SMOKE jack, is a very simple and com modious machine in a kitchen , so called, from its being moved by means of the smoke, or rarefied air, moving up the chimney, and striking against the tail of the horizontal wheel, which, being in. dined to the horizon, is thereby moved about the axis of the wheel, together with the pinion which carries the wheel, and this carries the chain which turns the spit. The wheel should be placed in the narrow part of the chimney, where the motion of the smoke is swiftest, and the greatest part of it must be made to strike upon the sails.
The manner of operation of this use ful machine is easily understood. The air which contributes to the burning of the fuel, and passes through the midst of it, is greatly heated, and expanding pro digiously in bulk, becomes lighter than the neighbouring air, and is therefore pushed by it up the chimney. In like man ner, all the air which comes near the fire is heated, expanded, becomes lighter, and is driven up the chimney. This is called the draught or suction ; but would, with greater propriety, be termed the drift of the chimney. As the chimney gra dually contracts in its dimensions, and as the same quantity of heated air' passes through every section of it, it is plain that the rapidity of its ascent must be greatest in the narrowest place. There
the fly should be placed, because it will there be exposed to the strongest current. This air, striking the fly vanes obliquely, pushes them aside, and thus turns them round with a considerable force. If the joint of meat is exactly ba lanced on the spit, it is plain that the only resistance to the motion of the fly is what arises from the friction of the pivots of the upright spindle, the friction of the pinion and wheel, the friction of the pi vots of the horizontal axis, the friction of the small end of the spit, and the friction of the chain in the two pulleys. The whole of this is but a mere trifle. But there is frequently a considerable inequa lity in the weight of the meat on differ ent sides of the spit : there must there fore be a sufficient overplus of force in the impulse of the ascending air on the vanes of the fly, to overcome this want of equilibrium, occasioned by the unskilfulness or negligence of the cook.