text: Figure 4.3 illustrates various degrees of 
  static stability by a ball placed on various 
  surfaces. Positive static stability 
  is shown by 
  the ball in a trough; if the ball is displaced 
  from equilibrium at the bottom of the trough, 
  tendency to return to equilib- 
  there is an initial 
  the ball 
  rium.  If it is desired to “control” 