As the air enters directly from outside the house, it often carries with it particles of dirt, soot, etc. This may be remedied by placing a pan containing _a shallow layer of water_ at the lowest part of the tube, or by placing _cotton wool_ at the point of entry of the tube into the room. The tray of water soon dries up and is rarely replaced, while the cotton wool diminishes the amount of entering air. It is very useful however in cold weather, or when fogs occur. A _gauze funnel_ is sometimes inserted in the tube, or a sheet of gauze arranged diagonally across the tube from its highest to its lowest point. The gauze does not keep out minuter particles of dust, and requires occasional cleaning. All Tobin’s tubes, like other ventilating openings, should be made to open, so that their interior can be frequently cleaned. _Summary as to Domestic Ventilation._—Open windows, doors, and fire-places may be in most instances trusted. If gas is used as an illuminant, they should be combined with special arrangements for carrying off the products of combustion from the room. For delicate people, and especially in small rooms, outlet ventilation into the chimney breast combined with a Sheringham’s valve on the opposite wall is desirable. =Artificial Ventilation.=—Artificial ventilation may include two important and very different measures. In one of them currents of air and an exchange of pure for impure air are effected by means of various forms of heating apparatus. In the other mechanical measures are used for the same purpose,—the air being either driven out of the room or drawn out of it. In this chapter we shall consider only the =mechanical means of artificial ventilation=. There are two kinds, the first being known as ventilation by aspiration, or the _vacuum_ system; and the second as ventilation by propulsion, or the _plenum_ system. In =Ventilation by Aspiration= the foul air is drawn out of the room by machinery, its place being supplied by fresh air, which may be warmed before entry or not. This plan and the next have been employed chiefly in connection with large buildings, such as hospitals, etc., and in mines. The extraction of foul air may be effected by—(1) _a steam-jet_, which is allowed to pass into a chimney, and sets in motion a body of air more than 200 times its own bulk. Tubes from each room of the building are connected with this chimney, and the strong upward current extracts the air from them. This plan is useful in factories, where there is a superfluous supply of steam. (2) _A fan or screw_ may also be used. The vanes of the fan, when set in motion by electrical or some other motive power, produce a powerful current of air, which can be regulated according to requirements. As in the last plan, the aspirating influence of the fan may be exerted over a system of rooms, by means of connecting tubes.