The vent position can be changed from {HEAD ~ an equilibrium position} to {TAIL ~ a nonequilibrium position} 
to
The vent position can be changed from { HEAD ~ an equilibrium position } to { TAIL ~ a nonequilibrium position }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL