Source : Webster's Revised Unabridged Dictionary (1913)

Hygroscope \Hy"gro*scope\, n. [Gr. ? wet + -scope: cf. F.
   hygroscope.] (Physics)
   An instrument which shows whether there is more or less
   moisture in the atmosphere, without indicating its amount.

Source : WordNet®

hygroscope
     n : hygrometer that shows variations in the relative humidity of
         the atmosphere