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'Kinnell's 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 Tossell's 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.