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