# 1911 Encyclopædia Britannica/Ventilation

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​VENTILATION (Lat. ventilare, from ventus, wind), the process and practice of keeping an enclosed place supplied with proper air for breathing; and so, by analogy, a term used for exposing any subject to the winds of public criticism. The air which we breathe consists chiefly of two gases, oxygen and nitrogen, with certain small proportions of other gases, such as carbonic acid (carbon dioxide), ozone and argon. Oxygen, which is the active and important constituent, and on which life and combustion depend, forms about one-fifth of the whole, while nitrogen, which is inert and acts as a diluent, forms nearly four-fifths. Of this mixture each adult person breathes some 2600 gallons or 425 cub. ft. in twenty-four hours. In air that has passed through the lungs the proportion of oxygen is reduced and that of carbon dioxide increased. Of the various impurities that are^ found in the air of inhabited rooms, carbonic acid gas forms the best practical index of the efficiency of the ventilation. The open air of London and other large inland towns contains about four parts by volume of the gas in 10,000 of air. In the country, and in towns near the sea, two to three and a half parts in 10,000 is a more usual proportion. Authorities on ventilation usually take four parts in 10,000 as the standard for pure air, and use the excess over that quantity in estimating the adequacy of the air supply. But they differ as to the proportion to which the carbonic acid may be allowed Standard of purity.to rise under a good system of ventilation. It is generally admitted that the air in which people dwell and sleep should not under any circumstances be allowed to contain more than ten parts in 10,000. This has been accepted as the permissible proportion by Carnelley, Haldane and Anderson, after an extensive examination of the air of middle and lower class dwellings.
The rate at which an adult expires carbonic acid varies widely with his condition of repose, being least in sleep, greater in waking rest, and very much greater in violent exercise. As a basis on which to calculate the air necessary for proper ventilation we may take the production of carbonic acid by an adult as 0·6 cub. ft. per hour. Hence he will produce per hour, in 6000 cub. ft. of air, a pollution amounting to one part of carbonic acid in 10,000 of air. If the excess of carbonic acid were to be kept down to this figure (1 in 10,000), it would be necessary to supply 6000 cub. ft. of fresh air per hour; if the permissible excess be two parts in 10,000 half this supply of fresh air will suffice; and so on. We therefore have the following relation between (1) the quantity of air supplied per person per hour, (2) the excess of carbonic acid which results, and (3) the total quantity of carbonic acid present, on the assumption that the fresh air that is admitted contains four parts by volume in 10,000:—
Some investigators have maintained that, in addition to an increased proportion of carbonic acid, air which has passed through the lungs contains a special poison. This view, however, is not accepted by others; J. S. Haldane and Lorrain Smith, for instance, conclude “that the immediate dangers from breathing air highly vitiated by respiration arise entirely from the excess of carbonic acid and deficiency of oxygen” (Journ. Path, and Bact. 1892, 1, 175). Carbonic acid, however, is not the only agent that has to be reckoned with in badly ventilated rooms, for the unpleasant effects they produce may also be due, to increase of moisture and temperature and to the odours that arise from lack of cleanliness. Again, though there may be no unduly large proportion of carbonic acid present, the air of an apartment may be exceedingly impure when the criterion is the number of micro-organisms it contains. This also may be greatly reduced by efficient ventilation. Comparisons carried out by Carnelley, Haldane and Anderson (Phil. Trans., 1887, 178 B, 61) between schools known' to be well ventilated (by mechanical means) and schools ventilated at haphazard or not ventilated at all showed that the average number of micro-organisms was 17 per litre in the former, and in the others 152. Results of great interest were obtained by the experiment of stopping the mechanical ventilators for a few hours or days. Tested by the proportion of carbonic acid, the air of course became very bad; tested by the number of micro-organisms, it remained comparatively pure, the number being, in fact, scarcely greater than when ventilation was going on, and far less than the average in “naturally ventilated” schools. This proves in a striking way the advantage of systematic ventilation.
In the ventilation of buildings four main points have to be considered: (1) the area of floor to be provided for each person; (2) the cubic capacity of the room required for each occupant; (3) the allowance to be made for the vitiation of the air by gas or oil burners; and (4) the quantity of fresh air which must be brought Ventilation of buildings. in and of vitiated air that must be extracted for each individual. The first will depend upon the objects to which the room is devoted, whether a ward of a hospital or a school or a place of public assembly. The purity of the air of a room depends to a great extent on the proportion of its cubic capacity to the number of inmates. The influence of capacity is, however, often overrated. Even when the allowance of space is very liberal, if no fresh air be supplied, the atmosphere of a room quickly falls below the standard of purity specified above; on the other hand, the space per inmate may be almost indefinitely reduced if sufficient means are provided for systematic ventilation. Large rooms are good, chiefly because of their action as reservoirs of air in those cases (too common in practice) where no sufficient provision is made for continuous ventilation, and where the air is changed mainly by intermittent ventilation, such as occurs when doors or windows are opened. With regard to the third point, in buildings lighted by gas or oil the calculations for the supply of fresh and the extraction of foul air must include an allowance for the vitiation of air by the products of combustion. The rate at which this takes place may be roughly estimated in the case of gas by treating each cubic foot of gas burnt per hour as equal ​ to one person. Thus an ordinary burner giving a light of about twenty candles and burning 4 cub. ft. of gas per hour vitiates the air as much as four persons, and an incandescent burner as much as one and a half persons. A small reading lamp burning oil uses the air of four men; a large central table lamp uses as much air as seven men.
As to the fourth point there is great diversity of opinion. To preserve the lowest standard of purity tolerated by sanitarians, ventilation must go on at the rate per person of 1000 cub. ft. per hour, and 3000 cub. ft. per hour are required to preserve the higher standard on which some authorities insist. E. A. Parkes advised a supply of 2000 cub. ft. of air per hour for persons in health and 3000 or 4000 cub. ft. for sick persons. In the case of a public assembly hall no great harm will occur to an audience occupying the room for a comparatively short time if 30 cub. ft. of air per minute are provided for each person. The United States book on school architecture gives a practical application to its remarks on this subject as follows:—
The amount of fresh air which is allowed to hospital patients is about 2500 cub. ft. each per hour. Criminals in French prisons have to content themselves with 1500 cub. ft. per hour. Assuming that we care two-thirds as much for the health of our children as we do for that of our thieves and murderers, we will make them an allowance of 1000 cub. ft. each per hour, or about 16 cub. ft. per minute. Forty-eight children will then need an hourly supply of 48,000 cub. ft. Definite provision must therefore be made for withdrawing this quantity of foul air. No matter how many inlets there may be, the fresh air will only enter as fast as the foul escapes, and this can only find an outlet through ducts intended for that purpose, porous walls and crevices serving in cool weather only for inward flow. What, then, must be the size of the shaft to exhaust 48,000 ft. per hour? In a shaft 20 ft. high, vertical and smooth inside, with a difference in temperature of 20°, the velocity will be about 21/2 ft. per second, or 9000 ft. per hour; that is, it will carry off 9000 cub. ft. of air per hour for every square foot of its sectional area. To convey 48,000 cub. ft., it must have a sectional area of 51/2 sq. ft.
A general idea of the floor area, cubic space and fresh air supply per inmate allowed by law or by custom in certain cases is given in the table below:-
| Class of Building. | Floor Area in Feet per Person. | Cubic Capacity in Feet Per Person. | Cubic Feet of Fresh Air supplied and Foul Air extracted per Person. |
| Schools | 9 to 10 | 200 | 1,800 |
| Barracks | 70 | 720 | 1,800 |
| Prisons | 90 | 800 | 1,800 |
| Concert halls and theatres | 9 | 108 | 2,000 |
| Billiard and smoke rooms | | | 2,000 |
| Hospitals | 120 | 1,440 | 2,000 to 3,000 |
| Public libraries | 20 | 2,400 | 2,500 |
| Turkish baths. | 70 | 800 | 5,000 |
| Workshops | 120 | 1,440 | 5.000 |
| Cowsheds, per cow | 90 | 1,100 | 10,000 |
| Stables; per horse | 120 | 1,600 | 12,000 |
The supply of fresh air indicated in the table should not be regarded as entirely satisfactory, for the standard of purity suggested is low, and ought to be exceeded, but it might deter many from moving in the matter if a proper and higher standard were to be laid down at first.
One of the most important points is the proper warming of the fresh air introduced into buildings, for unless that be done, when a cold day occurs all the Ventilating arrangements will probably be closed. The fact should not be lost sight of that the air in a room may on the one hand be quite cold and yet very foul, and on the other, warm and yet perfectly fresh. To avoid draught the air should enter through a large number of small orifices, so that the currents may be thoroughly diffused. This is done by gratings. The friction of their bars, however, seriously diminishes their capacity for passing air, and careful experiments show conclusively that very ample grating area is required to deliver large volumes. The same remark applies to extracting-flues. Owing to the small size and the roughness of the surface the velocity of the upward current is small, and the quantity of air that passes out is often much less than is requisite.
Means of Ventilation.—In order that the atmosphere of a room should be changed by means of air currents, thereby securing proper ventilation, three things are necessary; (1) an inlet or inlets for the fresh air, (2) an outlet or outlets for the vitiated air, and (3) a motive force to produce and maintain the current. In systems which are distinguished by the general name of mechanical or artificial ventilation special provision is made for driving the air, by fans, or by furnaces, or by other contrivances to be described more fully below. In what is called natural ventilation no special appliance is used to give motive force, but the forces are made use of which are supplied by (1) the wind, (2) the elevated temperature of the room's atmosphere, and (3) the draught of fires used for heating.
Natural Ventilation.—The chief agent in domestic ventilation is the chimney; when a bright fire is burning in an open grate, it rarely happens that any other outlet for foul air from a room need be provided. The column of hot air and burnt gases in the chimney is less heavy, because of its high temperature, than an equal column of air outside; the pressure at the base is therefore less than the pressure at the same level outside. This supplies a motive force compelling air to enter at the bottom through the grate and through the opening over the grate, and causing a current to ascend. The motive force which the chimney supplies has not only to do work on the column of air within the chimney, Chimney draught. in setting it in motion and in overcoming frictional resistance to its flow: it has also to set the air entering the room in motion and to overcome frictional resistance at the inlets. From want of proper inlets air has to be dragged in at a high velocity and against much resistance, under the doors, between time window sashes and through many other chinks and crevices. Under these conditions the air enters in small streams or narrow sheets, ill-distributed and moving so fast as to form disagreeable draughts, the pressure in the room is kept so low that an opened door or window lets in a deluge of cold air, and the current up the chimney is much reduced. If the attempt is made to stop draughts by applying sand-bags and listing to the crevices at which air streams in, matters only become worse in other respects; the true remedy of course lies in providing proper inlets. The discharge of air by an ordinary open fire and chimney varies widely, depending on the rate of combustion, the height and section and form of the chimney, and the freedom with which air is entering the room. About 10,000 cub. ft. per hour is probably a fair average, about enough to keep the air fresh for half a dozen persons. Even when no fire is burning the chimney plays an important part in ventilation; the air within an inhabited room being generally warmer than the air outside, it is only necessary that an up-current should be started in order that the chimney should maintain it, and it will usually be found that a current is, in fact, passing up.
When a room is occupied for any considerable length of time by more than about half a dozen persons, the chimney outlet should be supplemented by others, which usually take the form of gratings in the ceiling or cornices in communication wit flues leading to the open air. These openings should be protected from down-draught by light flap valves of oiled silk or sheet mica.
With regard to inlets, a first care must be to avoid such currents of cold air as will give the disagreeable and dangerous sensation of draught. At ordinary temperatures a current of outer air to which the body is exposed will be felt as a draught if its velocity exceeds 3, or even 2 ft. per second. The current entering a room may, however, be allowed to move with a speed much greater than this without causing discomfort, provided its direction keeps it from striking directly on the persons of the inmates. To secure this, it should enter, not horizontally nor through ratings on the floor, but vertically through openings high enough to carry the entering stream into the upper atmosphere of the room. where it will mix as completely as possible with warm air before its presence can be felt. A favourite form of inlet is the Sheringham (fig. 1). When opened it forms wedge-shape protection, into the room, and admits air in an upward stream through the open top. It should be placed at a height of 5 or 6 ft. above the level of the floor. Other inlets are made by using hollow perforated blocks of earthenware, called air bricks, built into the wall; these are often shaped on the inner
Fig. 1.-Sheringham Air Inlet. ​ side like an inverted louvre-board or venetian blind, with slots that slope so as to give an upward inclination to the entering stream.
In another and most valuable form of ventilator, the Tobin tube, the fresh air enters vertically upwards. The usual arrangement robin of Tobin tube (shown in front elevation and section in tube fig. 2) is a short vertical shaft of metal plate or wood which 6 ft. Its
leads up the wall from the floor level to a height of 5 or air-grating in the wall' from its upper end which is freely open the current of fresh air rises in a smooth may be given to the tube: if placed in a corner it will be triangular or segmental' against a flat wall a shallow rectangular form is most usual or it may be placed in a channel so as to be fiush Wlth the face of the wall; a lining of wood forming a dado may even be made to serve as a Tobin tube by setting it out a little way from the wall. The tube is often furnished with a regulating valve, and contrivances may be added for cleansing the entering air. A muslin or canvas lower end communicates with the outer air through an it stream. Various forms of section
FIG. 2.-Tobin Tube. bag hung in the tube, or a screen stretched diagonally across it, may be used to filter out dust; the same object is served in some degree by forcing the air, as it enters the tube at the bottom, to pass in closing contact with the surface of water in a tray, by means of a defecting plate.
These complications have a double drawback: they require frequent attention to keep them in order, and by putfaiug resistance in- the way of the stream they are apt to reduce the efficiency of the ventilation The air entering by a Tobin tube may be warmed by a coil of hot pipes within the tube or by a small gas-stove (provided, of course, with a fiue to discharge outside the products of combustion), or the tube may draw its supply, not directly from the outer atmosphere, but from a hot-air fiue. The opening. should always be about the level of a man's head, but the tube need not extend down to the floor: all that is essential is that it should have sufficient len h to let the air issue in a smooth vertical current without eddies (fig. 3).
These inlets are at once so simple and effective that no hesitation need be felt in introducing them freely in the rooms of dwelling-houses. When no special provision is made for them in the walls, the advantage of a current entering vertically may still be 1n some degree secured by help of certain makeshift contrivances. One of these, suggested by Dr Hinkes Bird, is to open one sash of the window a few inches and fill up the opening by a board; air then enters in a zigzag course through the space between the meeting rails of the sashes. Still another plan is to have a light frame of wood or metal or glass made to fit in front of the lower sash when the window is opened, forming virtually a Tobin tube in front of the window.
As an example of the systematic ventilation of dwelling-rooms on a large scale, the following particulars may be quoted of arrangements that have been successfully used in English barracks. One or more outlet-shafts of wood fitted with flap valves to prevent down-draught are carried from the highest part of the room, discharging some feet above the roof under a louvre. The number and size of these shafts are such as to give about 12 sq. in. of sectional area per head, and the chimney gives about 6 sq. in. more per head. About half the air enters cold through air-bricks or Sheringham valves at a height of about 9 ft. from the floor, and the other half is warmed by passing through fiues behind the grate. The inlets taken together give an area of about 11 sq. in. per head. A fairly regular circulation of some 1200 cub. ft. per head per hour is found to take place, and the proportion of carbonic acid ranges from 7 to 10 parts in 10,000.
In the natural ventilation of churches, hails and other large rooms we often find air admitted by gratings in the floor or near it; or the inlets may consist, like Tobin tubes, of upright flues rising to a height of about 6 ft. above the floor, from which the air proceeds in vertical streams. If the air is to be warmed before it enters, the supply may be drawn from a chamber In public buildings. warmed by hot-water or steam pipes or by a stove, and the temperature of the room may be regulated by allowing part of the air to come from a hot chamber and part from outside, the two currents mixing in the shaft from which the inlets to the room draw their supply. Outlets usually consist of gratings or plain openings at or near the ceiling, preferably at a considerable distance from points vertically above the inlet tubes. One of the chief difficulties in natural ventilation is to guard them against down-draught through the action of the wind. Numberless forms of cowl have been devised with this object, with the further intention of turning the wind to useful account by making it assist the up-current of foul air. Some of these exhaust cowls are of the revolving class, made to various designs and dimensions and put in rotation by the force of the wind. Revolving cowls are liable to fail cf E" by sticking, and, generally speaking, fixed cowls are to be W ° preferred. They are designed in many forms, of which Buchan's may be cited as a good example. Fig. 4. shows this ventilator in horizontal section: aa is the vertical exhaust flue through which the foul air rises; near the top this expands into a polygonal chamber, bbbb, with vertical A sides, consisting partly of perforated sheet-metal plates; outside of these are fixed vertical curved guide-plates, c, c, c, c; the wind, blowing between these and the polygonal chamber, sucks air from the centre through the perforated sides. The efficient working of an exhaust cowl, however, depends almost entirely upon the favourable conditions of the wind. The two t ings t at su y motive orce in automatic or rihiiural ventilation by W/ND means of exhaust cowls and similar appliances-the difference of temperature between inner and outer air, and the wind—are so variable that even the best arrangements of inlets and outlets give a somewhat uncertain result. As an example, it is evident that on a hot day with little movement in the air this mode of ventilation would be practically ineffectual. Under other conditions these automatic air-extractors not infrequently become inlets, thus reversing the whole system and pouring cold air on the heads of the inmates of the apartment or hall. To secure a strictly uniform delivery of air, unaffected by changes of season or of weather, it is necessary that the influence of these irregular motive forces be as far as possible minimized, and recourse must consequently be had to some mechanical force as a means of driving the air and securing adequate ventilation of the building.
Artificial Ventilation.—Buildings may be mechanically ventilated on the vacuum system, the lenum system, or on a system combining the best points of botii. In nearly every case of the application to modern buildings of mechanical means of ventilation the combined system in one form or another is adopted. In the vacuum system the motive force is applied at the outlets; the vitiated air is drawn from the rooms, and the pressure of the atmosphere in them is slightly less than the pressure outside. Upon the foul air being withdrawn fresh air finds its way in by means of conveniently placed inlets. In the plenum the motive force is applied at the inlets; fresh air is forced in and drives the vitiated air before it until it escapes at the outlets provided. The pressure within the room is greater than outside. The plenum method has distinct advantages: it makes the air escape instead of coming in as a cold draught at every crevice and casual opening to the outer air; it avoids drawing foul 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 fans; in the vacuum method suction is produced by fans or by heating the column of air in a long vertical shaft through which the discharge takes place. Water 'ets and steam jets have also been employed to impel or extract the air. Whatever system of ventilation is adopted, it is most important that windows capable of being widely opened should also be provided to aerate at frequent intervals the whole building, either as a whole or in sections, and they should be so arranged' that no corner can be left stagnant or unswept by the purifying current. The Victoria Hospital at Glasgow and the Royal General Hospital at Birmingham are, however, ventilated on the plenum system without the aid of open windows, with what are said to be satisfactory results. In the case of hospitals, it is evident that aeration by means of open windows could not in Great Britain be effected except on warm and sunny
FIG. 4.-Sectional Plan of Buchan's Exhaust Cowl. ​ days, but in the case of concert halls, theatres and similar buildings, it is possible (and most essential) thoroughly to aerate the building between each occupation.
The extraction of foul air should in most cases be effected at the top of a room or building, so as to utilize the natural tendency of warm air to' rise; but at Birmingham and elsewhere the Sjgzj' outlets are near the floor, the fresh air being brought in "dated half-way up the walls and directed towards the ceiling. 3” The air inlets should be Tobin tubes or similar devices, placed some 4 or 5 ft. above the iioor, and so arranged that the air should be passed in contact with radiators or pipes to warm it before entering. In the case of a building for one of the American legislatures, the warmed fresh air is allowed to enter on the level in front of the desk of each member, so that he secures a proper volume of fresh air for his own use before it is breathed by his neighbour. The introduction of rapidly revolving, but silent, fans, driven by electricity, is a great advance whlgch places Fvithir; the riach olglthe en ineer or arc itect the means or so vin the ro em Fans' of ii/epitilatiog of buildings, hand has been foga larglgrixtzent res onsible for the rapi progress o the art o Venti ation. e an and) motor combined extend the advantages of positive mechanical ventilation to all who have access to electric current, with the further benefit that the extreme simplicity of the electric driving of the fans greatly facilitates the control and distribution of ventilating effect. The moderate power required by these fans for a given duty has contributed greatly to their extended use. They should deliver into a chamber of considerable size, so that the velocity of the air may become reduced before it passes into the distributing Hues. The question of silence in running, in such places as houses of parliament, law courts, churches and chapels, is of paramount importance, and no fan should be accepted until it is proved by actual working to be noiseless.
In some instances revolving pumps of the Root's blower type are used (see BELLOWS AND BLow1NG MACHINES). At the Dundee College a battery of five of these blowers, each discharging over 150,000 cub. ft. of air per hour, is driven easily by a gas engine of two horse-power. The air is passed through two filters of coarsely woven fabrics which serve to remove all particles of impurity. The rooms are heated by having coils of Perkins's high-pressure hotwater pipes (see HEATING) in the main distributing flues. The inlets are flat upright tubes extending ug the side walls to a height of nearly 6 ft., and open at the top. utlets are generally provided in the end walls, one group near the ceiling, another a few feet from the foot. They are fitted with 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 aratively 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 proportioned to do this wit out allowing the velocity with which air enters to exceed 6 ft. per second.
The water-spray ventilator is a mechanical ventilator using a. jet of water to impel the air. A nozzle at the top of a circular Wafer' air-shaft delivers a conical sheet of water, which impinges ZZ-;; on the sides of th.e shaft a little way below and carries star 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 stove is often added to heat the supply. In the early days of mechanical ventilation extraction by a hot-air a more common mode of ventilating hospitals and other public buildings than now. The heat was applied by a shaft was
gffxc' furnace or stove at the bottom of the shaft, or by coils hot a;' of hot-water or steam pipes. In the lecture theatre of the sham Paris Conservatoire des Arts A. ]. Morin employed this means of extraction, and arranged that the fresh air should enter through the ceiling and the foul air be drawn off through the Hoor 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.
In theatres and similar buildings clusters of gas jets or sunlight burners, fixed at the ceiling level at the base of a metal shaft which is connected with the open air, serve as effective ventilating agents Eyuextracting the foul air which collects in the upper part of the a
To ensure the admission of the desired amount of air into a !'O0m, and to arrive at the proper allowance of inlets and outlets, it is The necessary /to ascertain the direction and velocity of the measurh movement of the air through the m. The quantity ofair ment passing through a given opening is found by multiplying “fam the area of the opening expressed in square feet by the velocity of the current of air stated in lineal feet per minute, the product being the number of cubic feet passing per minute. Where the air is admitted through gratings only the clear area should be calculated, the amount of solid material being deducted from the gross superficies of the grating. The velocit of the air current may e determined by means of an anemometer fg.v.).
We may conclude with a short summary of the methods adopted of ventilating a. number of typical buildings of various classes of different countries.
The Smallpox Hospital at Bradford consists of two wards, 75 ft. by I5 ft., placed back to back, with a space of about 3 ft. between them enclosed by walls forming a foul-air chamber of the same length as the wards, and reaching to the ceiling. At this level are outlets for the vitiated air-one over each bed. A furnace at the base of a tall shaft withdraws through these outlets the air which passes through the furnace on its way to the outer air. The windows are tightly closed and fresh air enters from a chamber below through gratings in the floor at the foot of each bed.
The New York General Hospital was stated in 1875 to contain 163 beds. In the wards there is one window to each bed, each pier between the windows containing a foul-air extracting flue running from the base of the building and connected in the roof with large trunks leading to an exhaust fan. The heating is by steam coils placed in the basement in such a way that by a valve the cool fresh air can be sent either through or around the heating coil. The warmed fresh air is conveyed through an air-tight iron pipe fitted in each extracting shaft and is admitted to the wards through slits in the window-sills forming a jet directed upward on the principle of Tobin tubes. The outlet openings for the foul air are placed one beneath each bed, with extra outlets for occasional use at the top and base of the external walls. The placing of the freshair supply pipes in an inaccessible position inside the foul air ducts cannot be approved for hospital ventilation, as it is quite possible that in time, through the decay of the pi e joints or of the pipes themselves, communication may be established between the fresh and foul air, thus entirely upsetting the system of ventilation. The City Hospital of Hamburg, containing 130 beds, was opened in 1890. The buildings are one storey high and are heated on the ancient Roman hypocaust principle. Beneath the entire floor run longitudinally a number of brick and concrete Hues about 30 in. square, covered on the top with marble tiles, forming the floor of the wards. In these flues are placed the steam heating pipes. Warmed fresh air is admitted through large radiators in the centre of the wards, the vitiated air escaping through o enings in the ridge of the roof. Mr H. Percy Adams adopted a similar by ocaust method for warming the chapel and the dining-hall at the }I){i g Edward VII. Tuberculosis Sanatorium at Midhurst, Sussex, except that -the radiators are omitted from the centre of the rooms, and placed in recesses in the side walls.
In the Houses of Parliament at Westmiilstery which were designed and built for the public business in 1836, considerable attention is devoted to the question of the purification of the air, but the arrangements are lamentably antiquated and ineffectual in their working. The supply of fresh air is drawn by fans from the terrace at 'the river front, and, after being warmed and moistened or cooled by water-spray or blocks of ice, as the temperature may require, passes through exceedingly tortuous and restricted air passages to the various chambers, where it is admitted through large gratings in the floor, which are covered by porous matting to prevent draughts. The outlets for the vitiated air are in the ceilings of the apartments, and from these the air has to be dragged down to the base of the ventilating shaft in the Victoria tower, where an up-current is maintained by a large furnace.
The French Chamber of Deputies, according to a report made by M. Frélat in 1891, is much overcrowded, the allowance of floor space for each member being only 30 square centimetres. The apparatus is powerful enough to change the air every six minutes, but to avoid draughts it can only be worked slowly. Fresh air is driven down by a fan through openings in the ceiling, and vitiated air removed at the floor, giving a downward system of ventilation. For the ventilation of the new Sessions House at the corner of Newgate Street and Old Bailey, London, opened in 1907, an elaborate system on the plenum downward principle was installed The fresh air, drawn in at the basement by powerful fans, passed in turn through purifying screens, on which water was constantly playing, and over steam-heated coils, before entering the distributing trunks; into these sufficient cold air also was admitted to reduce it to the required temperature. Branch ducts conveyed this warmed fresh air to the points of inlet just below the ceiling. The outlets for the vitiated air were placed near the floor level, an electric fan drawing it up and discharging it at the roof. It was claimed that 600 tons of filtered and warmed or cooled fresh air were passed through the building every hour.
In the Capitol at Washington in .America the upward system is installed. Fresh air, warmed by coils in the basement, is delivered by means of fans through openings in the floors of the various chambers and galleries, and the extractors are placed in the ceilings. This foul air passes out of the building through louvre ventilators placed on the roof ridge. Some of the vitiated atmosphere, however-that from the corridors and galleries-is drawn by means of a fan to the basement and blown up a lofty shaft. The Grand Opera House in Vienna is ventilated on a most elaborate and complete system, the arrangements there giving excellent results. The scheme for heating and ventilating this ​building was designed by D. Böhm. The building measures 397 ft. by 299 ft., and the theatre will hold about 2700 persons. Ventilation is effected by two fans, the lower for propulsion, the upper for extraction. The latter is aided also by the heat produced by the great pendant which has ninety burners. The heating is effected by steam, and the air enters the hall at a temperature of from 63° to 65° F., the points of entrance being at the floor and the risers of the seating. Each gallery and compartment of the theatre, including the stage, has a separate installation of heating apparatus and supply duct so that any one portion may be warmed and ventilated independently of the rest. The velocity of the incoming air is between 1 and 2 ft. per second. The driving fan in the basement sends air into the building at the rate of 1059 cub. ft. per head per hour by means of electricity. The temperature in different parts of the house can be observed in a central control office, and here also are the levers which control the valves regulating the air supply, both hot and cold. During a performance the superintendent of heating and ventilation is on duty in this office and secures to each part of the building its proper supply of fresh air at a proper temperature.
For the ventilation of mines see Mining, and for that of railway tunnels see Tunnel.
Authorities.—The following are the principal publications on ventilation: J. S. Billings, Ventilation and Heating; Leeds, Treatise on Ventilation; Carpenter, Heating, and Ventilating Buildings.  (J. Bt.)
* ↑ In calculating the cubic capacity per person the height should not be measured beyond 12 ft. above the floor.
* ↑ When the air is not filtered, and when it has been warmed before entering, the vertical direction of the stream is readily traced by dust, which is deposited on the wall in a nearly upright column, spreading slightly fan-wise as it rises. With cold air the deposit of dust is comparatively slight. The difference is due to the fact, noticed and explained by Mr John Aitken, that air quickly deposits any suspended particles when it is brought into contact with a surface colder than itself, but retains them in suspension if the surface be warmer than the air. Another domestic illustration of the same fact is given by the greater dustiness of walls and furniture in a stove-heated room than in a room heated by an open fire.
* ↑ For an account of tests of various forms of ventilating cowls, see S. S. Hellyer, The Plumber and Sanitary Houses.