202  Miscellaneous  Implements  Exhibited  at  Park  Royal. 
The  principle  is  largely  the  reverse  of  the  railway;  thus  the 
sleepers  are  replaced  by  feet  carried  at  the  ends  of  the  spokes 
of  the  driving  wheel  of  the  engine  and  placed  successively 
on  the  ground,  as  required,  and  again  removed.  Each  of 
these  feet  carries  a wheel  or  roller  upon  which  rails  mounted 
on  the  engine  (or  carriage)  slide.  A man  or  an  animal  when 
walking  so  })laces  his  feet  in  succession  upon  the  earth  that 
they  may  form  foundations  over  which  he  may  move.  The 
effort  required  for  his  forward  motion  is  ap])lied  through  these 
foundations,  but  the  actual  motion  takes  place  in,  about,  and  over 
the  ankle,  a beautiful  form  of  ball  and  socket  joint.  So  in  the 
Fig.  Hi— Internal  Mechanism  of  Pedrail  Steam  Tractor  (Article  I(X)0). 
pt'drail  the  action  of  a man  or  animal  walking  is  mechanically 
imitated.  The  feet  have  ankles  about  wbich  the  forward 
motion  may  take  place.  Rods  and  springs  replace  the  sinews 
and  muscles  which  enable  each  foot  to  adapt  itself  to  the 
inequalities  of  the  ground  on  which  it  is  placed,  whilst  the 
spokes  of  the  wheel  take  the  }>lace  of  the  legs  and  complete 
the  simile. 
It  is  impossible  in  the  space  at  my  disjiosal  to  describe 
all  tbe  mecbanical  details ; suffice  it  to  say  that  each  foot 
with  its  ankle  and  roller,  makes  a jioint  upon  which  the  rail 
can  slide. 
