The second mode of effecting ventilation, namely, by the use of a tube or chimney opening into the air from the upper part of an apartment, depends for its action on the ascensive power possessed by a lofty aerial column. As the "draught" of a furnace-chimney carries up the smoke, &c., more rapidly if the chimney be very lofty, so does a lofty chimney exceed a low one in carrying off vitiated air : and for the same reason, even if no chimney, properly so called, be provided, a lofty room, furnished with appropriate openings in its ceiling, will furnish a draught to carry off impure air more rapidly than a low room ; and in many of our public buildings this arrangement is deemed sufficient.
Dr. Reid, when the temporary House of Commons was being pre pared, introduced the system of an artificial draught. A circular shaft was built, 120 feet high, and from 8 to 11 feet in diameter. A large fire was maintained at the bottom of this shaft, which caused a powerful draught; and the draught sucked out all the vitiated air from the building. by a tunnel connecting the basement story with the shaft. The vitiated air in the House ascended by apertures into a space above the ceiling, and thence descended by pipes to the tunnel which conveyed it into the shaft. The experience derived from this arrangement led to many improvements in the ventilation of the pre sent Houses. 3fr. Goldsworthy Gurney here employs mechanism, by which fresh air is drawn in from some of the open courts, and after being filtered through a wire-gauze screen, is made to pass over steam heated boxes ; it is moistened by jets of spray, and enters the floors of both Mouses through holes covered with horsehair cloth. In sum mer the air is cooled by the wet spray, and does not come into contact with any heated surfaces. Besides this mode of warming the two principal chambers, there are steam-pipes to various parts of the building. A large coke fire produces a draught, which drives the vitiated air out of openings near the ceiling.
The modern application of the two main principles of ventilation— the production of a current by a rotating fan, and by the ascensive force of heated air—are very varied. We can only notice a few of them. One of the simplest and best known is Dr. Arnott's chimney valve, for 'ventilating ordinary sitting-rooms. The air of a room, vitiated by breathing, smoking, etc., ascends, but cannot easily escape, because there are no openings near the ceiling ; it contaminates all the air above the level of the chimney opening. A chimney valve corrects this. A hole is made from the aide of the room into the chimney, near the ceiling. An iron door or valve is fixed in the opening, so nearly balanced as to open or close with great ease. When opened,
the air in the upper half of the room enters by virtue of the draught in the warm chimney.
Lesure's method of ventilation, introduced in 1857, draws in cold air from the atmosphere, warms it, discharges it near the ceiling, and sends off the impure air from a room without mixing with the pure warm air. To effect this, a fire is lighted in a small stove or furnace ; the products of combustion are carried at once through pipes into the chimney ; and air enters through a hole in the outer wall, by a channel to a receptacle beneath the fire-place, into the interior of a casing sur rounding the stove, but not in contact with any part of the fire or stove. The air becomes heated during its passage, end at the same time moistened by steam from a basin of water placed within the case ; it escapes through an opening into the room, by lateral apertures parallel with the ceiling. As the upper parts become filled with warm air, the foul air finds its way to the fire and the chimney. In Wemyss's plan, introduced in 1859, an adjustable gridiron slide is fitted up in the wall near the ceiling, the inner aide flush with the walL The slide governs a passage into a metal chamber, fitted into a hole In the wall behind the slide. The chamber has two passages, one to admit the passage of fresh air, the other of foul. There is a venti lating fan at the outer end of the chamber. A cord easily moves the sliding valve, so as to make it open or close ; and the chamber is made telescopic, to suit it to the thickness of the wall. Idainnel's patent ventilator, introduced in the same year, is intended to ventilate carriages and ship. as well as houses and public buildings. It is planned to obviate certain evils arising from having the abduction tubes, to draw off foul air, near the roof or ceiling, and the induction tubes, for admitting fresh air, near the floor ; in all such arrangements there is a cold current in the lower part of a room, productive of many inconveniences. To remedy this defect, Mr. M'Kinnel proceeds as follows. Two concentric tubee rise from the ceiling of the room to be ventilated, the inner one to a greater height than the outer. Both have access, in various ways, to the outer atmosphere. The vitiated air of the room escapes through the inner tube, while the pure air from with out descends through the annular space between the two tubes. Both pasmges have wire gauze screens to ward off rain, soot, fie.. ; and valvular mechanism to regulate the currents. A lower apartment may be ventilated in a similar way, by having the ascending and descending flues in the outer walls, and a horizontal tube between the ceiling of the room and the floor of that next above it.