122 GREENHOUSES 



maintaining a supply of fresh air, but is 

 utilized as a method of controlling tempera- 

 ture and humidity. (4) Greenhouses, be- 

 cause of their transparent roofs, are much-- 

 more liable to sudden or violent changes in 

 temperature (especially in days of alternate 

 clouds and sunshine) than are dwellings, and 

 the necessity for ventilation in order to 

 equalize the temperature is evident. (5) 

 Greenhouse plants are, as a rule, particular- 

 ly sensitive to cold drafts, and ventilation 

 cannot be left to the indiscriminate opening 

 of doors. 



Systems of Greenhouse Ventilation. 



There can hardly be said to be any well de- 

 fined systems of greenhouse ventilation, as 

 compared with the so-called systems of 

 ventilation for public buildings. Greenhouse 

 ventilation rests on the principle that warm 

 air has a tendency to rise, and since the air 

 within the greenhouse is considerably warm- 

 er than that outside, during both summer 

 and winter, the question of changing the air 

 presents no serious problem. It is only 

 necessary to provide a means for the warm 

 air to escape. The cooler air from the out- 

 side easily finds its way into the house 



