WALLS AND VENTILATORS. 



35 



17. Two types of sash-bars. 



made of brick or stone, but unless they are very thick or are 

 hollow they are likely to be colder than a well built board 

 wall upon posts. The space represented by the thickness of 

 the posts affords an admirable 

 dead air space. Nearly all com- 

 mercial forcing-houses are upon 

 posts, and it is commonly said r- 

 that such walls will last as long ^ 

 as the plate will. This is proba- 

 bly true, but the plates, as usually 

 made, are unnecessarily short 

 lived. A forcing-house should 

 stand fifteen or twenty years 

 without extensive repairs, if well 

 built ; and if the side walls are 

 of masonry (stone or cement), the plates well made, the 

 roof so well supported that it cannot sag, and the whole 

 freshly painted every year or two, the structure should 

 stand nearly a life time. Good board walls are shown in 

 I I Figs. 20 and 25. In each, there is an air- 



space between the posts. In Fig. 20, there 

 are two air-spaces. The sheath ings cover- 

 ing the posts are shown at bb, the post 

 at p, and the siding at c. This wall, if 

 well built, is no doubt as warm as a 12- 

 inch solid brick wall.* 



Ventilators. The house should have 

 ample provision for ventilating it, yet it is 

 easy to get the openings so large that the 

 is. A plain sash- temperature of the interior may be lowered 

 too suddenly and too far when the sash are 

 lifted. In windy days, it is impossible to open very large 

 sash at all without letting in too much cold air to the 

 plants immediately underneath. Many small sash are pref- 



* Experiments with greenhouse walls have been made at the Minne- 

 sota Station (Bull. 7) and the Massachusetts Station (Bull. 4). 



4 FORC. 



