The Collaborative International Dictionary of English v.0.48:

Dasymeter \Da*sym"e*ter\, n. [Gr. dasy`s rough, thick + -meter.]
   (Physics)
   An instrument for testing the density of gases, consisting of
   a thin glass globe, which is weighed in the gas or gases, and
   then in an atmosphere of known density.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dasymeter
    n 1: densimeter consisting of a thin glass globe that is weighed
         in a gas to determine its density