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the pulpit, P. A nearly horizontal and forward movement of the warm air vitiated by the breath of those sitting behind is then experienced for some seconds, succeeded by an upward current when the cold air which descended from the dome had become heated, and a similar state of things was repeated at intervals. It was only during the wave-like pulsations of the air and when there was a movement upwards that there was any appreciable decrease of pressure in the building. Owing to the height of the dome, and the large area underneath, the form of the intermittent air currents was that of the up and down movements of the plunger of a pump chiefly. The large outlet in the dome should have two valves—one to be closed in cold weather, and the other capable of accurate adjustment, and much warmed fresh air ought to be admitted near the floor level.
Fig. 2o is a type of many of the chapels and halls built forty or more years ago, before the necessity of conveying the foul air out of the building through metal tubes, which passed from the ceiling to the top of the roof, was recognised. These buildings were ceiled at the wall-plate, and the foul air was simply admitted through gratings into the roof. If the building was large there were several gratings, but in nearly all cases the area of the outlets into the roof was from 5 to 2o times as great as the area of the inlets for fresh air. In the winter, two out of the three openings shown in the sketch poured cold air into the building at intervals, and if a door was opened wide for a second or two and an up current started, a few seconds after the door had been closed a re-action occurred, and a great downpour of cold air supervened. Whilst the door was open, the movement of the air was naturally in the direction of the pulpit. The chancels in most churches, the pulpits in most chapels, and the platforms in most halls are at the end farthest from the main doors. The front of the building, therefore, is usually most exposed to the wind and pressure in the open atmos
phere, and, under these circumstances, in the cases of chapels and halls especially, the leakage of cold air from the roof occurs more at the exposed end than at the other. Generally speaking, therefore, the opening near the door will be the one from which a down draught will be chiefly experienced, and the movement of the air in these buildings is usually from the front to the back—from the entrance door towards the pulpit or the platform, as the case may be. Hence it often occurs that whilst the preacher experiences the warm current of air moving towards the pulpit, he also has to breathe the atmosphere already vitiated by the breath of the audience.
In those churches where the roof is ceiled until the second bond of the principal is reached and there are high buildings near the pulpit end, a down draught not unfrequently occurs through the gratings above the preacher's head. This is usually induced by the action of the wind, which upsets our calculations as well as the ventilation of a building. The movement of air through large openings into the roof is very erratic and rarely follows any fixed rule, as it is at the mercy of the wind and other currents in the atmosphere of the roof above the ceiling. The general effects of gratings which open into the area of the roof are discomfort in winter with serious colds, but in summer and in weather when the temperature of the air outside is 6o° F. they aid ventilation, and some buildings of this kind are remarkably cool in hot weather, chiefly those, of course, in which the ceilings are high. Neither the ceilings nor the gratings opening above them are to be recommended, and chapels and halls are rarely built like this now. Where such gratings exist and intolerable draughts are experienced, valves or covers of wood which are close-fitting should be fixed near the gratings in the roof and these opened as much as will best suit the ventilation during cold weather.
Mission halls and low buildings without any roof ventilation and dependent upon the windows being opened in winter, are traps for propagating pulmonary disease; and whilst the free will of the subject should be as far as possible respected by the State, it is a question whether some supervision by the Local Authority should not be exercised over all buildings where the public assemble, in order to secure the comfort and well-being of the audience, and the preservation of health. Fig. 21 is a rough drawing which represents the classrooms and assembly hall of a public school. There are 1o classrooms on each of the two floors, and there is a large air outlet in every classroom close to the ceiling at the nearest end to one of the two shafts, S S. These shafts are of large area, 7 feet square, the idea being that the larger the shaft the more air would circulate when the temperature of the air outside was moderately warm. The prevailing winds blow from A to B. In consequence of the large area of the shafts, down currents prevail in cold weather, and intermittent air currents are formed in the large hall, and during windy and cold weather in the classrooms also. The building is heated by hot water pipes and coils throughout; and the fresh air comes in beneath the windows and through gratings in the outer walls. When the wind blows sharply in cold weather from A to B, the air inlets and gratings in the end of the wall A are under increased wind pressure, and vast volumes of air get in through them giving rise to much discomfort. To obviate this, the valves of the gratings are frequently closed, and become further clogged with dust in consequence. When the deluge of air got into the classrooms in the end A, the air outlets swept much foul air into the air shaft, S, and, as the volume of air admitted into the classroom through the inlets on either side was decreased by reason of the aspiration of the wind upon them, the foul air which travelled into S from the classrooms at the end, A, passed back to those on either side of the building and into one end of the hall, forming down currents which at times drove the inlet air back into the outer atmosphere. During