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pressure of the outer air is exerted upon them; but it will be found very frequently that they are either kept shut, or they are choked by dirt.
It is certain that not more than one-fourth, and often not more than one-sixth, of the total ventilating force in a church or public building is spent in raising the internal column of air, much of the other three-fourths being used up in forcing air through the crevices around the doors, windows, etc. The friction of the air so forced increases according to the square of the velocity, and when this fact is duly weighed, how fatal to the introduction of fresh air is the practice of closing every chink and crack by india-rubber tubing! For if by this proceeding one-half of the area of the cracks and crevices is cut off, the building will not only lose one-half the air which previously got inside, but actually three-fourths the original quantity. How necessary it is, therefore, to see that every inlet and grating is kept clean and open, and that there are sufficient apertures for air to get into the building; but it is fully realised that this is only practicable when the air which enters through such inlets is warmed as it gets into the structure.
If, however, three-fourths of the ventilating pressure is expended in forcing air into a building, it follows that the pressure which exists inside at the floor level must be less than that on the outside—in other words, there must be a partial vacuum inside the building, and, in the majority of instances, there is.
If the hall is low, and there is much outlet space at the top, then the tension on the inside air is practically nothing at intervals, and very little at other times. But, generally speaking, there is a very appreciable tension, or partial vacuum in churches, chapels and halls, notwithstanding the leakage from high windows and from the roof. It is this roof leakage which is so mischievous, and so frequently ignored probably on account of its being little understood. It is generally agreed that the air which gets in at a high window or through the roof is only forced inwards at that point by a pressure equal to the remaining air column in the building. Let it be assumed that a church is 5o feet high from the floor to the apex of the roof. Through the loss of a slate a volume of air issues into the building 1o feet below the apex of the roof. If the ventilating pressure in this building is 2 oz. upon every square foot, then the ventilating force through which air will enter at 1o feet below the
apex of the roof will be or two-fifths of an ounce of
5o ventilating pressure upon every square foot, and this is a
correct measurement of the ventilating pressure at that point. But there is a factor of much importance which has not been considered, and which has, probably, never been previously explained. If what has been stated before is correct, and three-fourths of the ventilating power has been used up in forcing air through the crevices around doors and windows, then less than one-fourth of the difference in density due to the heated atmosphere has been added as pressure to the air inside the building, because a portion of the internal pressure is used up in giving velocity to and in overcoming the friction of the air passing through the outlets in the roof. In a church 5o feet high, the difference in pressure between the air inside the building and that on the outside after the outer pressure was used up in trying to force air into it, would represent the difference between the weight of a column of
. xi x 2 internal and external air about *7 feet h1gh—i.e.,^- or
about 11 oz. per square foot. If, therefore, the air entering
the roof 1o feet from the apex as already noted follows the
law of falling bodies and streams to the floor of the church, it
would exert a pressure equal to a height not of 1o feet
but of 37 feet. This is assuming, of course, that the
cold air streaming in would fall as a solid body and not
separate much and become warmed as it fell, and so lose some of its pressure. In those buildings that are high and where there are two outlets, it is the most natural thing possible that one ventilator should send a flood of cold air downwards in order to neutralise the difference of pressure between the outer and inner atmospheres, and the additional volume of air so admitted and partially warmed would be forced through the other ventilator at a high velocity. The ventilating pressure which has been calculated for the church 5o feet high, is the low pressure obtained during the spring and autumn months of the year when the air outside is about 5o°, but during very cold weather in winter that pressure will be much increased. In the spring and autumn months less is heard of down draughts, because the air outside is not very heavy, but once the temperature falls to 35°, or less, the difference of pressure left in the building, after the outer atmosphere has expended its power in forcing air into it, would, in many instances, be as much as 3 oz. per square foot, were it not for the volume of air which pours down from the crevices in the roof and from the other roof outlets. Unless a corresponding volume of air is admitted at the floor level, it shows how baneful to ventilation are the large top exits, some of which are not exits in cold weather, but inlets. From the above deductions it is clear that three-fourths of the ventilating pressure is generally used up in friction due to the small apertures through which air is forced into a building. Whilst this is true in most cases, great care must be exercised that the air is not introduced through apertures which are too large, otherwise, as will be seen in the next chapter, the action of the wind will lead to unpleasant results.
The influence which the elastic properties of air exert upon the internal ventilation of large buildings, and how alternating air-currents 1 are formed, will be next considered. When a
'The substance of what follows was communicated to the British Association at the Glasgow Meeting, 1go1.
building has too much outlet space but no ventilator fixed either upon the ridge or within the side of the roof, the air gathers in the portion which is ceiled either at the first or second tie of the principal. In nearly every case a grating or gratings communicates with the air space enclosed, and if the building is high, a flood of cold air will pour down through the one which offers the least resistance. The point of least resistance is usually determined by the form of the building itself, although during strong winds the circumstances may be somewhat altered. The easiest path for the upward movement of air is naturally in the centre of the building free from walls and other obstructions, and the grating generally found to have an upward current passing through it is the one in the centre. The position of the doors may make a difference in some cases, but this is the natural result. If there are three ventilators on the roof, or if there is much free way through cracks and fissures, it often happens, especially where the entrance doors face the preacher, that the cold air descends most upon him; least in the centre of the church, and then in increasing quantity at the end where the entrance is. If the door is on the side of the church, the centre of the building has usually the least down draughts, unless the headroom is very considerable, the two ends being more equally and generally affected. This is how down draughts occur, and the difference in density between the air outside the building and the air inside is sufficient to account for these. But persons who frequent large churches and halls cannot fail to notice that where there are severe down draughts, there are other and equally unpleasant experiences. The cold wave of air is followed by a hot and oppressive atmosphere, which may last for a few or several seconds, then there is a period of slight relief followed by one of greater oppression, and subsequently the cold wave appears again. At intervals, these experiences are repeated, and form what are styled "alternating air currents ".
Several years ago, the author proved the existence of such alternating air currents experimentally, and the cause of the alternations, in attending to the ventilation of a large public hall. The hall was very high, 4o feet to the ceiling, and had three sun-burners with tubes passing through the apex of the roof, and these tubes were surrounded by large cylinders for the exit of air from the building. Being desirous of measuring the actual difference of pressure of the air outside a building and the air inside, and thinking so high a hall would afford a good opportunity, a pressure recording instrument was fixed to see what the difference was. The temperature outside registered only 3oo F., and the difference of pressure was naturally considerable, but, surprising as it was, at intervals of about a minute, the pressure inside the hall increased until it was not only equal to that of the air outside, but actually very appreciably above it. The building was well heated by hot water coils for the experiments, and every inlet was opened to the full. The pressure indicator was examined frequently and always with the same results. Taking the point of the least internal pressure as the first observation, it took about half a minute to reach the point of highest internal pressure, and rather less than half a minute after to reach the point of least pressure again. The first five seconds after the least pressure was reached, there was a gradual rise, followed by double such an interval of more rapid increase; then there were a few seconds of lesser increase, followed by a lengthened period during which the instrument remained almost steady. When the reduction of internal pressure began again, much cold air still descended, and there were ten or more seconds during which the reduction in pressure was gradual, then, for about half that period, a very rapid decrease occurred, followed by several seconds when the instrument was steady and almost stationary at the point of least pressure.
From the results of these and subsequent experiments, it