diff --git a/utils/spring-mobility-model.h b/utils/spring-mobility-model.h
index 553ab9d..700c994 100644
--- a/utils/spring-mobility-model.h
+++ b/utils/spring-mobility-model.h
@@ -68,8 +68,11 @@
   void 
   Update (void) const;
 
+  static void
+  UpdateAll ();
+
 private:
-  double m_epsilon;
+  static double m_epsilon;
 
   double m_nodeMass;
   double m_nodeCharge;
@@ -78,14 +81,13 @@
   double m_dampingFactor;
 
   static double m_totalKineticEnergy;  
+  static bool m_stable;
+  static EventId m_updateEvent;
   
   mutable Vector m_position;
   mutable Vector m_velocity;
-  mutable bool m_stable;
   mutable Time m_lastTime;
 
-  EventId m_updateEvent;
-
   std::list<Ptr<MobilityModel> > m_springs;
 };