224 GREENHOUSES 



size outside creates a flow upward in the 

 chimney. This difference increases with the 

 height of the chimney, and if the difference 

 in temperature increases the velocity is more 

 rapid. Locations high above sea level require 

 higher chimneys than those near sea level, 

 on account of the rarety of the atmo- 



Fig. 119. Chimneys should extend above the roofs of 

 adjacent buildings 



sphere. For example, at Denver, Col., (5,300 

 feet) the height should be about 20 per cent, 

 greater than at sea level. 



Chimneys should be vertical if possible and 

 the inside should be smooth and free from all 

 obstructions. They should also extend well 

 above the roofs of adjacent buildings, particu- 

 larly when there is danger of a "down 

 draft." Round chimneys present less sur- 

 face per cubic capacity than do square chim- 

 neys, and are thus more efficient. For the 

 same reason square flues are better than ob- 

 long flues. 



