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