AN air-conditioning unit is usually a small cabinet, the only external evidence of its purpose being two small grilles. In Engineering of July 9, a description is given of one meant to stand on the room floor and placed near a window as the air inlet can then be readily arranged. It is about 40 in. high, 30 in. wide and 18 in. deep. It is electrically operated, and the supply cable can be plugged into any existing power point such as one used for a radio cabinet. With the air-cooled type this is all that is required, but with the water-cooled type connexions are needed with the water supply. In both summer and winter, the incoming air is first filtered ; the inlet is a permanent fixture and does not interfere with opening the window for cleaning. Otherwise the window is kept closed to keep out the noise of street traffic. The fan is driven by a motor independent of the fans used for cooling and so can provide ventilation without either heating or cooling, if the outside air is in the desired condition. The cooling of the air and the removal of excess moisture by condensation is effected by a special refrigerator. The condensate is then re-evaporated and carried outside, so no drainpipe is required in the air-cooled unit. For increasing the moisture in the air, the water in the humidifier tank is vaporized by an electric immersion heater. The relative humidity is settled by a hygrostat which automatically controls the heater. A small neon light indicates when the heating elements are working, and reminds the occupant to consider whether it is advisable to turn off the heat at night when leaving the conditioning plant running.