Webster's 1828 Dictionary
HYDROPNEUMATIC
a.[Gr. water, and inflated, breath, spirit. ] An epithet given to a vessel of water, with other apparatus for chimical experiments.
Webster's 1913 Dictionary
HYDROPNEUMATIC
Hy `dro *pneu *mat "ic, a. Etym: [Hydro-, 1 + pneumatic: cf. F.hydropneumatique.]
Defn: Pertaining to, or depending upon, both liquid and gaseous substances; as, hydropneumatic apparatus for collecting gases over water or other liquids.
HYDROPNEUMATIC GUN CARRIAGE
HYDROPNEUMATIC GUN CARRIAGE Hy `dro *pneu *mat "ic gun carriage. (Ordnance )
Defn: A disappearing gun carriage in which the recoil is checked by cylinders containing liquid and air, the air when compressed furnishing the power for restoring the gun to the firing position. It is used with some English and European heavy guns.
The Wordsmith Dictionary