316 WARMING AND VENTILATING OCCUPIED BUILDINGS. 



In theaters, in consequence of tbe great number of passages, tlie 

 difference should be 65° to 72° to secure the necessary discharge. 



46. Insufficiency of natural ventilation. — It also follows from what 

 precedes that in proportion as the difference of temperatures becomes 

 less the velocity of the circulation of air diminishes, which explains 

 why, if in winter the natural ventilation produced simply by the exoess 

 of the temperature of occupied places — usually kept at about 00° — over 

 the external temperature is sufficient in most cases to secure a proper 

 change of air when well-proportioned ventilating-flues have been put 

 up, it is no longer so in spring and still less in summer. In these 

 seasons, natural ventilation becomes inefficient, and, as besides it is not 

 always possible to keep the windows open, it becomes necessary to have 

 recourse to artificial ventilation whenever it is deemed necessary to 

 obtain a regular change of air. 



47. Accidental reversal of the motion of tlie air. — When the shafts are 

 not kept at a sufficiently high temperature, it often happens that the 

 motion of the air becomes reversed, and the flues introduce the external 

 cold air instead of causing the discharge of the foul air. This effect is 

 frequently produced in arrangements where, by the introduction and 

 discharge of air, natural ventilation cannot be counted upon, notwith- 

 standing the difference of height in the entry and discharge shafts. 

 This is frequently observed in reception-halls, where, several rooms 

 being thrown into one, the chimneys of some, being heated more or less, 

 serve as discharge-flues, while others bring in cold air. The same effects 

 are also i^roduced in places which are only ventilated a part of the day, 

 such as lecture-rooms, theaters, &c. It then often happens that the 

 motion of the air is periodically reversed. The cold air enters by the 

 discharge-flue, and the warmer air of the places which have been occu- 

 pied escapes through the openings for the admission of air. This 

 reversal, which produces a useless cooling effect, may be prevented by 

 placing in the air-pipes and channels doors or registers, which may be 

 shut when the ventilation is to be discontinued. 



Finally, it is also necessary to close the communication with the places 

 to be ventilated when starting the fire, mentioned further on, placed at 

 the bottom of the ventilating-shaftfor producing draught, so as to avoid 

 a down-draught, as in the preceding case, filling the room with smoke. 

 A special air- supply should be reserved for this fire. 



48. Insufficiency of tcindoiv -openings. — It is generally believed that 

 opening the windows of a large room will produce a com^olete change of 

 air, and many physicians think that in hospitals the opening of a cer- 

 tain numbers of windows placed on opposite sides, will have that effect. 

 This is not as true as supposed; and in summer, when the air is still and 

 there is no wind, it often happens that the complete opening of five or 

 six windows, on opposite sides of a large reception-room, coach-house, 

 railroad-station, or riding-school, produces but a very imperfect change 



