The Collaborative International Dictionary of English v.0.48:

Hypsometer \Hyp*som"e*ter\, n. [Gr. ? height + -meter.]
   (Physics)
   An instrument for measuring heights by observation of
   barometric pressure; esp., one for determining heights by
   ascertaining the boiling point of water. It consists of a
   vessel for water, with a lamp for heating it, and an inclosed
   thermometer for showing the temperature of ebullition.
   Hypsometric

WordNet (r) 3.0 (2006):

hypsometer
    n 1: an altimeter that uses the boiling point of water to
         determine land elevation