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was found that when the atmosphere in a large building is very hot, the period of least pressure referred to is extremely trying, because the quiescent foul air is re-breathed, and the effects are increased probably by the electrical conditions due to the sudden expansion of the air; but when the fresh air commences to pour down, the oppression decreases until the point of greatest pressure is nearly reached. During the period of comparative rest at the highest pressure the dense air is being heated and expanded, and, until it begins to move upward, the breath of the audience is inhaled, and a feeling of oppression experienced. When a large volume of air descends, the waves flow over one portion of the building, and, owing to the sudden compression, not much of the fresh air reaches the level of the sitters, so that during the periods of greatest pressure the air is so beaten back as to become very foul, whilst during the period of least pressure the conditions are even worse.
A point of much interest is the fact that, owing to its elastic properties, the denser and colder air descended and compressed the layer underneath so much by falling through so great a height, that the pressure due to the density and velocity of the descending current upon so elastic a body gave rise to a greater pressure inside the building than there was outside. The coldness of the night, and the heating of the building inside, considerably helped these results, which the author never succeeded in getting in a low building. It was found frequently, however, that alternating air currents at their greatest pressure in high buildings in winter, beat back the inlet currents near the ground level, and in several instances an anemometer held in a narrow opening in a doorway leading to a church turned rapidly inwards indicating an up current, and then stopped and subsequently turned outwards during the periods of greatest internal pressure.
In the hall mentioned, there were a number of hot water coils near the outer walls with air inlets behind them. These
were tested with a very delicate anemometer, and during the period of greatest internal pressure some of the air in the building was forced out through the fresh air inlets, the great elasticity of the air doubtless having much to do with this.
In high churches having much outlet space in the roof and heated by hot water pipes, where alternating air currents abound in winter, it often happens that the fresh air inlets are checked and act as outlets during the period of greatest pressure; and when the period of reduced pressure commences they act as inlets again. When, however, the period of rapid decrease in pressure is reached, and the temperature of the air outside is very low, such a deluge of cold air ascends the gratings that the audience cannot stand it, and the consequence is that the inlets are shut altogether, and the ventilation of the building considerably impaired.
Where sun-burners are fixed underneath ventilators, and air shafts passing through the roof, it is not very difficult to watch the progress of down draughts and intermittent air currents, by carefully observing how the gas flames are affected. If there is a continuous and powerful up current, the flames point upward and become elongated. If the up current is violent, as it is where powerful alternating currents are formed in consequence of the top exit space being much too great for the low temperature of the air outside, it may be, as it was in the case of the public hall to which reference has been made, that the luminosity of the gas is more or less lost owing to the flames being rendered blue by the quick movement of the air. If the building is high and has two such sun-burners and much top outlet space, then, in cold weather, the flames of the two burners will appear irregular, showing that there is sometimes an up current and sometimes a down draught. In less cold weather it will be found that one of the sunburners will show a continuous up current, and the other admit a more or less severe downpour of cold air at intervals, according to the frequency with which the doors are opened, and the temperature of the air outside. A careful observer can tell from the flames how matters stand, but, unfortunately, the tubes from the sun-burners are not responsible for all the mischief. Then again, the majority of churches and public buildings are not furnished with sun-burners, and those which are, are rapidly becoming fitted with the electric light. In order to try and prevent down draughts, it not unfrequently happens that when the electric light has been installed the sun-burners are used to cause an up current. This is a great waste, and can be obviated without much difficulty.
It must not be forgotten, therefore, that in high buildings the top outlets require to be under perfect control, otherwise, when a reasonable supply of fresh air has been provided near the floor level, this supply will be rendered inoperative by the action of the alternating air currents in the building. Excessive top outlet space is always the cause of the mischief, and, when the greatest pressure is formed inside the building, no air can come in at the bottom. When the least internal pressure occurs, the suction on the air inlets near the floor level is so great, and so much greater because of the elastic nature of the air, that in spite of the best provision for warming the air as it enters the buildingj the fresh air will pass in so rapidly that persons cannot endure it in very cold weather, and the air inlets will be shut.
By careful observation, it is alike possible and interesting to note the form which the waves of air assume under the influence of intermittent air currents. These waves will vary in length and depth according to the height and other dimensions of the building, as well as upon the position from which the greatest downpour of cold air comes. The church which is described in sequel, and shown in Fig. 19, p. 85, having one large outlet above the dome, is so high and spacious that the cold air falling and giving rise to intermittent air currents compresses the lower layers of air and forms waves like those shown in Fig. 1. These waves have horizontal undulations, and their crests, for the most part, are not high. The falling cold air compresses the layer underneath, and at the same time spreads outwards as it falls. Fig. 1 represents the waves formed by the cold air streaming through the opening in the top of the dome. A few seconds later, and the falling air has compressed the
atmosphere in the building to the utmost extent, and immediately afterwards the cold air becomes heated, expands, and gives rise to the upward movement, when the position of the crests of the wave will be, naturally, reversed. The doors are opened frequently to admit persons, and the pressure of the air coming in presses against that just above the
level of the seats and forces it in the direction of the pulpit, P. When this occurs, the lower waves during the downpour of cold air become shorter, and have much higher crests in consequence of the forward motion, and the form of the undulations somewhat resembles Fig. 2—the short waves with the high crests being next to the pulpit end.
The church mentioned in Chapter V., and shown in Fig. 16, has a considerable down draught from one of the ventilators, whilst there is a strong up current through the other. The movement of the air is chiefly from one end of the building to the other, or pulpit end; but there is a lateral movement induced by the circulation of hot air, and in this case the waves of the intermittent air currents approach in form to Fig. 2.
In theatres and buildings which are very high compared to the width or diameter of the free air space or uninterrupted column of air, and where a huge chandelier is fixed under the dome at the top, it frequently happens that the outlet space is
very excessive when compared with the area ot the inlets, and alternating up and down movements of the air are very marked. These movements may not inaptly be compared to those of a spiral spring, Fig. 3, when it is compressed and released, only that the waves would be more horizontal at the points facing those portions of the building where the chief open spaces occurred.
Owing, however, to the elastic nature of the air, the wave movements are not mechanical, but responsive to the various local influences which surround them. It will be evident from the illustrations that the waves must vary in length and form according to the circumstances mentioned.