Prof.  A.  Cayley  on  a  Bicyclic  Chuck.  365 
of  the  magnetic  force  was  noticed  by  me  in  the  year  1841,  when 
I  had  put  up  magnetical  instruments  of  new  construction  and 
with  very  light  needles  which  were  enclosed  air-tight ;  with  the 
heavy  bars  formerly  in  use  movements  of  such  small  durations 
could  not  be  observed. 
[To  be  continued.] 
XLY.   On  a  Bicyclic  Chuck.     By  A.  Cayley,  F.R.S.* 
THE  apparatus,  although  I  have  called  it  a  chuck,  is  con- 
structed, not  for  turning,  but  for  drawing ;  viz.  it  rotates 
horizontally  on  a  table  (being  moved,  not  from  the  inside  by  the 
axle  of  the  lathe,  but  from  the  outside  by  a  handle-frame),  carry- 
ing a  drawing-board  which  works  under  a  fixed  pencil  supported 
by  a  bridge.  Two  points  of  the  drawing-board  describe  circles ; 
and  the  curve  traced  out  on  the  drawing-board  is  consequently 
that  described  by  a  fixed  point  upon  a  moving  plane  two  points 
of  which  describe  circles;  or,  what  is  really  the  same  thing,  it  is 
the  curve  described  on  a  fixed  plane  by  a  point  rigidly  connected 
with  two  points  each  of  which  describes  a  circle.  The  apparatus 
is  at  once  convertible  into  an  oval  chuck  of  nearly  the  ordinary 
construction  ;  viz.  it  may  be  arranged  so  that  the  curve  described 
on  the  drawing-board  shall  be  an  ellipse. 
Bottom  plane  is  a  rectangular  board  (1)  (see  figure)  about 
30  inches  by  24  inches,  having  in  the  middle  a  sliding-piece  (2) 
carrying  a  block  (3). 
Second  plane  contains  two  circular  segments  (4)  fixed  to 
the  bottom  plane,  serving  as  an  axle  for  the  moving  piece  (5) 
next  referred  to,  and  allowing  the  block  (3)  to  move  between  them. 
And  in  the  same  plane  we  have  a  moving  piece  (5)  in  the  form 
of  a  rectangle  with  a  circle  cut  out  thereof,  rotating  about  the 
segments  (4),  and  having  upon  it  a  groove  in  which  works  a 
sliding-piece  (6)  carrying  a  block  (7) ;  there  is  in  this  block  a 
circular  hole,  D.  The  second  plane  includes  also  two  sides  (8) 
of  a  handle-frame,  which  two  sides  slide  along  two  of  the  sides 
of  the  piece  (5) . 
Third  plane  consists  of  a  rectangular  piece  (9)  rotating  about 
an  axle  fixed  to  the  block  (3),  and  having  a  sliding-piece  (10) 
in  which  is  a  circular  hole,  C.  The  third  plane  includes  also  the 
before-mentioned  block  (7),  having  upon  it  the  hole  D;  and  it 
includes  also  the  remaining  two  sides  (11)  of  the  handle-frame, 
and,  let  into  the  same  so  as  to  be  flush  therewith  on  the  upper 
surface,  two  slips  (12)  completing,  in  this  plane,  the  handle- 
frame. 
"We  have  thus  on  a  level  the  sides  (11),  (12)  of  the  handle- 
frame  and  the  holes  C,  D,  where  C  rotates  about  the  point  B, 
*  Communicated  by  the  Author. 
