_A Lifting-Jack_ is a contrivance for raising great weights by force from below; also called _a jack-screw_. From its derivation from Jack, equivalent to lad or boy, has arisen its modern use as denoting a contrivance which is subject to rough usage. It is operated by _a screw_, whereas—_a hydraulic jack_ is a jack or lifting apparatus operated by some _liquid_, usually oil, acting against a piston or plunger, the pressure on the liquid being produced by a force pump. _The hydraulic jack_ consists of, 1, a cylinder; 2, a ram or plunger; and 3, a pump. One of these machines is shown and described in the Glossary, page 24, another is illustrated by Fig. 138. The Fig. 137 on the opposite page shows the inside view of Fig. 138 but on a different scale. _The names of the parts_ are particularly to be noted. [Illustration: FIG. 138.] Movable hydraulic, or screw, jacks serve on numerous occasions most effectively for lifting or propping-up of less accessible parts. Eye-bolts and jack-bolts are arranged for, in all parts that are likely to be handled, to facilitate and accelerate the work in necessarily crowded quarters. _The base or foot_ is usually made of cast iron or cast steel and may be either round or square to suit requirements. _The cylinder_ is bored from a seamless steel ingot and having a thread upon its lower end is screwed into the base. _The ram_ is also a tube of seamless steel having a thread at the top and is screwed into the head or cap which is made either of cast iron or cast steel. The lower end of the ram has a thread inside to receive the pump plug which contains the delivery valve, while upon its outside is placed the cup leather packing and the ram packing ring. The pump for operating the ram is from five-eighths to three-quarters of an inch in diameter depending upon the capacity of the jack, and has a plunger packed with a cup leather. A suction valve is contained within the plunger. A short arm is fitted upon a socket which enters through the side of the head or reservoir. This arm is connected by a pin to the pump inside the ram while the outer end of the socket has a tapered rectangular hole through it to receive the jack-lever. A leather collar packing makes the socket tight where it enters the side of the reservoir.