79)  under  the  receiver  of  an  air-pump,  and  exhaust  the  air.  The  air
within  the  globe  expands,  and  escapes  from  the  small  orifices  a  and  c
at  the  extremity  of  fhe  two  arms.  But  this  motion  of  the  air  is
attended  by  an  opposite  motion  of  the  arms  and  globe,  and  a  rapid
rotation  is  caused.
A  man  in  a  boat  weighing  one  ton  pulls  at  one  end  of  a
rope,  the  other  end  of  which  is  held  by  another  man,  who


80  GENERAL  DYNAMICS.
weighs  twice  as  much  as  the  first  man,  in  a  boat  weighing
two  tons :  both  boats  will  move  towards  each  other,  but
in  opposite  directions ;  if  the  resistances  which  the  two
boats  encounter  were  the  same,  the  lighter  boat  would
move  twice  as  fast  as  the  heavier,  but  with  the  same
momentum.
If  the  boats  are  near  each  other,  and  the  men  push  each
other's  boats  with  oars,  the  boats  will  move  in  opposite
directions,  though  with  different  velocities,  yet  with  equal
momenta.
The  opposite  impulses  received  by  the  bodies  concerned
are  usually  distinguished  by  the  terms  action  and  reaction.
We  measure  these,  when  both  are  free  to  move,  by  the
momenta  generated,  which  is  always  the  same  in  both
bodies.
71.  Third  Law  of  Motion.  —  To  every  action  there  is
an  equal  and  opposite  reaction.
The  application  of  this  law  is  not  always  obvious.
Thus,  the  apple  falls  to  the  ground  in  consequence  of  the
mutual  attraction  between  the  apple  and  the  earth.  The
earth  does  not  appear  to  fall  toward  the  apple.  But,
as  the  mass  of  the  earth  is  enormously  greater  than  that
of  the  apple,  its  velocity,  for  an  equal  momentum,  is
proportionately  less.
EXERCISES.
