I am trying to create a simple RTS camera that scrolls when the mouse is at the edges of the screen, here is my code:

public float scrollSpeed = 5.0F;
    public float scrollBuffer = 10.0F;
    public float damping = 2.0F;

    private Transform _tr;

    void Awake () {
       _tr = transform;
    }

    void Update () {
       Vector3 newPos = _tr.position;
       if (Input.mousePosition.x >= Screen.width-scrollBuffer)
         newPos.x = Mathf.Lerp (_tr.position.x, _tr.position.x + scrollSpeed, damping * Time.deltaTime);
       if (Input.mousePosition.x <= scrollBuffer)
         newPos.x = Mathf.Lerp (_tr.position.x, _tr.position.x - scrollSpeed, damping * Time.deltaTime);
       if (Input.mousePosition.y >= Screen.height-scrollBuffer)
         newPos.z = Mathf.Lerp (_tr.position.z, _tr.position.z + scrollSpeed, damping * Time.deltaTime);
       if (Input.mousePosition.y <= scrollBuffer)
         newPos.z = Mathf.Lerp (_tr.position.z, _tr.position.z - scrollSpeed, damping * Time.deltaTime);

       _tr.position = newPos;
    }

I want the camera to come to a smooth halt when the mouse moves away from the screen bounds, but the "damping" method I have implemented above doesn't seem to work. Does anyone know what I am doing wrong?