VENTILATION AND HEATING. 1 15 



of air, which is suitably warmed while on its way into the 

 room, and to make the doors shut snugly, and to have 

 double windows, as then both the admission of fresh air 

 and the regulation of heat will be better secured. But 

 it is a serious question whether, with all our modern ap- 

 pliances, conveniences, and luxuries, we have better air 

 in our houses, and take cold less frequently, than our 

 ancestors who depended more on the fireplace, even if 

 they did " roast on one side while they froze on the other." 

 Fireplaces are expensive as mere heaters, but they are 

 excellent ventilators. 



Ventilating Flues around a Smoke Flue. If small 

 ventilating flues could be built around the flue of the main 

 heating apparatus, and connected with the various rooms 

 of the house, air could be drawn from these rooms by 

 ascending currents created by the heat of the central smoke 

 flue. Such flues surrounding smoke flues, would have the 

 added advantage of protecting the house from fire through 

 the too common " defective flue." 



The General Principles of Ventilation. Of the forces 

 that operate to renew the air two are natural, diffusion 

 and the wind ; and two are artificial, warm air shafts and 

 fan systems. 



Diffusion. Gases tend to mix. We know that if a 

 bottle containing an odorous substance be opened in 

 a room where there are no air currents the odor tends to 

 spread equally through the room. So if a person is in 

 one corner of a large room, where there are no inlets 

 or outlets, and no currents, as he uses the oxygen immedi- 

 ately around him, the oxygen farther away will diffuse 

 toward him so that he will continue to get oxygen till the 

 amount of oxygen in the room is nearly exhausted. So, 



