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Fig. 81.—The air enters through a special inlet and is warmed as it passes through hood surrounding the stove.

Fig. 82.—Chimney with a passage behind fireplace, or grate, in which the air is warmed as it enters.

question in cold weather is because the air furnished must be not only pure, but warm. To keep cold air out often means to keep foul air in. Heating with hot air, by which system pure air is passed over a furnace, and fresh air constantly admitted, may be a good method (Figs. 80, 81), but is often a dismal failure because it dries out the air, which in turn dries out the skin. To prevent this, wide vessels of water should be set at the inlets. Dry air is cooling. Why? Dr. Barnes proved that moist air at 65° is as comfortable as dry air at 71°. Air saturated with vapor at 60° will only be 50 per cent saturated at 80°. Such air dries out the mucous membrane of eyes, nose, and throat. Heating by hot water circulating in pipes, or by steam, gives no means of introducing fresh air, and is likely to cause worse ventilation than any other method. The radiators should stand close to windows or other fresh-air inlet, that the air may be heated as it enters, and the outlet for air should be farthest from the radiators. The same rules apply to heating by stoves. An oil stove for heating is an inconceivable