so caused. Then, in the majority of instances, the valve fits very slackly, because it was thought of slight importance, and great volumes of air leak around it in cold weather.

In many instances where ventilators are fixed, the roof is ceiled either at the second principal bond or at the wallplate, and the metal tubes of the ventilators may consequently be from 8 to 20 feet in length at their termination above the roof. For the convenience of shortening the ropes, and getting at the valves, the latter are almost always fixed at the lowest end of the tubes, the result being that from 8 to 20 feet of cold air column above the valve is a perfect nuisance, when the velocity of the air escaping through the ventilator is low, or when the area of the tube is reduced by closing the valve. Let Fig. 23 represent a building ceiled at the wall-plate, having a ventilating tube which measures 18 feet in length from the ceiling to the top of the roof. The hall is 30 feet high, and if the valve, instead of being fixed at the bottom, were placed at the top of the tube, and the cracks and crevices had been properly adjusted, the ventilating power of the building should be measured by a column of air equal to 30 + 18 or 48 feet in height. But with a valve at the bottom of the tube, the 18 feet of ventilating pressure can only be added when the valve is fully open and there is enough air passing through the tube at sufficient velocity to keep the outer air from entering. If the valve is one-third open, two-thirds of the 18 feet tube or more may be filled with cold air, and the greater part of the ventilating pressure will be lost. And this is neither the only nor the most serious fault, perhaps, since air being so very elastic, the cold air in the 18 feet of tube when the valve is less than half open would compress and overcome the heated air, and give rise to alternating air currents which are so detrimental to ventilation. All roof ventilators, therefore, should have appliances for closing them perfectly, and these are best fixed

Butterfly

at the mouth of the tubes at their upper end. valves are most unsuited for the purpose, because when partly or even full open, they favour the formation of the down as well as the up current, owing to the manner in which they divide the tube.

The rope connecting the valve of each ventilator with the part of the building where it is to be fixed and regulated should be of steel strand wire, and the lower end of the wire rope should be attached to some arrangement connected with an indicating board showing how far the valve or cover should be opened according to the temperature outside the building. If some such arrangement is not furnished for the caretaker, it is not possible for him to gauge the ventilators and regulate the air outlets so as to prevent down draughts, and to place the building in the best condition for ventilation.

The question might be asked, however, "Why not close up the ventilators permanently or get rid of them altogether, if the area of the crevices in the roof is sufficient?" The ventilating power of a building depends as much upon the temperature of the air outside as upon the height of the building. When the air outside is 35° F. and the air inside is 65° F., there is more than enough power afforded by the difference in density between the two air columns to ventilate a high building, and force the foul air through the cracks in the roof. When, however, the outside temperature is 55° F., and the artificial heat is discontinued, the friction with which the foul air meets in passing through the crevices in the roof, reduces the volume of air which gets into the building very appreciably, hence it would be a decided advantage to have ventilators on the roof through which the foul air could escape freely and without encountering much friction. Furthermore, the heat of the sun in the summer warms the roof so greatly that ventilating tubes materially assist the upward current of the air, and, for these reasons,

it is desirable to have ventilators, provided they can be closed perfectly.

Old-fashioned louvre ventilators are not desirable unless they can be nicely regulated, because, if large enough for summer ventilation, the leakage of cold air during the winter months is sufficient to cause severe down draughts, and give rise to up and down movements of the air in a building. Where a large louvre is fixed, from which cold currents of air descend, and where gas jets are used to assist the up current because the ventilation of the building is top heavy, such defects can be cured by a suitable double valve arrangement for regulating the outlet area in cold weather, and connected with an indicator board for accurate adjustment according to the temperature of the air outside. If the caretaker possesses an instrument for detecting down draughts, it will be possible for him to regulate the valves and avoid down currents of air even in the coldest weather, if proper arrangements have been made for adjusting and closing the valves.

Some buildings are fitted with silk or mica valves underneath a louvre arrangement. The gas jets are distributed, necessarily, inside the building, and not immediately under the ventilating shafts, but the force or pressure for ventilating is not enough in low buildings to raise the valves when the outside temperature is above 50° F.; and such valves or flaps are decided and serious hindrances to ventilation after artificial heat has been discontinued in the spring and throughout the summer and autumn. The ventilation would be much improved by removing the valves, and placing a tube of reasonable diameter provided with a close-fitting cover at the upper end and connected with an indicating arrangement as already mentioned, or, should the opening be very large, by adopting a double valve, the smaller being for winter and the whole opening for summer use.

Some of the roof ventilators upon the market are said to

entirely prevent down draughts. It is difficult to assume that the makers are ignorant or that they do not know that the statement is untrue, but such assertions are as false as they are misleading; and, furthermore, if the ventilators possess any real exhaust power they must cause down draughts in consequence of the general lack of inlet air.

What is wanted in a roof ventilator is a covering to prevent the rain falling into the inside tube, and one which instead of suddenly exhausting the air will, on the contrary, as perfectly as possible equalise the pressure which may have been disturbed by the force of the wind, and the air currents above the roof. If suction is to be applied in the roof of a building then it should be done by mechanical means alike continuous and reliable, and not dependent upon the vagaries of the wind. In the absence of such mechanical aid, nothing should be allowed to interfere with the ventilating pressure of the building, which cannot be assisted by any form, fashion or shape in the ventilator itself.

So much was said of late years about the "self-acting properties of pneumatic ventilators that the real ventilating force available to ventilate a building was rarely mentioned. When so-called natural ventilation was referred to it was generally in relation to the aspiration of the wind. Now that. the aspirating action of the wind is known to be so erratic and uncertain, more attention is being given to the ventilating force upon every building which results from heating the air inside, and which is increased by the heat from the lungs and bodies of the audience as well as by the moisture in the breath. This ventilating force is reliable. It varies in strength according to the temperature of the air outside, but if this ventilating power is studied and made use of effectively, after placing the inlets and outlets under proper control, the caretaker ought to be able to ventilate his building in a much more satisfactory and certain manner than heretofore.

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From what has been said, it is evident that the first thing to be done to improve the ventilation of present buildings is to provide warm fresh air and introduce it at the floor level. At first sight this seems an insuperable difficulty in most instances. Where hot water pipes are laid below the floor in the aisles, etc., the inlet air shafts generally enter and meet the pipes at too great a distance from each other, and the difficulty is to warm the volume of air which rushes up all in one place. The lower pipe should be covered with finely perforated zinc so as to cause the cold air to be more evenly distributed, or, if the air inlet is large, sheets of perforated zinc should be curved downwards and placed under the iron grating above the pipes, then the weight of the gratings will keep the zinc sheets level on their outer edges. The perforated sheets must be cleansed frequently to prevent choking. A little care in this respect will do much good, and, by nicely regulating the outlet spaces in the roof to prevent the ventilation being top heavy, the results will be fairly satisfactory. On examination, it will be found that the fresh air inlets are generally closed, because of the draughts experienced in winter for want of the air being distributed all along the pipes.

Where air is heated inside the building by passing down one grating over hot iron plates and up another grating, very little can be done to improve matters. If the floor of the build. ing lies close to the ground, hot water pipes should be laid if possible, and a large volume of air introduced and carefully distributed over the whole length of the pipes, which should be laid in front of the communion rail, and in other convenient spaces in the aisles and near the walls. The outlet spaces at the top of the building should be carefully regulated, for, if the ventilation is top heavy, the warmed air arising from the pipes will simply shoot upward towards the roof and cold air descend in the centre of the church between the aisles. It is most important, therefore, that the roof outlets shall be well regulated.