When air is admitted by special ducts they should be so arranged that air may be evenly distributed over the room. The relative position of the inlets to the outlets is a question upon which the engineers are not exactly agreed. It is obvious, however, that their arrangement must admit of a complete change of air at proper intervals and that draughts will not be established directly between inlets and outlets. It is generally conceded that the best results are obtained when the inlet is placed above near the ceiling, and the outlet placed directly below near the floor. In this way there seems to be a more equitable distribution of the entering air and less likelihood of a direct draught between the inlet and outlet. The outlet should never be directly opposite the inlet, since the air will pass directly through the room and there will be very little mixing with the room air. If the room is crowded it is more desirable to admit the fresh air from beneath, but when this is done there must be a great number of inlets. In this way the fresh air is admitted more directly into the breathing zone and at the same time advantage is taken of the natural air currents in the room. There are many patent devices on the market for ventilating through the windows. Such devices are very good and may be used to an advantage. Usually they are so built that the air is directed toward the ceiling as it is admitted. The same desired end is accomplished by placing a board under the lower sash as described above. Mechanical Ventilation By far the most satisfactory method of ventilating large buildings is the mechanical method. This is not practical for small buildings or homes because of the special devices that are required and the expense of operation. Mechanical ventilation may be accomplished by the plenum system, the vacuum system, or by the combined plenum and vacuum systems. The most desirable results are obtained when both systems are used and used in connection with the heating system. Heating and ventilating are so closely related that they must be considered one with the other. A poorly ventilated room is more difficult to heat and an improperly heated room is difficult to ventilate. The =plenum system= consists of a fan to force the air into the rooms. These fans are run by water motors or electricity whereby the air is forced through ducts into the rooms. This system is made necessary by the great buildings, basements and large steamships, which could not be inhabited if natural ventilation was the only method employed. The =vacuum system= consists of suction fans whereby the vitiated air is drawn out of the rooms and replaced by fresh air through the natural openings. In this way natural ventilation may be made more efficient, but this method in itself is not adequate for large buildings.