Abstract: A cylinder of adjustable length comprises a cylinder body, two pistons slidable in the cylinder body, and a rod which is connected to both pistons and projects from one end of the cylinder body. A dividing wall forming part of the cylinder body defines a first chamber which faces one of the pistons and a second chamber which is separate from the first chamber and in which the other piston is slidable. The two chambers are filled with a fluid, preferably grease, and communicate with each other through a duct controlled by a remotely-controllable valve. The rod and the cylinder body can be locked in any relative position by the closure of the valve to prevent the passage of the fluid from one chamber to the other.