Source : Webster's Revised Unabridged Dictionary (1913)

Weatherglass \Weath"er*glass`\, n.
   An instrument to indicate the state of the atmosphere,
   especially changes of atmospheric pressure, and hence changes
   of weather, as a barometer or baroscope.

   {Poor man's weatherglass}. (Bot.) See under {Poor}.

Source : WordNet®

weatherglass
     n : a simple barometer for indicating changes in atmospheric
         pressure