Jack, Hydraulic, a small portable hydraulic press arranged with a force-pump working by hand, and used for the purpose of lifting heavy bodies. A screw-jack has the same object, but works on the principle of the screw. A strong screw with square threads is held in a vertical frame and turned by means of a hand lever. It thus rises, and may be made to lift up heavy weights under which it has been inserted. Its multiplying power is increased by lengthening the lever, and by diminishing the pitch of the screw, but usually its efficiency is much less than that of the hydraulic-jack by reason of the great loss of energy in overcoming friction.