Plethysmograph apparatus comprising a measuring cell and {HEAD ~ a reference cell} constructed in {TAIL ~ such manner} that exterior pressure variations cause equal pressure variations in both cells 
Plethysmograph apparatus comprising a measuring cell and { HEAD ~ a reference cell } constructed in { TAIL ~ such manner } that exterior pressure variations cause equal pressure variations in both cells
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH