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Rh 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 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 5 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:-

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, 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

. 1.-Sheringham Air Inlet.