A well tool (10) for applying a pulling or a pushing force to an object in an interior of a well bore comprising: a) an inner elongated member (50,62,66); b) an outer elongated member (64); c) a sealed interior defined between the inner elongated member (50,62,66) and the outer elongated member (64); and d) an actuation means (58,62) defined at least partially within the sealed interior; wherein the actuation means (58,62) is adapted to reversibly move the outer elongated member (64) longitudinally over the inner elongated member (50,62,66) and wherein the inner elongated member (50,62,66) and the outer elongated member (64) are arranged such that a volume of the sealed interior occupied by the inner elongated member (50,62,66) remains substantially constant as the inner elongated member (50,62,66) and the outer elongated member (64) move relative to each other.