Abstract: The apparatus of the invention is applicable to gasholders and comprises a mercury seal, e.g. for a gas holder, which seal provides fluid-tight sealing in an annular space formed between a cylinder and a piston. The mercury is deposited in a reservoir which is inside the piston but which communicates with the annular space outside the piston. In the reservoir, the mercury is subjected to the inside pressure P of the cylinder, and it moves up inside the annular space when the difference between the inside and outside pressure P and Po increases, thereby forming an adaptive fluid seal.