Louvre ventilators are made of slips of wood, glass, etc., very much like Venetian blinds-which latter make good ventilators if you open the top of the window, and turn the blind so that the laths slope upwards.

Windows have been constructed so that when the top sash is lowered it pulls down a contrivance made of louvres, but this is rather a complicated arrangement. Louvre ventilators, made of glass, are largely used, and can be put into a window instead of one of the panes. You can open or shut them, and put them at any angle you please. The disadvantage that they have is that the metal framework is liable to rust so that they will not work, but with care they do very welL

Windows which slope when they open are used sometimes, and they afford a very good means of letting a large volume of air into assembly rooms, etc., which are liable to be overcrowded.

In some cases, especially where French casement ' windows are used, Cooper's circular disc ventilator is fixed* It is entirely made of glass-a circular disc with five holes in it, corresponding with five holes in the pane. The disc can be turned so that the holes in it are or are not opposite to those in the window-pane, so that air can be admitted or not at pleasure. One advantage of this ventilator is that it has no metal framework, and so cannot get rusty.

In conclusion, I will mention the vertical shaft plan, which was re-introduced a short time ago into this country by Mr. Tobin. Its action depends upon the principle described just now; wherever you make an opening into the outer air, air will come in, by reason of the greater pressure outside, and if you admit air into a room through a vertical shaft, the air will rise up through the shaft and come into the room like a fountain, but no draught will be felt. One advantage of this plan is that you can filter the air in various ways, by passing it through cotton-wool, or you can pass the air over the surface of water placed in the tube leading to the vertical shaft, deflecting the air on to the surface of the water by causing it to strike against sloping pieces of metal In that way the suspended particles in the air are driven, as it were, on to the surface of the water, and the air passes on into the vertical tube which rises up into the room. That is a very admirable contrivance, though not so simple as some of the methods I have before described. I should add that it is patented.

Now let us consider the ways in which foul air gets out of rooms. In the first place, a very large quantity of air passes out of all rooms by the chimney, whether there is a fire or not, provided that fresh air can come into the room somewhere else. Whenever the air outside is colder than the air inside, the air will go out through the chimney ; that is partly due to the diminution of pressure, which causes an up-draught in the chimney whenever air is moving over it, which is nearly always the case. It has been thought by many that it* would be advisable to have some means by which the air at the top of an apartment, which is always warmest, could be drawn off; and one of the earliest contrivances to effect this was Neil Arnott's valve, which is a valve placed in a box, opening into the chimney; it is a very light metal valve, which can only open towards the chimney, so that whenever the pressure of the air in the room is greater than that of the air in the chimney, it opens and allows air to escape from the room into the chimney, and when the contrary is the case it shuts, and so prevents the air in the chimney from coming into the room, and when it is not required it can be fastened by means of a string attached to it. The disadvantages of that valve are, that after a time it gets out of order, and does not close so easily, and so a certain amount of air and blacks from the chimney get into the room ; it also makes a clicking noise, which is unpleasant, especially in a bedroom. You can sleep if there is a clock in the room, because the ticking is regular, and you get accustomed to it, but it is extremely unpleasant and disturbing to have an irregular ticking or clicking. That valve was improved upon by Boyle, who, instead of employing a metal flap to be blown by the air towards the chimney, used a series of little pieces of talc, which acted in precisely the same way as Arnottfs valve ; but if you blow against these little flaps from behind with a sudden strong gust, the air catches underneath their edges, and opens instead of shutting them. A better plan than either of these is to have a separate shaft, side by side with the chimney, so that the air is warmed by contact with the flue, and an up-current is promoted in the shaft which communicates with each room, and so air is extracted without any possibility of blacks getting into the house.

The next is known as M'Kinnell's ventilator. In this there are two tubes, one inside the other. They are let into the ceiling of a room, and made to end outside, at different heights; the heated air escapes through the inner tube, and the cold air from the outside comes down between the two tubes, and is deflected horizontally into the room by a metal rim placed round the end of the inner tube, and parallel to the ceiling. The action of that ventilator is very much increased if a gas jet be placed below the inner tube. It is upon this principle that railway lamps are made.

Another ventilator which is constructed upon the plan of M'KinnelTs is one in which the outer tube is provided at its upper part with vanes, which catch the wind, and so the air is deflected down into the space between the two tubes and into the room through perforations which are just below the ceiling.

A modification of these contrivances may be used in rooms where there is another room above, so that you cannot have tubes going up into the outer air; it consists of a like arrangement of tubes, and they end between the ceiling and floor of the room above, between the joists upon which the floor rests; and on each side of the house there are air bricks let in so that the air comes in at one side, and blows out at the other. These are known as TosselTs ventilators.

In artificial ventilation, by means of which air is forced into places, or drawn out, by machinery, the chief agents that are used are large fans, consisting of metal vanes placed round an axle, like the spokes of a wheel-either large propelling fans, by which air is driven into the places and allowed to get out as it can, or by channels leading to flues; or extracting fans, which draw air out of the places, and allow it to come in from the outside through channels provided for it. In this way many of our mines are ventilated.

Drawing air out by means of large furnaces is also generally considered to be a part of artificial ventilation, but the principles are precisely the same as those described with reference to the ventilation of apartments.

In sunlight ventilators there is a large extraction shaft around the tube which carries away the products of combustion of the gas, and the ventilating power of this shaft is sometimes increased by connecting pipes with it conveying the products of combustion from the gas-burners in various parts of the building.

In buildings warmed by hot-water pipes the air admitted may be warmed by allowing it to pass over heated pipes, and the shaft through which the pipes pass from the boiler in the basement to the upper parts of the house may be used for the purpose of extracting air.