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of those streets. The wind blowing down the main street, M. S, meets with less resistance than that flowing over the housetops, and the aspiration out of the streets at right angles is greater, horizontally, at the ends adjoining the main street than it is in the vertical direction across the housetops, the consequence being that a sharp twist is given to the air drawn out of the side streets as it joins the wind blowing down the main thoroughfare. The high crests of the wind waves also give a sudden upward jerk to the air in the main street as it passes every cross road because of the larger volume of air which it meets and of the less resistance as at U U U. If a house on the corner of two streets happens to be higher than the others, the wind effects are generally very marked upon it, as shown by the fantastic chimney pots usually fixed in such a situation.
The church, C, Fig. 6, or a large public building so situated, will be subject to the same wind effects, and the aspiration of the wind will influence two sides of the building at the same time. If the door, D, is in the main street, and there is a ventilator at V on the roof and both are open, the suction at the door as the wind is blowing along the street, M. S, will cause a down draught from the ventilator when the windows are shut. If the window W2 or W3 is open, the air will be drawn towards the door. If W2, W3 and W4 are open, then air will pass out through W4. If the space B at the back of the church is narrow, and panes are open in the end window, the outward aspirating effect will not be great when the door is shut, but, if the space between the church and the houses is wide, then the aspirating effects of the wind blowing above W1 will be considerable, and air will be drawn out of the building. There is a large window over the door, D, and when the door is shut, the ventilators in the window, if open, will be aspirated by the wind blowing down the main street, M. S, and much air can be drawn out of the building. As the church is higher than the buildings adjoining, the pressure upon the windows W5, W6 and W7 will be slightly inwards.
Having obtained this information, how should the window openings be arranged to afford the best summer ventilation? —the movement of the wind, though slight, being in the direction of_ the arrows. W1 should be full open; W8, full open; W2, full open; W3, full open; W4, half open. After noting the temperature outside, W5 and W7 should be regulated so that the draught is not unbearable; always remembering that the movement of the air will be accelerated after the audience assembles. The same remark applies to W6, which can, however, be more open than the other two. If all the window ventilators are full open, the force of the movement of air in the direction of from W5 to W2 will throw the breath back upon the audience, and the value of W1 and W8 will be much lessened.
As another example of summer ventilation, a similar church is situated at C1; it is higher than the houses at H, so there will be upward suction over the area between the side of the church and the houses, but the wind will blow dead on with all its force against the side where the windows W2, W3 and W4 are situated. As the tops of the houses are above the ventilating panes in W1, they must not be opened too wide, and a down current should be avoided, especially when the wind is strong. If the door, D, can be opened somewhat during service it would be very helpful to ventilate the church in hot weather when a slight breeze is blowing in the direction of the arrows; and the apertures in the windows above should be open to their full capacity. The openings in W2, W3 and W4 must be regulated so as to avoid over strong air currents being produced by the wind pressure upon them. The apertures in the windows 5, 6 and 7 should act as outlets, but had better not be opened too full else the draughts from 2, 3 and 4 will be much more pronounced.
C2, Fig. 6, is a small church, but the door and window above face the wind. Windows 5, 6 and 7 should be judiciously opened, as the air movement will be from W6 to W3. The church being higher than the houses, and the wind pressing against the door and window above, these may be used to let air into the building in summer. W2, W3 and W4 will act powerfully as outlets and should be regulated in accordance with those on the other side of the church—W2 and W3 being generally more open than W4. The wind blowing along the sides and ridge of the roof will aspirate air out of the area B, and so out of the apertures in W1, which should be opened as wide as possible, but not sufficient to cause intermittent air currents.
The church C2 is so situated that it can be ventilated admirably in summer with very slight wind, and C and C1 are not difficult to work. If the wind, however, blows from the opposite direction, C becomes very like what C1 is when the wind blows as shown by the arrows, only that the houses in front of C will break the force of the wind at W2, W3 and W4. If a church is surrounded by buildings as high as itself, the problem becomes more difficult, and a large high roof ventilator will be most serviceable if the church or building is not upon a street corner, or in an open position. When wind blows over housetops, it causes suction upon all those which are not high enough to catch the impact of the wind, and, as in the case of the open tube alluded to in this chapter, the flowing wind will aspirate powerfully as it blows over the open roof ventilator. If a church or hall is surrounded by buildings higher than itself, the only chance to ventilate it in summer without mechanical aid, is by the upward suction of the wind; and it will be found most effectual not to throw windows near the top of the building too open, whilst those near the floor level should be as wide as possible and the doors open also in warm weather. The aspirating power of the wind is greater near the top of the building than at the floor level, because the air is subjected to less friction. If a church, G3, or a hall, as the case may be, is closed in by buildings on three sides, and the door opens on to the street with a window over it, there should be ventilators high up of good size for outlets, and these, and the door if possible, should be opened wide in hot weather.
After observing the wind aspiration upon apertures in windows, one will be prepared to learn that the suction upon other inlets and outlets by the wind will very frequently overturn the provisions made for ventilating buildings. These overturning effects may be, and often are, accelerated by the arrangement of the heating apparatus in a building. Let S A, Fig. 6, represent the first floor over a free library used as a science and art school. The front door, D, faces the prevailing winds and is kept open. The library and rooms on the ground floor are heated by hot water pipes, and there is a staircase that goes up to the science and art classrooms on the first floor. There were no folding doors to this staircase, the consequence being that much of the heated air from the ground floor was forced up to the passages on the first floor where the air pressure was so great that the Tobin's shafts, indicated by X X X, acted most powerfully as outlets.
In the church, C2, there are Tobin shafts at the point XXX, and when the wind is blowing strongly in the direction of the arrows, and the door D shut, the Tobin shafts will be so powerfully aspirated by the wind that they will act as outlets.
Let the block T H, Fig. 7, represent a town hall situated in its own grounds. The prevailing winds blow in the direction of the arrows, and there are Tobin shafts, X X X, on the two floors, on all sides of the building. The heating is done by radiators in each room, and the foul air outlets pass into the passages near the ceiling and also through the gratings in the outer walls. The pressure of a moderately strong or strong wind blowing against the front of the building drives air up the Tobin shafts in immense volumes, and so much trouble