a means to rotate said control ring so as to change {HEAD ~ the position} of {TAIL ~ said vent} 
of
a means to rotate said control ring so as to change { HEAD ~ the position } of { TAIL ~ said vent }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL