JACK

, in Mechanics, is an instrument in common use for raising heavy timber, or very great weights of any kind; being a certain very powerful combination of teeth and pinions, and the whole inclosed in a strong wooden stock or frame BC, and moved by a winch or handle HP; the outside appearing as in fig. 1, here annexed.

In fig. 6, pl. 12, the wheel or rack-work is shewn, being the view of the inside when the stock is removed. Though it is not drawn in the just proportions and dimensions, for the rack AB must be supposed at least four times as long in proportion to the wheel Q, as the figure represents it; and the teeth, which will be then four times more in number to have about 3 in the inch. Now if the handle HP be 7 inches long, the circumference of this radius will be 44 inches, which is the distance or space the power moves through in one revolution of the handle: but as the pinion of the handle has but 4 leaves, and the wheel Q suppose 20 teeth, or 5 times the number, therefore to make one revolution of the wheel Q, it requires 5 turns of the handle, in which case it passes through 5 times 44 or 220 inches: but the wheel having a pinion R of 3 leaves, these will raise the rack 3 teeth, or one inch, in the same space. Hence then, the handle or power moving 220 times as fast as the weight, will raise or balance a weight of 220 times its own power. And if this be the hand of a man, who can sustain 100 pounds weight, he will, by help of this Jack, be able to raise, or sustain a weight or force of 22000lb, or about 10 tons weight.

This machine is sometimes open behind from the bottom almost up to the wheel Q, to let the lower claw, which in that case is turned up as at B, draw up any weight. When the weight is drawn or pushed sufficiently high, it is kept from going back by hanging the end of the hook S, fixed to a staple, over the curved part of the handle at h.

Jack is also the name of a well-known engine in the kitchen, used for turning a spit. Here the weight is the power applied, acting by a set of pulleys; the friction of the parts, and the weight with which the spit is charged, make the force to be overcome; and a steady uniform motion is maintained by means of the fly.

See the fig. of this machine, pl. 12, fig. 7.

Smoke Jack, is an engine used for the same purpose with the common Jack, and is so called from its being moved by means of the smoke, or rarefied air, ascending the chimney, and striking against the fails of the horizontal wheel AB (plate 12, fig. 8), which being inclined to the horizon, is moved about the axis of the wheel, together with the pinion C, which carries the wheels D and E; and E carries the chain F, which turns the spit. The wheel AB should be placed in the narrow part of the chimney, where the motion of the smoke is swiftest, and where also the greatest part of it must strike upon the sails.—The force of this machine depends upon the draught of the chimney, and the strength of the sire.

Jack-arch, in Architecture, is an arch of one brick thickness.

Jack-head, in Hydraulics, a part sometimes annexed to the forcing pump.

JACOB's-Staff, a mathematical instrument for taking heights and distances; the same with the Crossstaff; which see.