WARMING AND VENTILATING OCCUPIED BUILDINGS. 311 



the mean temperature of the surrounding air, that air expands, its 

 density diminishes, and each cubic foot, no longer weighing as much as 

 the same volume of the rest of the air, is pressed upward by a force 

 equal to the dift'erence of densities. 



Thus, in summer, the air in contact with the window-panes becoming 

 warmed, and, in winter, the same eifect being j)roduced by stoves, lights, 

 and even by the people in the room, the air, becoming less dense, rises 

 toward the ceiling. 



If, for example, tlie mean general temperature of any place is 61°, the 

 density of the air is 0.000077 pound, and if owing to the action of the 

 sun the air in contact with the windows becomes raised to 79°, its den- 

 sity becomes 0.000074, and each cubic foot of that air is forced upward 

 by a force equal to 0.000077—0.000074=0.000003 pounds. 



36. Frequency of the preceding effects. — The effects which we have just 

 mentioned frequently manifest themselves in a very unpleasant way 

 in winter, when a person is seated by a window, in a room where the 

 general temperature is high, and still more sensibly in large halls, 

 lighted by skylights. In the latter case, it is often necessary, to avoid 

 these difficulties, while retaining the lights, to heat the space between 

 the roof and the ceiling to 80° or 104° by means of stoves, examples of 

 which will be given further on. 



On the contrary, glazed ceilings, recently introduced into some 

 theaters and i3alaces to admit the light from a large number of gas jets, 

 also heat the room to a very unpleasant degree. 



A similar effect is produced in summer in railroad-stations, courts, 

 workshops, and in large buildings covered with glass roofs in which 

 sufficient ventilation has not been provided. The temperature in such 

 places often rises to 104°, 110°, and more. 



37. Unstable equilibrium of air. — Cooling and warming effects similar 

 to those just mentioned are constantly taking place in dwellings, the 

 air is never at rest, and the slightest variation in temperature and press- 

 ure produces almost endless motion. The air, then, is always in unsta- 

 ble equilibrium. 



38. General principles of ventilation.— Gh^uge of air in occupied 

 places is only rendered necessary by the alteration produced in it by 

 respiration, bodily exhalations, the heat given out by the occupants, by 

 the lights, or by these different causes combined. The many observa- 

 tions which I have made, and a comparison of experiments made by 

 different engineers and by myself, have led me to the following conclu- 

 sions, which I regard as proper to serve as fundamental principles in 

 the formation of plans for the ventilation of occupied buildings, and 

 especially of hospitals: 



1. Ventilation is designed for the removal of foul air and the substi- 

 tution of fresh air. 



2. The principal object of ventilation is the immediate withdrawal of 

 foul air. It should, in general, act as near as possible to the points 



