Abstract: Power transmission apparatus for controlling the elevation of a gun barrel. Two telescopic members are provided, one being secured to the gun base and the other to the gun barrel. The first and second parts define a first piston and a hollow piston rod attached thereto with the first piston being slidable in a cylinder defined by the second part. A second piston having an associated rod is also provided and such second piston is slidable inside the first piston rod. First and second chambers are thus formed, the first being filled with an incompressible fluid such as hydraulic fluid and the second chamber is filled with a highly compressible fluid such as a gas.