mitting air through the door is sometimes useful. It resembles his window ventilator almost exactly; a long slit is cut through the door, a perforated metal plate FIG. 8.

Vertical Tube Ventilator.

placed outside and a flat plate fixed parallel to the door inside and in front of the slit, thus giving the air as it comes into the room an upward direction. An admirable plan for the admission of air into rooms

is by means of vertical tubes-an old system, but one which has been brought into prominence of late years by Mr. Tobin. A horizontal aperture is made through the wall into the outer air just above the floor, and then a vertical pipe carried against the wall to a height of from five to six feet. The cold air is thus made to ascend like a fountain into the room. It does so in a compact column, which only perceptibly spreads after it has got some height above the mouth of the tube. It then mixes with the warm air at the top of the room, producing no draught at all. In spite of the vertical height through which the air has to pass before it emerges into the room, a considerable amount of soot and dust of various kinds is brought in by it. This may be obviated by placing a little cotton wool in the interior of the tube. This, however, although a very efficient plan, has the serious disadvantage of im

Water Tray for Vertical Tube Ventilator.

peding the current of air. A better one is that in which a tray containing water is placed in the horizontal aperture

in the wall, the entering air being deflected on to the surface of the water by metal plates. The greater part of the dust is thus arrested by the water, which can be changed as often as necessary. In warm weather ice may be placed in the tray. Another plan is to place in the vertical tube a long muslin bag with the pointed end downwards, and kept in shape by wire rings. This provides a large filtering area, and offers very little resistance to the passage of air. The bag may be taken out and cleansed from time to time.

Several contrivances have been devised for the admission of air close to the floor, just behind a perforated skirting board. Among these are Ellison's conical ventilator, mentioned in the last chapter, and Stevens' skirting board ventilator, in which metal cups are placed in front of the inlet openings, and so distribute the air that no draught is felt. I think however, that it is advisable only to admit warmed air at a low level into rooms, but there is no reason why such openings should not be made high up in the rooms-behind cornices, for example. Prichett's paving, made of agricultural pipes, may also be used for making walls and partitions, and is obviously applicable for ventilating purposes, whether used as inlet or outlet.

We now come to speak of exit shafts and valves. The first and most important of these is the chimney, about which I have already spoken. I need only add here that it is advisable to do without the use of cowls upon chimneys wherever it is possible. If the chimney can be made high enough it will not require a cowl, and if it cannot, a

simple conical cap is generally sufficient to prevent down draughts. There is no doubt, however, that several fixed chimney cowls for preventing down draughts not only do so, but produce an up draught in the chimney when the wind blows down upon it, as can be readily shown by an experiment with a model. A small piece of wool is made to ascend in a glass tube by blowing vertically down upon the fixed model cowl placed upon the top of it. Of revolving cowls for chimneys, the common lobster-backed

cowl is probably the best. Of the many cowls which have been invented with the object of increasing the up draught in exit shafts of various kinds, some are fixed, as Boyle's,

Buchan's, Stevens's and Lloyd's, and some revolving, as Scott, Adie and Co's., Howarth's, Stidder's, Banner's, and

the one invented by Mr. Boyle, but discarded by him some years ago. Whether any of these cowls increase the up current in exit shafts is a matter which is still under investigation, but it is easy to show that the common rough experiment, by means of which they are supposed to do so, is entirely fallacious. Cotton wool is drawn up a tube at

least as easily by blowing across it in a slanting direction as by blowing through a cowl placed on the top of it. The fixed cowls have the advantage that they cannot get out of order. The revolving cowls have the disadvantage which is common to all apparatus with moving parts, that they

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