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Rh 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

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.