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