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