The quantity of water required to humidify the air of a house will depend _first_, on the temperature and humidity of the outside air; _second_, on the cubic contents of the building; _third_, on the rate of change of air in the building. If the ventilation is good the rate of atmospheric change is rapid and the amount of water in consequence must be correspondingly increased. The data included in the following table showing the relative humidity and amount of water required were taken from a seven-room frame dwelling in Fargo, N. D., during particularly severe winter weather. The relative humidity determinations were made with a hygrodeik each day at noon. The house was heated by a hot-air furnace arranged to take its air supply from the outside. The air supply is recorded under Cold-air intake. The furnace was provided with a water pan for humidifying the air supply. The amount of water evaporated each day is recorded in the column headed Evap. in 24 hours. The outside temperature ranged from -12°F. to -21°F. The weather was clear and calm except the last day, Jan. 12, which was windy. The higher humidity on that day was no doubt due to the greater amount of heat required from the furnace and the consequent evaporation of the water from the water pan. The humidity determinations made by a hygrodeik, as before explained, are only approximately correct but sufficiently exact for practical purposes. The temperature is given in degrees Fahrenheit. In the table it will be noticed that the outside air was used only a part of the time because of the severity of the weather. Attention is called to the quantity of water required to keep the humidity at the amount shown. This averages 27-1/2 quarts per day. At the time these observations were made the physics lecture-room at the North Dakota Agricultural College averaged 18 to 20 per cent. saturation during class hours, with observations made from a similar instrument. This is a steam-heated room with only accidental means of adding water to the air. The result was an atmosphere 3-1/2 per cent. above that of Death Valley. HOT-AIR FURNACE Readings taken at 12 o’clock noon each day