{TAIL ~ An acoustic gas monitor} 6 wherein {HEAD ~ said wall section} consists of material which is selectively permeable to said gaseous substance 
{ TAIL ~ An acoustic gas monitor } 6 wherein { HEAD ~ said wall section } consists of material which is selectively permeable to said gaseous substance
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH