Abstract: A hydraulically operated prime mover comprises a liquid reservoir having a relatively large surface area at an upper level and connected via a lock to a waste liquid area at a lower level, the lock containing a relatively closely fitting deep-draught buoyant float which extends above the flotation level by an amount at least equal to the draught and an energy transfer device which extracts the potential energy from the float over substantially the entire stroke when it rises and falls between its two buoyant positions in the lock corresponding to the upper and lower liquid levels.