FRESH AIR 327 



habit. Undoubtedly our American houses during the winter months 

 are usually kept too hot to maintain the highest efficiency of the in- 

 dividual. 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 tempera- 

 ture is better where physical work is being done. The following tem- 

 peratures 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 thermom- 

 eter 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 mer- 

 cury in such an instrument depends on two factors : first, the tempera- 

 ture of the air ; and secondly, the amount of evaporation of the water im- 

 mediately 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 effi- 

 cient simple instrument for the determination of humidity is the com- 

 bination 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 main- 

 tenance 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. 



