Jack rabbit (Zo["o]l.), any one of several species of large American hares, having very large ears and long legs. The California species (Lepus Californicus), and that of Texas and New Mexico (L. callotis), have the tail black above, and the ears black at the tip. They do not become white in winter. The more northern prairie hare (L. campestris) has the upper side of the tail white, and in winter its fur becomes nearly white. Jack rafter (Arch.), in England, one of the shorter rafters used in constructing a hip or valley roof; in the United States, any secondary roof timber, as the common rafters resting on purlins in a trussed roof; also, one of the pieces simulating extended rafters, used under the eaves in some styles of building. Jack salmon (Zo["o]l.), the wall-eyed pike, or glasseye. Jack sauce, an impudent fellow. [Colloq. & Obs.] Jack shaft (Mach.), the first intermediate shaft, in a factory or mill, which receives power, through belts or gearing, from a prime mover, and transmits it, by the same means, to other intermediate shafts or to a line shaft. Jack sinker (Knitting Mach.), a thin iron plate operated by the jack to depress the loop of thread between two needles. Jack snipe. (Zo["o]l.) See in the Vocabulary. Jack staff (Naut.), a staff fixed on the bowsprit cap, upon which the jack is hoisted. Jack timber (Arch.), any timber, as a rafter, rib, or studding, which, being intercepted, is shorter than the others. Jack towel, a towel hung on a roller for common use. Jack truss (Arch.), in a hip roof, a minor truss used where the roof has not its full section. Jack tree. (Bot.) See 1st Jack, n. Jack yard (Naut.), a short spar to extend a topsail beyond the gaff. Blue jack, blue vitriol; sulphate of copper. Hydraulic jack, a jack used for lifting, pulling, or forcing, consisting of a compact portable hydrostatic press, with its pump and a reservoir containing a supply of liquid, as oil. Jack-at-a-pinch. (a) One called upon to take the place of another in an emergency. (b) An itinerant parson who conducts an occasional service for a fee. Jack-at-all-trades, one who can turn his hand to any kind of work. Jack-by-the-hedge (Bot.), a plant of the genus Erysimum (E. alliaria, or Alliaria officinalis), which grows under hedges. It bears a white flower and has a taste not unlike garlic. Called also, in England, sauce-alone. --Eng. Cyc. Jack-in-a-box. (a) (Bot.) A tropical tree (Hernandia sonora), which bears a drupe that rattles when dry in the inflated calyx. (b) A child's toy, consisting of a box, out of which, when the lid is raised, a figure springs. (c) (Mech.) An epicyclic train of bevel gears for transmitting rotary motion to two parts in such a manner that their relative rotation may be variable; applied to driving the wheels of tricycles, road locomotives, and to cotton machinery, etc.; an equation box; a jack frame; -- called also compensating gearing. (d) A large wooden screw turning in a nut attached to the crosspiece of a rude press. Jack-in-office, an insolent fellow in authority. --Wolcott. Jack-in-the-bush (Bot.), a tropical shrub with red fruit (Cordia Cylindrostachya). Jack-in-the-green, a chimney sweep inclosed in a framework of boughs, carried in Mayday processions. Jack-in-the-pulpit (Bot.), the American plant Aris[ae]ma triphyllum, or Indian turnip, in which the upright spadix is inclosed. Jack-of-the-buttery (Bot.), the stonecrop (Sedum acre). Jack-of-the-clock, a figure, usually of a man, on old clocks, which struck the time on the bell. Jack-on-both-sides, one who is or tries to be neutral. Jack-out-of-office, one who has been in office and is turned out. --Shak. Jack the Giant Killer, the hero of a well-known nursery story. Jack-with-a-lantern, Jack-o'-lantern. (a) An ignis fatuus; a will-o'-the-wisp. ``[Newspaper speculations] supplying so many more jack-o'-lanterns to the future historian.'' --Lowell. (b) A lantern made of a pumpkin so prepared as to show in illumination the features of a human face, etc. Yellow Jack (Naut.), the yellow fever; also, the quarantine flag. See Yellow flag, under Flag.