and  Registering  the  Force  and  Direction  of  the  Wind.       35 
When  the  wind  changes,  the  pin  fixed  on  the  vertical  axis 
moves  forward  till  it  comes  to  one  of  the  stops ;  then,  pushing 
on  the  stop,  it  moves  the  crown  wheel  and  causes  the  disks  to 
revolve  and  raise  the  weight.  As  soon,  however,  as  the  weight 
has  passed  the  top,  it  falls  over,  moving  the  crown  wheel  and 
causing  the  stop  to  gain  a  little  on  the  pin ;  the  disks  then 
remain  at  rest  till  the  pin  has  again  overtaken  the  stop. 
The  disks  are  so  adjusted  that  contact  is  always  commenced 
after  the  weight  has  begun  to  fall,  and  finished  before  the  weight 
has  quite  done  falling. 
Thus  a  perfectly  uniform  and  steady  contact  is  obtained. 
The  Dial  and  Registering  Instrument  (figs.  3  and  4). 
The  Dial. — On  the   stand  of  the  dial  instrument   are  two 
electromagnets  placed  horizontally.     Opposite  the  poles  of  these 
are  armatures  hinged  to  the  base;  these  form  parts  of  levers,  the 
upper  parts  of  which  are  of  thin  brass. 
At  the  top  of  each  lever  is  a  kind  of  brass  claw  hinged  so  that 
it  can  rise  a  little,  but  prevented  by  the  hinge  from  falling  below 
a  certain  position.  Fixed  rather  lower  down  on  the  lever  is  an- 
other brass  claw  of  the  form  shown ;  opposite  to  the  claw  is  a 
ratchet-wheel  with  sixteen  teeth  (as  shown).  When  a  current 
passes  in  the  magnet  the  armature  is  attracted,  the  point  moves 
the  ratchet-wheel  on  one  tooth,  the  hinge  allowing  it  to  rise  over 
the  top  of  the  wheel ;  at  the  same  time  the  stop  (or  lower  claw) 
prevents  it  moving  more  than  one  tooth.  When  the  current 
ceases,  the  armature  is  drawn  back  by  a  spring ;  a  set  screw  pre- 
vents the  armature  going  back  too  far.  The  two  wheels  and  the 
hand  are  fixed  on  one  axis,  the  wheels  being  placed  as  near 
together  as  the  thickness  of  the  magnets  will  allow. 
A  slight  spring  with  a  friction-roller  at  the  end  of  it  is  so 
arranged  that  the  roller  rest3  between  two  of  the  teeth  of  one 
of  the  ratchet-wheels ;  this  keeps  the  wheel  always  in  the  right 
position  for  receiving  the  push  on  the  lever.  The  wheels  are 
made  adjustable  on  the  axis  by  means  of  set  screws,  partly  that 
the  roller  in  putting  one  wheel  into  position  may  also  adjust  the 
other,  and  partly  that  the  hand  may  be  made  to  rest  at  the  even 
points. 
One  magnet  drives  the  hand  in  one  direction ;  the  other  drives 
it  in  the  other  direction. 
This  is  the  indicating  part  of  the  instrument;  it  can  be  used 
without  the  registering  part.  When  registration  is  required  the 
following  arrangement  is  used. 
The  Printing  apparatus. — The  axis  of  the  hand  is  prolonged 
backwards,  and  a  type- wheel  embossed  with  the  sixteen  points  of 
the  compass  is  fixed  on  it  in  such  a  position  that  the  point  to 
D2 
