Jack. A very useful implement in kitchens, sometimes turned by the force with which smoke and rarified air ascend in a chimney. The Smoke-jack consists of a horizontal wheel, the radii of which are placed obliquely, so as to permit the smoke to pass through them; the axle then turns a small crank-wheel, which works into another at right angles, and at the end is a pulley with a chain to turn a spit set before the fire, with any article intended to be turned and-roasted. Many jacks are, however, turned by means of a weight, which pulls round a cylinder, and thereby, with the intervention of a chain, rotates the spit.