124 GREENHOUSES 



venient in spring and summer. An in- 

 genious method is sometimes employed in 

 conservatories whereby the air is taken in 

 from below the benches and is warmed by 

 passing over the heating pipes. Thus the 

 danger of injury to the plants is greatly less- 

 ened. There is no evidence to show that 

 there is any special benefit to be derived from 

 these ventilators (Fig. 67). 



Overhead Ventilation. During the winter 

 practically all the ventilation of greenhouses 

 is accomplished by means of overhead 

 ventilators set in the roof at or near the 

 ridge. These ventilators are in the form of 

 sash hinged on the outside, and may be 

 closed down tightly over the sash bars or 

 opened to any degree desired. As the warm 

 air naturally rises, the opening of these 

 ventilators allows the warmest air of the 

 house to escape, and fresh cool air to filter 

 in through the crevices between panes of 

 glass without causing excessive drafts. 



Experience shows that these ventilators 

 need to be relatively narrow and practically 

 continuous along the whole length of the 

 house, rather than intermittent, as the pres- 

 ence of occasional large openings is more 



