Abstract: A hydrostatic bladder-type storage means for storing mechanical energy by a compressible media in a pressure bottle. An interior of the pressure bottle contains an elastically resilient bladder accommodating the medium. The pressurized medium is introduced into the bladder from one side of the bottle, with the bladder being surrounded by hydraulic oil which is adapted to be forced into and removed from a cavity located between the bottle and the bladder. A porous elastic body of foamed synthetic resinous material is provided in the bladder with the medium filling pores of the foam body. The foam body is intimately joined to the wall of the bladder and has a specific thermal capacity which is greater than the thermal capacity of the medium.