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