Natural Means of Ventilation Outside air may be polluted with dust, smoke and suspended matter and it is then necessary to free the air of these impurities before it enters the house. Especially is this true in cities and where large buildings are to be ventilated. The most satisfactory methods of ventilating a large building is the plenum and the vacuum systems. [Illustration: _Window Ventilation._] In the ordinary home, ventilation is through the natural openings such as windows and doors, although special openings may be provided which will admit fresh air and carry out the vitiated air. A great deal of air is admitted through the crevice and openings around windows and doors. A very simple method of airing a room in cold weather when a direct draught is objectionable is to place a board a few inches wide and as long as the width of the window beneath the lower sash. This prevents the air from coming through the open window, but permits it to enter between the upper and lower sashes. This also has the advantage of directing the air current toward the ceiling. This is very effective in the sick room and is so simple that any one can use it. [Illustration: _Window Mopper_] The size and shape of the room to be ventilated must be taken into consideration as well as the number of persons therein. The minimum amount of space allowed for each person has been placed by various authors at from 300 to 1,000 cubic feet, depending upon the nature of the work carried on, the size and shape of the room and the type of ventilation depended upon. In hospitals where fever cases are cared for 2,500 cubic feet are desired, while in government barracks each soldier is allowed 600 cubic feet. [Illustration: _Window Ventilation_] The necessity for a sufficient supply of pure air can not be over-emphasized. When the windows and doors are used to ventilate the house they should be thrown open at different times so that the house air may be completely changed. We must not, however, go to the other extreme and jeopardize the health of the occupants by keeping the temperature too low, yet it has been proven that cool, fresh air is more easily heated than warm vitiated air.