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which have no ceiling and are boarded to the apex of the roof become cooled more quickly than the walls, and the consequence is that where the interior of a church shows only a temperature of 5o0 or 55o through whatever cause, the upper portion of the roof will soon become cooler when air from the outside is admitted with each person coming in, and, in very severe weather, cold currents may descend from the roof even when there are no perceptible cracks.

Where the electric light has been installed it is often thought that the fault of these downpours of cold air is due to the heat being less than was given off by the gas previously used, forgetting that no gas was lit for the morning service. Electric light or any other light, it is necessary to make the church comfortable, and this cannot be done in any other way so effectively and so conducive to health, as by raising the temperature of the walls and inside material to 6o° before the time of assembly. Where the area of the top outlets of the building is too excessive, and not under control, it should be remedied, as the decrease in the coal bill would soon pay the cost, whilst the comfort which would follow would be greatly appreciated.

In some churches where the electric light is' installed, gas is used above the lower bond of the principals of the roof to heat the upper stratum of air in the building, and recently burners of the Bunsen type have been fixed in some churches. The products of combustion where the flames play upon iron gauze are deleterious, and the ordinary white gas flame is much preferable. Some persons think that gas burnt in a Bunsen burner gives more heat than if burnt in a luminous flame, but, for equal volumes of gas, the total heat evolved is the same. Once the roof of the building was under control, and it was possible to heat the walls as suggested, then it would be unnecessary to use gas to heat the air near the roof, and no expense in reason should stand in the way of this result being attained.

Vent1lat1ng Pressure.—When the air is warmed, the molecules of which it is composed are said to set themselves at greater distances—in other words, the volume of air involved expands and is consequently lighter bulk for bulk than air of a lower temperature. As air is one continuous ocean, it stands to reason that the lighter portion must always be lifted upward; so that in a building where the atmosphere is warmed artificially, there is a force on the outside always tending to raise the air upwards and cause circulation. According to the number and area of the inlets by which the outer air can gain access into a building will be the rapidity with which the internal air will be moved, and if these air inlets at the floor level permit an adequate volume of air to enter, then the ventilating force or power upon the building is sufficient for the time being. In churches and public buildings the greatest difficulty to obtain enough ventilating pressure is experienced in the autumn and late spring when the outer air is cold enough to cause a draught if a fresh air inlet is opened. If the air is about 55° outside and it is not possible to warm the air without making the audience feel hot, and the air is moist, the ventilating pressure of the building will be at its worst; and when, as will be shown presently, it is seen that three-fourths of the ventilating pressure may be used up, and most frequently is, in overcoming the friction the air encounters in getting into the building, very little pressure is left by which the air in the interior can be moved, and expelled through the outlets. When, in addition to the friction mentioned, it is pointed out that in the majority of churches and halls by far the largest volume of air which gains access to the building comes in at the top, and, by so doing, reduces the actual ventilating pressure available to force air in at the floor level on the outside, it will become evident, so far as so-called natural ventilation is concerned, that the usual appliances provided do not succeed in using the full ventilating power to the best advantage.

Some persons suggest that air inlets of the Tobin type would remedy this state of things. There are many churches where they are fixed, and in nearly all cases if persons sit close to them they are not opened at all in cold weather, because the flood of cold air is intolerable. There are buildings where they have not been opened for years. What is wanted is warm air at the floor level; and sham air inlets are worthless, and Tobin shafts are sham inlets when fixed close to where persons are sitting, and where they must be kept closed.

From what has been stated, it will appear evident that the ventilating power of a building depends upon the difference in density between the column of air inside a structure and a column of outside air of the same height. This force available for ventilating can be readily calculated. A church or hall is 5o feet high to the apex of the roof, and as there is no ceiling, it has 5o feet of headroom. The temperature of the air inside is 65° F., and that of the air outside is 5o°. Fifty cubic feet of air at 5o° F. weigh 3"8o lbs., and 5o cubic feet at 65° F. weigh 378 lbs. The difference in weight between the outside and the inside column of air is therefore '11 lb. per square foot, or nearly 2 oz. pressure upon every square foot of air surface.

A hall with 3o feet of headroom has a ventilating pressure

under similar conditions, 5-="066 lb. per square foot.

The hall with 5o feet of headroom has a ventilating pressure 11 times as great as a hall with 3o feet of headroom.

What is the meaning of the term ventilating pressure as applied to a public hall? It is simply this, that the outside air at the floor level of the hall will not only balance the air inside at the floor level, but will raise it, and force it upwards with a pressure equal to 2 oz. exerted upon every square foot of surface—the pressure increasing with the height of the building, a rise of temperature inside, or by the lowering of the temperature of the outside air. When this theory of ventilating force became widely known, a somewhat natural, though mistaken deduction, was made. It was thought that if the ventilating force was exercised in raising the column of air in a building and propelling it upwards, what was wanted primarily was a free path or exit for the ascending air. It was said " there must be ample room to let the air out at the top, and then there will be ventilation ". It did not strike the advocates of roof ventilation that there should be facilities afforded to let a volume of air into the building equivalent to that which should pass through the roof outlets, nor was any calculation made of the extra volume required because of the cracks and interstices in the roof. When ventilators were fixed, and there was much interstitial space in the form of cracks and fissures in the roofs of churches, it was found, as a first experience, that more fresh air tried to get in near the floor level through the doors and windows to supply the place of that which had so freely made its escape at the top. Then the seat holders near the doors and windows complained loudly of draughts and threatened to migrate; and to pacify these, the church authorities, without understanding what they were doing, ordered thick woollen strips around the doors, and the close adjustment of the windows, whilst ere long the inventor assisted with india-rubber tubing to prevent all draughts. The church authorities did not seem to realise that they were carefully excluding the outside air, and so failing to supply the necessary volume to pass through the ventilators and other outlets in the roof of the building. The consequence was that little air got into a church or hall at or near the floor level, and what did get in was not sufficient to move through the openings in the roof and through the ventilators with sufficient velocity.

Most people think that the outside ventilating pressure upon a building has done its work when the apex of the roof or the mouth of the ventilator is reached, and that a sort of equilibrium exists there, so that no consideration is given to the fact that the pressure upon the air in the building is less than that upon the air outside, and that it is only the velocity of the air moving towards and through the top exits which prevents the outer air gaining admission into the building. Let that velocity fall below from 2 to 3 feet per second according to the height of the building, the simultaneous movement of the audience, as it sits down after a hymn, or a patriotic demonstration as the case may be, will be sufficient to upset the equilibrium, and cause down currents of cold air to enter through the roof exits. Owing to so many of the churches and public buildings having excessive top outlets when the air outside is very cold, and to there being so great a lack generally of fresh warm air admitted at the floor level, the velocity of the air passing outwards is too low, and at intervals of half a minute or more the roof outlets permit streams of cold air to descend, and give rise to those undulating and intermittent currents of air in large buildings which are so detrimental to good ventilation, and so unpleasant to the audience. If the building has no roof ventilator, and the velocity of the air as it escapes through the cracks and fissures in the roof is too low, streams of cold air enter in through the larger crevices until a point of internal pressure is reached when the outgoing air is expelled at too great a velocity to allow any further quantity of cold air to descend from the roof.

Let it be assumed that a church has 5o feet of headroom, and a ventilating pressure of 2 oz. per square foot of air surface. This pressure is most ample and will afford a velocity through a given opening at the apex of the roof of 1o feet per second without any difficulty, if the pressure can be exerted upon the air in the building at the floor level. But it cannot be exerted at the floor level, because the doors and windows have been most carefully adjusted. It is not forgotten that many buildings have fresh air inlets, and that the

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