In addition to this, provision should be made for a constant access of fresh air to the room. A simple and rather primitive method is by raising the lower sash by a strip of wood several inches in height and the exact width of the window. The air will then enter the space between the upper and lower sashes. Some such or any better method of ventilation should be in continuous use, day and night, when the room is occupied. So soon as the weather is sufficiently mild, some of the windows should be left open all the time. When the air of a room is fresh and pure, the human system is furnished with all the oxygen it can consume, and heat is thus introduced into the body, so that a lower external temperature is necessary for comfort. Hence, supplying a sufficient quantity of fresh air minimizes the amount of coal consumed, besides increasing the vigor of the body. The temperature of the house will depend on the occupation, age, and health of the inhabitants. With a sedentary occupation, a temperature of from 64° to 70° F. is the most suitable. The temperature of the bedroom at night should not be allowed to fall below 50° F. Every room in the house should be furnished with a thermometer. =The Injurious Effects of Overheated Air.=—A rise of temperature in the surrounding air diminishes the amount of oxygen consumed and the amount of carbon dioxid discharged; a fall of temperature has the opposite effect. In addition, this overheated air forms a hot jacket about the body, which prevents the radiation of heat necessary to keep the body in a healthy condition. For the same reason, when out-of-doors, furs should not be worn close up around the neck, and fur coats should only be worn in the extremely cold weather. Paper and rubber worn about the body act in the same way, by preventing the radiation of heat and moisture—practically steam jackets are formed; the skin is rendered very sensitive and susceptible of chilling on the slightest fall of temperature. =The Proper Degree of Moisture of the Air of the House.=—The air below the freezing-point is deprived of much of its moisture; brought into the house, and raised from 70° to 80° F., or drawn into the nostrils and raised to 98° F., it must take up its quota of moisture. This moisture must, therefore, be provided in the air of the house. In the case of houses heated by furnaces, some moisture is furnished by the water-pans of the furnace; but in the case of heating by hot air and steam pipes, there is less circulation of air, the air is very much drier, and generally of a very much higher temperature. A satisfactory method of furnishing these houses with the proper degree of moisture has not yet been invented.