This supposition, of four cubic feet of fresh air per minute for each individual in a room artificially lighted, of course involves the con dition that an equal quantity of vitiated air per minute must be allowed to escape ; and the inquiry naturally follows, How does this escape take place Carbonic acid gas is heavier than atmospheric air; but there are three circumstances which render respired air rather lighter than the general air of a room : namely, the existence in it of nitrogen and vapour, both of leas specific gravity than air, and the higher temperature of the respired air than the air of a room. From all these circumstances combined, it is found that respired air ascends to the upper part of the room ; and it follows that the ceiling, or some neighbouring part, is the proper place for an outlet.
In the majority of buildings erected, there is no account what ever taken of the means for insuring ventilation. The fire-places are constructed, the windows fixed, and the doors hung without a thought being paid to the means of effecting a constant change of the air contained within the apartments. But it is probable that in most English rooms, provided as they are with tolerably large open fire-places, and with doors which are frequently opened, the ventilation is sufficiently complete. The fresh air enters the room by the open window, the lower part of the open door, and any crevices which may exist at a small height from the floor ; while the vitiated and specifi cally lighter air escapes partly up the open fire-place and chimney, partly near the upper extremity of the open doors, and partly by crevices around the doors and windows generally. In crowded rooms, however, where the amount of vitiated breath bears a much larger ratio to the cubical contents, and where the doors are generally small compared with the height of the room, the impure air cannot escape by these means, and some arrangements must be made near the ceiling for the removal of the air. These methods are chiefly of two different kinds; the one by the use of a revolving wheel or fan, and the other by the action of a chimney or tube.
We sometimes see one of the upper panes of glass removed from a window, and its place supplied by a revolving fan, as a means of ven tilating the appartment. This fan or wheel is provided with radii formed like the sails of a windmill, or the blades of a screw-propeller, so that any force which sets the wheel in motion will cause a current of air to pass obliquely between them. But in this case the fan is made to revolve merely by the impulse of the air itself, and is not under the control of the inmates of a building. A more complete
exemplification is presented by the arrangement of the large cotton mills of the North. A fan-wheel, fau-veutilator, or wind-fan (for by all these names the contrivance is known), being placed in any con venient position, is set in motion by the steam-engine of a factory, and by its rotation draws out the vitiated air from a series of rooms with great rapidity. The following is one variety of wind-fan used in the factories. It consists of two cast-iron end-plates, A A, having a central circular opening, c c c, from the circumference of which the outline of each plate enlarges spirally, the point nearest the centre being near d, and that farthest off being under E (Fig. 1). These two parallel plates are connected by bolts, a a a; a mantle of sheet-iron being previously inserted into grooves cast in the edges of the end-plates so as to enclose a cavity with an elongated outlet at B, to which a pipe is attached for carrying off the vitiated air in any direction. Within this cavity a shaft, o, revolves, in bearings, b b, placed centrally in the frame-plates, A A, and cast in the same piece. On this shaft a boss is wedged fast, bearing five flat arms, c c c, to which are riveted five flat plates or wires, of the shape shown between a and a (Fig. 2), having a semi circular piece cut out of them on each side, about the size of the end opening. On one end of the shaft, c, beyond the box-bearing, the loose-and-fast pulleys, D, are fitted for receiving the driving-band, and for turning the wings in the direction shown by the arrow. Thus the air is driven before them out of the end orifice, B, while it enters by the side openings at cc c (Fig. I). By the centrifugal force of the revolving wings, the air is condensed towards their extremities, and makes its escape from the pressure through the orifice, 13, while it is continually drawn in at the sides by its tendency to restore the equilibrium.
At the Reform Club-house a steam-engine works a revolving fan, capable of throwing 11,000 cubic feet of air per minute into a sub terranean tunnel under the basement story; and the steam of con densation, from the small steam-engine which works the fan, supplies three cast-iron chests with the requisite heat for warming the whole building. A plan of ventilation has been adopted at the General Poet Office, in which the fan is used. The fan, worked by a steam-engine, draws fresh air down a shaft ; the air is filtered through fine wire gauze, and then sent up into all the rooms through pipes pierced with small holes.