  made  as  follows.  When  the  wheels,  &c.  of  the 
instrument  are  properly  chosen,  the  final  adjustment  is  usually 
effected  by  employing  flexible  spirals  of  a  given  material 
and  of  a  certain  number  of  turnings.  This  adjustment  can, 
however,  only  be  a  rough  one,  and  what  is  still  worse,  even 
if  a  perfect  adjustment  is  once  effected  it  is  certain  not  to 
remain  perfect  after  being  in  use  for  some  time  owing  to  the 
variation  in  the  flexibility  of  the  springs,  to  the  effect  of  slow 
oxidation,  moisture,  fatigue,  &c=  So  also  when  exchange- 
wheels  are  used,  the  speed  is  not  in  proportion  to  the  number 
of  teeth  in  the  wheel,  as  is  often  assumed,  because  the  work  to 
be  spent  in  moving  the  clock  parts  and  the  spindle  is  not 
directly  proportional  to  the  speed  of  each  revolution;  i.e., 
each  pair  of  wheels  must  have  its  own  governor  which  is  to 
be  specially  arranged  for  the  given  speed.  To  meet  all  the 
above  requirements,  the  springs  of  the  governor  are  fixed 
as  shown  in  fig.  II.  (p.  399),  and  by  turning  the  screws  d' 
and  d"  they  can  be  considerably  increased  or  diminished  in 
tension ;  the  adjustment  of  speed  can  therefore  be  carried 
out  with  great  sensitiveness  and  without  difficulty. 
At  the  end  of  the  cross  arm  (Ji)  a  screw  (u)  is  arranged, 
which  together  wdth  the  nut  (r)  forms  a  balance-weight  at 
the  end  of  the  cross  arm.  This  arrangement  gives  an  adjust- 
able and  extremely  accurate  means  for  securing  an  exact 
balance  of  all  parts  of  the  spindle  while  in  position,  which, 
besides  the  clock,  is  the  only  movable  part  of  the  instrument. 
This  is  also  a  point  of  great  consideration  in  chronographs 
of  great  speed,  because  greater  speed  does  not  necessarily 
mean  a  corresponding  greater  accuracy.  When  a  clock 
moves  different  parts  of  an  instrument,  not  only  does  the 
clock  move  those  parts,  but  those  parts  also  move  the  clock. 
Since  it  is  exceedingly  difficult  to  get  the  wall  of  a  hollow 
cylinder  of  a  uniform  thickness  throughout,  it  being  a 
sufficiently  difficult  task  to  get  its  upper  surface  true  alone, 
the  drum  is  never  or  almost  never  completely  counterbalanced 
