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