220 The Princijjles of Ventilation. 



2. Gravitation by reason of the necessary difference in temperature 

 of the air in-doors from that out-doors. 



Ventilation by perflation is the most simple of all means, consisting 

 in opening windows on opposite sides of the house. It is, of course, 

 only applicable when the weather is warm, and in perfectly calm 

 weather is only moderately successful. Perflation may be combined 

 with an apparatus for cooling the air, and is then almost always suc- 

 cessful. A method that I have tried with good results is, to hang 

 three or four wet towels from sticks, supported at one end by the 

 slats of the blind — the towels hanging perpendicularly to the window, 

 on the windward side of the house. The efi'ect is to cool the air by 

 the absorption of heat consequent on the evaporation of the water. 

 There is a distinct acceleration of the velocity of the incoming air, 

 as might be expected. This combination however, is really an artificial 

 method. 



The method by gravitation is less successful in its operation than 

 the one just described ; it consists in utilizing the difference in tempera- 

 ture commonly existing between the air in a house and that outside. 

 Doors and windows, at the bottom of the house, are opened and the 

 scuttle is also opened, or else the upper sash of windows in the upper 

 story are drawn down. In the case of a high, narrow house, this 

 method gives fairly good results. Of course, the kitchen has to be 

 shut off from the rest of the house, in order to keep culinary smells 

 from circulating, and for a like reason the dining-room must at certain 

 times be excluded from the benefits of the system. After a current 

 has been fairly established, it will be found an advantage to open 

 slightly, at the bottom, the windows of the lower stories on the wind- 

 ward side of the house, as these will then act as inlets. Like the 

 method of perflation, this plan cannot be depended on. Just when 

 ventilation and cool air are most needed, that is on close, hot, sultry 

 days, the difference in temperature becomes so small that the exchange 

 of air practically ceases. 



The artificial methods of ventilation are divisible into two principal 

 classes ; first, Those employing heat directly ; and, second. Those 

 employing motors of some sort. The principle involved in the first of 

 the cases is the same as in the natural gravitation method described 

 above; that is, hot air has a lower specific gravity than cold air, and 

 will rise if opportunity be given, while cold air supplies its place. In 

 the direct application of heat, the source is either indirect radiation, 

 direct radiation, or a combination of the two. It is to be noted that 

 whatever maybe the system of ventilation, in the cold months it must 

 include the heating of the building, and in the summer the cooling of 



