private void shrink()
    {
        // method to "fade" smoke object with each act
        if(getImage().getWidth() < 10 || (getWorld().getObjects(Ball.class).isEmpty() )) {
            // remove smoke when out of world
            getWorld().removeObject(this);
        }
        else {
            // "fades" in size with each act
            GreenfootImage img = new GreenfootImage(image);
            img.scale ( getImage().getWidth()-fade, getImage().getHeight()-fade );
            setImage (img);
        }        
    }