anchor: ParticleSystem:setLinearAcceleration
  Sets the linear acceleration (acceleration along the x and y axes) for particles.
  
  Every particle created will accelerate along the x and y axes between xmin,ymin and xmax,ymax.
  ParticleSystem:setLinearAcceleration( xmin, ymin, xmax, ymax )
  xmin number The minimum acceleration along the x axis.
  ymin number The minimum acceleration along the y axis.
  xmax (xmin) number The maximum acceleration along the x axis.
  ymax (ymin) number The maximum acceleration along the y axis.