HEATING AND VENTILATION. 698 HEATING AND VENTILATION. which it contains. Another cause of diminished cliange of air through the walls and the cracks oxygen is the burning of oil and gas for illumi- around chimneys and doors; but in wellcon- nalion. As an index of the impurities present, structed houses this may be very small in fairly and of the diminution of oxygen, carbonic acid cold weather, and jiractieally nothing in mild has been chosen. For convenience it is generally weather, while for schools, churches, and the like expressed in parts per 10,000, thus giving 4 it is of little consequence excejjt on very cold or jjarts per 10,000, instead of 0.04 per cent., as the windy days. This interchange may well be normal average amount in the air. In the purest called imturul ventilation, a term which may air the carbonic acid may fall to 2.2 parts; in also be extended to include all ventilation de- cities it may be normal at 4.5 parts; in occupied pending solely on the dift'ercnce between the in- rooms it may go to 33 parts per 10,000, or even ner and outer air. It may be added that natural higher. Without going into the calculation in- ventilation has been used quite extensively in volved, or considering the other impurities, it contradistinction to forced ventilation, and in may be said that when the air of a locality shows place of the term gravity ventilation employed an excess of carbonic acid above the normal for in this article; but when gi'avity ventilation is the locality as great as 2 parts per 10.000 it assisted by fires at the base of the stocks it, also, is considered to be polluted; also that it has been seems to belong properly under forced ventila- found desirable to supply each person in a room tion. The more recent nomenclature adopts with at least 30 cubic feet of fresh air per min- gravity in place of natural, and mechanical in ute, or 1800 cubic feet per hour. Obviously, the place of forced ventilation. Mechanical ventila- actual amount of air required will vary with the tton, as the name implies, depends upon ma- charactcr of the rooms and their occupants, two chinery for tiie movement of the air. It may be very important considerations being whether or used to introduce fresh air, to remove foul air, not the occupants are engaged in physical exer- or both. Where indirect heating is employed the cise, and the number of hours of continuous occu- warm-air and the fresh-air supplies may be pancy. After a careful consideration of the sub- identical, but in mild weather the combined sup- ject. including a weighing of authorities, Dr. ply does not move readily by gravity alone, so John S. Billings, in his work on ventilation, gives mechanical aid may be advantageous. With di- the following table: rect heating an independent air-supply is re- Cubic feet of air quired, and in cold weather it, also, must be perlionr heated. When not heated, such a supply may Hospitals S.GOOperbed need to be urged along by mechanical power. Legislative assembly halls 3,6noperseat With either indirect nr direct heating an air- Barracks, bedrooma, and workshops 3,000 per person ,, , , ,, . „^..„„ta<rp of nerfect control .Schools and churches 2,400 per person "'a^'^ "af ^ne gieat aaaniage oi periecr. control Theatres and ordinary halls of audience ...2,000 per seat of the fresh-air supply. If a hot-air blast is Office rooms 1,800 per person emploved, both heating and ventilation are under Sntas™m1.!!!i!:"*.™°"!.;V.V.;;V.:.-.;;.?:rrrperBon equallV good control, and it is claimed that the heating is more economically done. For the re- These figures are for use in planning new moval of foul air mechanical means are also ad- buildings, where no allowance is placed "on vantageous. as being more reliable than gravity, leakage through crevices or on bad construction The most common mechanical aids employed are of the building as a source of air-supply," and blowers or fans, placed in the air-ducts, and where it is "assumed that the walls will be reii- either forcing the air ahead of them or creating dered more or less impermeable by paper and a vacuum into which it rushes. Screw propellers paint." In the case of old buildings, where for and pumps ma.v also be used to force the air. any reason it is difficult to supply so much air The terms plenum, or inflating, and vacuum, or as recommended above, Dr. Billings says one- aspirating, are applied, respectively, to blowing in half the amount may be made to do, but it will and to sucking out air. The great danger in be unsatisfactory. He also emphasizes the fact either system is that the fans, blowers, or other that the whole ventilating and heating system air-movers will be too small, in which case the must be proportioned in accordance with the fig- friction losses will render them expensive to ures given, or any modification of them which operate, and the air current will be so rapid as may be adopted, including all air-ducts, inlets to cause draughts, or else an insufficient amount and outlets, blowers, and heat-generating appa- of air will be moved. There is also danger that ratus. a desire to economize in first cost will lead to Ventilation may be divided into two classes, air-ducts deficient in size, with the same gen- gravity and mechanical. Gravity ventilation is oral results. The proper elevations for introduc- based on the fact that warm air is lighter than ing fresh and removing foul air are still mooted cold, and hence is overbalanced and replaced questions. It is certain, however, that the rela- by the latter. In cold weather the inner is tion of the inlets and outlets should be so placed warmer than the outer air, so, if sufficiently as to .aive the nio.st perfect dift'usion of the fresh capacious inlets and outlets are provided, the air and to leave no sta.<jiiant foul air in the rooms, warm, foul air will be displaced by cool, fresh which will also tend to avoid heavy draughts, air. But in warmer weather the diflferences in lIany authorities favor the location of the in- teniperature, and consequently in pressure, are lets and outlets on the same side of a room, the less and the interchange lags. The currents in inlets to be above the heads of the occupants, the ventilating ducts may then be aided by a Tlie theory- is that the fresh air will ascend to slight heating, which may be eflfected in a and pass along the ceiling, and that the foul air variety of ways, such as a light fire, or even a will pass along the lower part of the room and gas jet or jets in the base of the ventilating then out. One of the chief objections to locat- shaft; or a coil of steam or hot-water pipes, ing inlets, for either heating or ventilation, in where feasible. Even if there be no ventilating the floor is that the ascending air carries up flues, there will be a more or less constant inter- floor-dust with it : and another is that unless the
