210. The comparative advantages of these opposing systems of ventilation may be briefly stated as follows:

1. The currents of warm air arising from human beings and containing the exhalations and products of respiration are warmer than the surrounding air, and naturally tend to move upwards. Thus they tend to assist upward ventilation and to impede downward movement. The amount of force required to overcome the rising tendency of these currents is so small, however, that it need not be considered in cases where fans are used for forcing the air, nor, in fact, in any case, except where ventilating chimneys having comparatively weak draft are employed.

2. The hot air and gases produced by gas burners, etc., have a strong upward tendency, and in some cases may be a valuable auxiliary to upward ventilation. When the downward system is used, these lights should be enclosed, and the products of combustion carried away to avoid contaminating the fresh air. Usually these hot gases can be utilized to increase the draft, by discharging them into the foul-air flues.

3. The direction of the ventilating movement has only a very slight effect in varying the distribution of the exhalations and products of respiration throughout the room. The tendency which these have to float upwards is counteracted in the downward system by the general movement of the air, and they are swept down to the floor and into the outlet flues. In the upward system, they are merely carried in the opposite direction. The quality of the air at the breathing level depends mainly upon the proportion of fresh air supplied.

4. In rooms that are likely to be fully occupied, it is difficult to locate the fresh-air openings in or near the floor so that the incoming currents will not impinge on some part of the bodies of the occupants and produce disagreeable sensations. The difficulty is aggravated by the tendency of air-currents to adhere to the surfaces, both vertical and horizontal.

In downward ventilation, the incoming currents are likely to be dissipated before they reach the lower part of the room, and consequently they may be comparatively few in number and larger in volume, and may have a higher velocity. The foul air may, without producing uncomfortable feelings, be taken out through floor openings at a higher velocity than could be allowed for inflowing currents.

5. The ascending air-currents employed in the upward system tend to carry up into the breathing zone the dust and other offensive matters which otherwise would lie upon the floor. As the upward flow is continuous, the dust is kept in constant circulation, and no opportunity is given it to settle. Downward currents, on the contrary, will carry it directly into the outlets and thus dispose of it.

6. In audience rooms of great height, having balconies and galleries, either system may be applied successfully, so far as the distribution of fresh air and heat is concerned, but the acoustic properties are likely to be better with the downward system, because the main body of air is more likely to be uniform in density and temperature, and free from perceptible currents.