1. _The entering air should be, if possible, of a temperature of 55° to 60° Fahr._ Whenever the temperature of a room differs from the external temperature by 10° Fahr., a draught is certain to ensue. It is impossible at all times to ensure the incoming air being of the temperature of 60°, without some artificial means of warming it. In this country it is seldom necessary to cool the incoming air, but this may be managed in artificial systems of ventilation by passing the incoming air over ice, or by using compressed air which becomes cooled on expansion, or by passing the incoming air through subterranean tunnels. 2. _The entering air should be pure._ When a room is hotter than the passages and kitchens, air from the latter, whatever may be its character, is drawn into the room. Similarly the ground-air under the kitchen-floor or the air from ash-pits may be drawn into the house, when no other means of ventilation are provided; and this is often followed by evil results. 3. _No draught or current should be perceptible_ from the incoming air, except when it is wished to flush the room with air, by opening the windows wide. It is a common complaint that a room is draughty, and, to remedy this, keyholes are stopped up, and mats are placed at the bottom of the door, etc. The draught can often be remedied by increasing the size and number of the openings through which air is admitted, so that the current of air is not concentrated and rapid. When this does not remedy it, the incoming air should be warmed. A feeling of draught is very often due to the radiation to and from a window, and disappears when a curtain or screen is placed between the radiating surface and the occupant of the room. 4. _The entry of air should be constant, not intermittent._ The occasional opening of a window or door will not compensate for the lack of a constant interchange of air, although it forms a very valuable adjunct, especially in the removal of organic particles which do not follow the law of diffusion. 5. _An exit should be provided for impure air_, as well as an entrance for pure air. The chimney furnishes this in most living-rooms, and diminishes the necessity for other means of exit. If the openings in a room for entrance and exit are properly regulated, a rate of 5 feet per second (about 3½ miles per hour) will provide sufficient air without any unpleasant draught in a room. For instance, if the opening measure 1 square foot, then a rate of 5 feet per second will give five cubic feet of air per second, that is, 18,000 cubic feet per hour. But as only 3,000 cubic feet are required, it follows that an opening one-sixth this size, _i.e._ 24 square inches, is sufficient for each individual. Reckoning the same amount for means of exit, 48 square inches is the size of the ventilating orifices required by each individual.