habit. Undoubtedly our American houses during the winter months are usually kept too hot to maintain the highest efficiency of the individual. We are in far better physical condition when surrounded by a house temperature of 65° to 68° F. than of 70° F. Some of the British authorities advise a house temperature as low as even 60° F. Young persons can live efficiently in a lower temperature than those of middle life, while aged persons require warmer air. A lower temperature is better where physical work is being done. The following temperatures of heated rooms are recommended by American ventilating engineers:
| Occupants at Rest | Degrees F. | Occupants Physically Active | Degrees F. | |
| Living rooms, offices, schools | 68 | Gymnasiums | 60 | |
| Lecture halls | 61-64 | Work shops, moderate exertion. | 61-64 | |
| Sleeping rooms | 54-59 | Work shops, vigorous exertion. | 50-59 | |
| Bath rooms | 68-72 |
As to humidity, a percentage of 60 with air of 68° F. is rational. But the amount of moisture that air is capable of absorbing varies greatly with the temperature, hence it is impossible to establish a single standard of humidity that can apply to a range of temperatures. The surest single index of the physiological quality of the atmosphere at any moment is the reading of the wet-bulb thermometer. In this thermometer the bulb is covered by thin muslin or silk soaked with pure water. The evaporation of the water cools the bulb. The position of the mercury in such an instrument depends on two factors: first, the temperature of the air; and secondly, the amount of evaporation of the water immediately surrounding the bulb, which in turn varies inversely with the amount of moisture in the air generally—the more moisture in the air the less evaporation from the bulb. The wet-bulb thermometer is thus an index, at once, of both temperature and humidity. The most efficient simple instrument for the determination of humidity is the combination of dry bulb and wet bulb thermometers known as the sling psychrometer, but a fairly satisfactory indicator for household use is the instrument sold commercially under the name of hygrodeik. For our living rooms a wet-bulb reading of 60° F. is favorable to the maintenance of a comfortable and efficient physiological state. We can usually keep the temperatures of our rooms within reasonable limits by the aid of our heating systems and air admitted through windows; but the humidity can not be so perfectly controlled without more elaborate means than most private houses are provided with. With the increase in size of our American buildings, whether apartment houses, office buildings, school houses or factories, the provision of ventilation by means of more or less elaborate apparatus has become a necessity, and the profession of heating and ventilating engineer has become one of dignity and importance.
