The melt circulation through the conduits is caused by the difference in the levels and densities of {HEAD ~ the melt} in {TAIL ~ these vessels} 
in
The melt circulation through the conduits is caused by the difference in the levels and densities of { HEAD ~ the melt } in { TAIL ~ these vessels }
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