The Collaborative International Dictionary of English v.0.48:

Hydropneumatic \Hy`dro*pneu*mat"ic\, a. [Hydro-, 1 + pneumatic:
   cf. F. hydropneumatique.]
   Pertaining to, or depending upon, both liquid and gaseous
   substances; as, hydropneumatic apparatus for collecting gases
   over water or other liquids.
   [1913 Webster]