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