{TAIL ~ The opening} has a bottom boundary whose height is adjustable, so that it can be adjusted to lie above {HEAD ~ the surface} of the glass melt 
{ TAIL ~ The opening } has a bottom boundary whose height is adjustable , so that it can be adjusted to lie above { HEAD ~ the surface } of the glass melt
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH