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When a building has been raised a few feet above the earth, it may be possible to send hot air (from a battery of pipes below and outside the structure) underneath the building. This is the most desirable method, as it is possible, then, to subdivide the air as it enters through the floor far better than it can be done from hot water pipes laid below the aisles. There is generally a width of 2o to 3o feet between the two aisles or footways occupied by the rows of chairs or seats as the case may be, and the centre of a building will not receive a proper air supply, as the tendency of the fresh air arising as it does from the pipes under some pressure and velocity is to go upward too quickly to spread much laterally.

A large number of buildings used for religious purposes have schoolrooms underneath with headroom from 1o to 16 feet. It is easy to ventilate a building which has a schoolroom underneath, but it is not so easy perhaps to convince the authorities that the breath of the children can be swept away and the schoolroom used as a warm fresh air distributor for the building above. Where there are schoolrooms underneath and ground enough adjoining to place a battery of pipes to warm the air before it enters the schoolroom, it is possible to ventilate the church over it almost perfectly. In many Nonconformist churches "Pleasant Sunday Afternoons" are held, and, if the building can be cleared of breath and foul air before the evening service, it stands to reason that the schoolrooms can be likewise purified, and the latter can be done more easily because the ventilating power of the building will be increased by the additional height of the schoolroom—-a most appreciable and useful adjunct. The ventilation of schoolrooms underneath chapels and halls is usually very bad indeed, because the headroom is so low, but if the ceiling and the floor of the building above were perforated with sufficient openings, the ventilation would be vastly improved. If the schoolroom is ceiled, it should have strips of coarsely perforated zinc run across the joists at intervals, and the air should pass through inlets in the floor to supply each pew. Other inlets should be fixed in the aisles, around the pulpit, in front of the communion rail, and, if possible, underneath the floor of the galleries, if there are any, into each pew there. The inlets should stand about two inches above the floor, and should be covered at the top to prevent the air shooting up, and also to keep them clean. No valves should be supplied whereby the pew occupants could close the inlets. Care should be exercised, however, that the inlets should be capable of being regulated underneath, so as to set them more or less open at first, in order that those nearest to the incoming air should not admit more than their share.

Neither sentiment nor prejudice ought to prevent the use of schoolrooms for the purpose indicated, and, if proper attention is given to the roof outlets of such buildings, then the ventilation will not require mechanical aid, and, even in the worst weather, it will be passable.

CHAPTER VII.

MECHANICAL AND HEAT-AIDED VENTILATION.

At the present time it is difficult to say which is most generally advocated, mechanical, or so-called natural ventilation. Now that the electric light is becoming so widely adopted, and electric currents can be obtained in small towns even, it is possible to get mechanical aids to ventilation in the form of electric fans, which, during the next fifty years, will revolutionise the ventilation in new buildings. Some of those who sell appliances for ventilation say that mechanical aids are of no value, and those who sell fans say, naturally, there is nothing like them for ventilating buildings. Common sense ought to give its verdict in favour of the fan, but common sense speaks of a thing as it finds it, and not always in accordance with its deserts. The fact is, up to the present, mechanical-aided ventilation has not proved successful, and, as so-called natural ventilation does not play quite so many pranks on account of its lesser power and cheaper working, it still meets with considerable favour. It is only fair, however, to enquire why it is that fans have been so unsuccessful, and it may assist one in forming a fair conclusion if the ventilation of a building having a fan in a low turret on the roof were examined to see wherein the defects lay. The building in question was large and ceiled at the wall-plate, and was about 35 feet high to that point. A large outlet in the centre of the ceiling communicated with a fan immediately above and underneath the turret upon the roof. This building is not given as a type of ventilation by mechanical power, but it will illustrate how such aid is rendered inoperative. Before the fan was fixed in this building, very little attempt had been made to introduce fresh air, and no provision was made for warm fresh air to come in at the floor level. As soon as the fan started it was found that the tension inside the building increased and became considerable, the result being that the leakage of air around doors, etc., caused severe draughts. The moment a door, either on the ground floor or in the gallery, was opened, floods of cold air rushed into the building, and the draughts were most unpleasant and disappointing. To obviate these, the doors were carefully fitted with felt to exclude the draughts, and glass screens were placed in front of the doors leading to the galleries, and by the side of the aisles adjoining the doors downstairs. When the chinks around the doors had been stopped, and the openings in the windows adjusted, so thoroughly were the draughts prevented, that the building was fairly air-tight; and it was really a mystery where the authorities expected the fresh air to come from. Worse draughts, however, were to be encountered. The fan being able to get only a very little air from below, exercised its power in placing the atmosphere in the building under the greatest tension possible, and it became so sensitive that the sitting down of the audience after standing brought a flood of cold air down through the central opening in spite of the revolving fan; and even when a slight wind was blowing, the building became subject to the intermittent air currents which have been already described. To obviate these down draughts from the central outlet, the fan was slowed down somewhat, and jets of gas were arranged just above the ceiling level under the outlet with a view to assist the up-current. " During the winter, most of the air getting into the building came down the central opening in fitful gusts at intervals. In warm weather, when the movable panes in each window could be opened, more air was available, both on the ground level and above the galleries. The movable panes were about 14 inches by 8 inches in area, and were regulated by cords fixed close to the seats. Some time ago the author was present at a crowded meeting in the building when the temperature of the air outside was about 53o F., and the fan was revolving slowly and regularly—all the window openings and doors being closed at the beginning of the meeting. The atmosphere soon got foul, and one of the audience opened a ventilator in one of the windows underneath a gallery downstairs. The air shot like an arrow towards the centre of the building, because of the reduced pressure inside, and was of little benefit to those near the opening. Soon afterwards, three similar ventilators were opened in the windows over the galleries, and these likewise shot streams of air towards the central outlet. The fan then obtained nearly all the air it required without much friction through the four openings, the consequence being that very little came in from the crevices around the doors, etc., and the atmosphere in most parts of the building was stagnant and very foul. Most persons unacquainted with physics would condemn the fan at once as giving rise to down draughts in winter, and being useless just at the very time of the year (spring) when the temperature outside (53o F.) was near its lowest ventilating value, and when, if ever, the fan ought to be of the greatest use. But the fact is, the fan was not to blame, as it did its work regularly, and its best under the circumstances. Mechanical, and heat-aided, or so-called natural ventilation, run upon similar lines, and what is essential to the success of the one is essential to the success of the other. It is no use trying to get air out of a building until provision is first made to let air into it. Had the authorities heated a large body of air, and allowed it to get into the classroom underneath the assembly hall> and then through numerous apertures in the floor into the hall itself, the fan would make sure that the

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