than the inside, as in December, when the inside air is thirty to forty degrees warmer than the outside. It is self-evident that if the ventilation is satisfactory in summer, it cannot be expected to answer in winter. In considering ventilation, the first question should be, How much air is vitiated in each room in a given time for each person occupying it? A consideration of this point gives us an answer to the converse problem, How much fresh air should be allowed to enter the room per hour? The opinions of experts differ on these points, but it may be taken as a general rule, that in a room occupied to its maximum capacity, 25 to 30 cubic feet of air should be extracted per minute for each adult. A rough guide is given to us by our sense of smell, for if a room has the slightest perceptible odour, its ventilation is imperfect. The difficulty, of course, is to extract the foul air and introduce fresh without creating draughts. The method of doing so is practically the same in all systems, the use of a shaft with numerous inlets opening into it at the top of the room to withdraw the air, an upward movement in it being caused by the warmth of the vitiated air itself, or by the use of a gas-burner or a stove inserted in it. Corresponding inlets for fresh air are necessary, and must have relation to the outlets. They should be a little larger in area, and well distributed over the room. If but one or two inlets are provided they will cause a current of air to run directly to the outlets, creating a draught, and leaving stagnant air in other parts of the room.
