<p>A longwall jack is provided having a telescoping cylinder and piston which can be expanded and pressurized with water or other fluids through a one-way valve in the head of the cylinder and which is provided with a pre-set pressure release means which will release fluid each time the pressure on the cylinder and piston exceeds the pre-set level and with a second release means combined with a retrieval means by which the pressure can be released and the jack retrieved by remote means.</p>
