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