It unfortunatelv happens that many eminent writers on ventilation, without carefully distinguishing between the kind of ventilation required, advocate the placing of air-inlets and outlets on opposite sides of rooms. It is true that to get good through ventilation, openings, such as windows and doors, should be situate on opposite sides of the same apartment, but for comfortable ventilation, particularly in cold weather, with a fireplace flue for the outlet, the inlet should be on the same side of the room. This also holds good whenever mechanical means are provided for securing ventilation, whether by extraction or propulsion.

If. however, neither open fires nor mechanical means are employed, and movements of the outer atmosphere are depended upon to secure change of air within a room, and the inlet and outlet openings are in outer walls, then undoubtedly they should be on opposite sides if practicable; if not, then on adjoining sides, because it would be difficult to form both inlet and outlet through a single wall in such a manner as to induce a constant current in one direction. The pressure of wind would generally be practically equal on the two openings at the same time, or at times it might vary, first acting on one, then on the other, so that at best an intermittent flow of air would be induced.

Experiment has demonstrated that air at normal temperature, moving with a velocity exceeding 5 feet per second, is generally felt as a draught, at 4 feet moat people would consider it uncomfortable, at 3 feet some would complain, but at 2 feet few would notice it, and with a velocity of 1 foot per second it would not be perceptible to anyone.

If air enters the room shown in Fig. 564 at the rate of 300 feet per minute - i.e. 5 feet per second, - its velocity may be reduced as it enters the apartment, by belling out the inlet-opening; the air will then quickly be diffused in the upper portion of the room, and, if there be only one exit-opening, no larger than the inlet, the velocity of the air in approaching it will increase to its original rate, but by enlarging the embouchure to (say) 4 square feet, the rate of air-tra\ .1 will at that point not exceed 300/4 = 75 feet per minute, or 1 feet per second, and will be considerably less throughout the habitable parts of the room.

This is presuming an air-tight apartment, but as in practice such is scarcely attainable, it is usual, when propulsion is employed for securing ventilation, to provide an outlet somewhat smaller than the inlet to allow for leakages, which, if small and well distributed, may tend to a more thorough distribution of the incoming air, and reduce the velocity at the main outlet.

By constantly keeping in mind these simple facts as to the relative positions and forms of inlets and outlets, discomfort from draughts may be, to a great extent, avoided, although, without mechanical power, it may not he possible to regulate the changes of air with absolute uniformity. The great aim of ventilation is, however, not to secure mathematical accuracy in the quantity of air supplied, but to obtain an adequate (era though variable) change of air without inducing objectionable draughts in the apartments vent dated. Whether this be done by mechanical or natural means, or by a propulsive «»r suctional tone, matters little so long as the current continues in one direction; and it may be laid down as a general rule, applicable in almost every case, that the air-inlet and the air-outlet should be on the same side of the room, and that both should be belled out towards the room, the air-inlet being best placed at about two-thirds of the height of the room, and arranged bo as to give the air a slight upward tendency, while the outlet should be at or near the level of the floor.

The problems of warming and ventilation are so interdependent, that the one subject cannot be fully considered without reference to the other; thus, ventilation is simplified when some form of warming apparatus is adopted in addition to open fires. It then Incomes possible, by means of suitable radiators, to warm the air entering the room and thus to prevent cold draughts, which are often dangerous to health. More will be said of this in a subsequent chapter.

The following simple experiments will illustrate some of the principles laid down in this chapter.

1. To show that change of air is principally brought about by variations in temperature, take a large glass vessel, fill it with pure air, and insert a few light particles, such as down; close it so that the air within cannot be changed, and by alternately warming different parts of the vessel, the air within may be made to circulate, the circulation being made apparent by the movements of the particles of down.

2. If a living animal or lighted taper be inserted, and the vessel again closed, it is only a question of time when the one will die and the other be extinguished, by exhaustion or contamination of the air within; a similar result may also take place even in a vessel with a single opening at the top, although a longer time will be required; but with an opening also in the bottom of the vessel, the taper would freely burn and the animal could live, thereby proving that actual change of air takes place, resulting principally, if not entirely, from the heat evolved either from the lighted taper or from the living animal within.

3. A similar glass vessel may also be employed to illustrate the varying effect of heat and sold upon the hygrostatic conditiou of air. Let warm air be admitted after it has had free access to moisture, then if the vessel be closed and placed in a colder position, it will become bedewed inside, or if only a portion of the vessel be considerably cooled down, dew will be formed within upon that colder part.

4. Further effects of condensation will be seen if the air within the vessel be contaminated with volatile organic matter while it is warm - say that a taper has been allowed to burn therein for a short time; - then if the vessel be closed and cooled down, a film will be deposited within consisting of the products of combustion and volatilization of substances composing the taper.

5. It is also useful to note that, if in either of cases 3 and 4 the vessel be again heated, the moisture or the film of animal matter will once more be volatilized and become invisible.