I've just viewed the National Geographic Channel's "Engineering Connections" program. specifically, the episode entitled "Airbus380", and that episode's discussion of the A380's landing gear shock absorbers, which used bicycle pumps filled with water to illustrate the basic principle of hydraulic shock-absorbtion, while pointing to the use of oil, as opposed to water, as the hydraulic fluid, due to oil's greater viscosity. I've found a mechanism that exploits the differential compressibility of oil and air to produce hydraulic shock absorption, pneumatic energy storage, and release of pneumatically stored energy in the form of hydraulic motion in opposition to the direction of application of the original energy input. I'm here referring to the hydro-pneumatic recoil mechanism of the famous Schneider Model 1897 75mm gun, which consists of a buffer cylinder, which, in turn, consists of a cylinder filled with oil and a piston which, during recoil, forces the oil through a duct into the recuperator cylinder, which then drives the recuperator piston (which is a floating piston) in the opposite direction (i.e., towards the recoil) to compress the air in the recuperator cylinder. The recuperator piston has attached to it, in the direction facing away from the recoil, a long, somewhat conically-shaped rod (with the flare facing away from the recoil) which passes through a diaphragm so that the whole piston/throttling rod/diaphragm assembly constitutes a simple, graduated valve. Once recoil is fully absorbed, the compressed air in the recuperator cylinder expands to return the gun to battery. Neat, huh? EDIT: At about the same time, the Russians came up with a recoil mechanism consisting of a single cylinder containing a stack of toroidal hard rubber gaskets. It worked almost as well as the hydro-pneumatic Schneider system, but, unlike the Schneider system, was not scalable to pieces much larger than 75mm light field guns. P.S.: The Schneider 75 would have been the perfect infantry support weapon had it only had a split trail and an elevation rack with arc sufficient to turn the gun into a howitzer.