54. The mechanical effects of the wind which are of interest in the heating and ventilation of buildings are:

1. The increase of the atmospheric pressure upon the windward side of buildings, and the corresponding decrease upon the opposite side.

2. The increase of the draft of chimneys and flues.

3. The reversal of currents, called blow-downs.

4. The formation of eddies and whirls, which deposit dust and snow in undesirable places.

5. The increase of evaporation and the consequent drying and cooling effects.

55. When the wind blows against a building, the pressure of the air upon the windward side will be greater than upon the leeward side. The air which is within the building will tend to flow out through every crevice and flue into the area of low pressure which exists upon the leeward side of the building. If, at the same time, there is no leakage of air into the building from the windward side, the atmospheric pressure within the building will fall slightly. But, if there are any openings or crevices upon the windward side, the air will flow from the area of high pressure upon the outside to the inside of the building, where the pressure is at, or below, the normal.

If the leeward side of the building is tight, and the wind can leak through the windward side only, then the pressure within the interior of the building will be slightly increased. This extra pressure will ooze out through every crevice upon the leeward or neutral sides of the building. It will escape through the ventilating flues also. Thus, a considerable amount of air will pass through the building notwithstanding the tightness of the leeward side.

If, on the contrary, the windward side is tight and the leeward side is leaky, only a small leakage will occur, and that will be from the interior toward the leeward side.

Thus, the amount of air which will be driven through a building by the wind (leaving out of consideration the effects upon chimneys, etc.) will depend mainly upon the tightness of the windward side, and not upon the condition of the leeward side.

The pressure which is exerted by the wind per square foot of obstructed area is greatly modified by the shape of the surface.

Thus, the pressure upon a sphere, or a hemisphere with convex side towards the wind, is about one-half of that upon a flat surface of equal diameter.