VENTILATION 161 in as a cold draught at every crevice and casual opening to the outer air ; it avoids drawing foul and mouldy air from sewers and basement ; and with it, more easily than with the other, one may guard against the disturbing influence of wind. In the plenum method the air is driven by pumps or by fans ; in the vacuum method pumps are rarely if ever used : suction is produced by fans or by heating the column of air in a long vertical shaft through which the discharge takes place. Water jets and steam jets have also been employed to impel the air. Extraction by a hot-air shaft is a common mode of ventilating hospitals and other public buildings. 1 Heat is applied by a furnace or stove at the bottom of the shaft, or by coils of hot-water or steam pipes, which should not extend up the shaft farther than ran be helped. In the lecture theatre of the Paris art conservatory, ventilated by Morin, where this means of extraction is employed, fresh air enters through the ceiling and foul air is drawn off through the floor from under the seats ; this reversal of the natural direction of the current is of course only possible when a sufficient external motive force is applied. The House of Commons furnishes another example : there the air, after being warmed and moistened, or cooled by water spray, as the state of the atmosphere may require, is admitted through large gratings in the floor, which are covered by porous matting to prevent draughts ; outlets from the top of the House lead by flues to the Victoria tower, where a furnace maintains the current in an up-cast shaft. 2 In theatres and other buildings lighted by clusters of gas jets or sun-lights at the ceiling the lights may be turned to account as effective ventilating agents by letting the foul air escape through shafts placed over them, which they heat at the base. 3 What is known in America as the Ruttan or Smead system of ventilation, successfully applied in many schools there, employs a hot-air shaft to furnish motive power. In warm weather a stove at the base of the shaft is used to heat the column ; in cold weather the exhaust air from the rooms is so much warmer than the atmosphere outside that the up-cast shaft acts without additional heating. This is in fact an example in which the classification of systems into natural and artificial breaks down. The supply of fresh air is warmed as it enters by passing through chambers containing tubular metal stoves ; the outlets are at or near the floor level. A curious feature in the arrangements is that the foul air, in passing to the up-cast shaft, is drawn through the privies, where it desiccates all discharges. Extraction by fans presents no features requiring special remark. A favourite fan for the purpose is the Blackmail propeller, the nearly flat form of which allows it to be readily placed in walls and partitions. One of these fans, 4 feet in diameter, when driven at a speed of about 330 revolutions per minute, is said to discharge 15,000 cubic feet of air per minute with an expenditure of one-horse power. Though this is a good performance, it should be observed that for ventilating purposes, where air has to be driven in large volume with low velocity and under low pressure, fans, while they have the advantage of being less bulky, are less efficient than pumps, for they require that the air in passing through them should move much faster than in other parts of its course, and much of the energy of this motion is wasted in eddies. When fans are used to blow air into buildings, they should deliver into a chamber of considerable size, that the air may become nearly still before it passes into the distributing flues. Loss of power may be avoided to some extent by receiving the air in a channel which gradually enlarges as it leaves the fan. The plenum method, with fans to drive the air, is exemplified on a large scale in the ventilation of St George s Hall, Liverpool, 4 where there are four large fans in the basement, driven by a 10- horse-power steam-engine. The building is heated by passing the air through chambers containing coils of hot-water and steam pipes ; after the air is warmed it is moistened by injecting steam, and pro vision is made for washing it by water-spray before it reaches the fans. When fans are used, either with suction or with pressure, the amount of the current is not strictly independent of those variable motive forces which are the sole agents in natural ventilation ; the case is analogous to that of an electric circuit in which several .sources of electromotive force are at work, assisting or opposing one another. The fan may be the main agent in circulating the air ; but differences of temperature, and at times the action of the wind, may make large variations in the resultant effect. The case is different when pumps are used. A certain quantity of air is delivered at each stroke, and the only effect of these irregular forces is to make the power required to drive the pump sometimes greater and sometimes less. Provided there are no casual inlets and outlets, the amount of air supplied is known with certainty ; the ventila- 1 For examples, see Morin, op. cit., or 1 roc. Inst. Mech. F.ng., 1867, p. 61. 2 The arrangements are similar to those introduced by Or Reid in the temporary Houses of Parliament, and described in his Treatise on Ventilation and Warming; see also Tomlinson s Warming and Ventilation, p. 265. In recent years pumps have been added, through which air may be forced into the building ; but the hot shaft is generally used. 3 See, for example, Morin s account of the ventilation of the Theatre Lyrique, Paris. 4 Proc. Inst. Mech. Eruj., 1863, p. 194. tion under these conditions is sometimes described as positive. Positive Good recent examples of positive plenum ventilation are to be plenum found in Dundee University College and in a number of schools veutila- in Dundee and Aberdeen, where the arrangements have been de- tion. signed by Mr W. Cunningham. Some of these are ordinary double- acting reciprocating pumps, driven generally by water engines. The pumps are rectangular wooden boxes, stiffened by iron ribs, and provided at top and bottom with inlet valves, consisting of a number of short waterproof cloth flaps working against a vertical wooden grid. The piston, which is also of wood, has a vertical travel ; it is held iu place and worked by wire ropes above and below, which lead over pulleys to the water motor ; and the piston is balanced by a counterweight on the descending branch of the upper rope. A piston 5 feet square, with a stroke of 5 feet, works at 20 strokes per minute and delivers 150,000 cubic feet per hour. In other instances, where the volume of air is greater than could easily be dealt with by common pumps, Mr Cunningham uses re volving pumps of the Root s blower type (shown in transverse vertical section through the revolving Dundee College a battery of five ing over 150,,000 cubic feet of a gas engine of two-horse-power. coils of Perkins s high pressure hot - water pipes in the main dis tributing flues. The inlets are flat upright tubes extending up the side walls to a height of nearly 6 feet, and open at the top. Ample proof of the advantage that results from giving a vertical direction to the entering current is supplied by the success of Mr Cun ningham s arrange ments, where this form of inlet is ex clusively adopted. Al ternative outlets are generally provided in the end walls, one group near the ceil ing, another a few- feet from the foot. They are fitted with "pistons" in fig. 8). At the of these blowers, each discharg- air per hour, is driven easily by The rooms are heated by having Fia. 8. Revolving pump for ventilation. doors which allow one or other to be closed ; the high-level outlets are used in warm weather, when the fresh air that comes in is com paratively cool ; the low-level ones are used in cold weather, when the fresh air, having been heated before it enters, would tend to rise and pass out too directly if the outlets near the ceiling were open. The outlet shafts communicate with a louvred tower or turrets on the roof. Each room receives a volume of air equal to its cubic capacity in about 12 minutes, so that the atmosphere is completely changed five times in an hour. The inlets are propor tioned to do this without allowing the velocity with which air enters to exceed 6 feet per second. The " jEolus " water-spray ventilator of Kind and Mestern is an Water- example of a mechanical ventilator using a jet of water to impel the spray air. A nozzle at the top of a circular air-shaft delivers a conical veuti- sheet of water, which impinges on the sides of the shaft a little way lator. below and carries down with it a considerable stream of air. This ventilator is used either to force air into rooms or to draw it out ; in the former case a small gas-stove is often added to heat the supply. For the ventilation of greenhouses and hot-houses, see vol. xi. p. 231. The advantage of ample and systematic ventilation is Organic not to be measured only by the low proportion of carbonic matter acid it secures. Carbonic acid is not the only test of . ... . ,T , -, r . micro- vitiation ; it is not even the most dangerous impurity, organisms Another criterion of the foulness of close air is the amount in air. of oxidizable organic matter it contains; still another, and a most valuable one, is the number of micro-organisms, especially of bacteria. The micro-organisms may be de termined by Hesse s method of slowly passing a given volume of the air to be examined through a tube coated inside with beef jelly ; the germs are deposited on the nutrient jelly and each becomes in a few days the centre of a very visible colony. In outside air the number of micro-organisms, as tested in this way, varies greatly : it
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