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