WALLS AND VENTILATORS. a5 
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 
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 “iron oe 
without extensive repairs, if well 77, Zwo types of sash-bars. 
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 
Figs. 20 and 25. In each, there is an air- 
space between the posts. In Fig. 20, there 
t are two air-spaces. The sheathings cover- 
ing the posts are shown at bb, the post 
| at p, and the siding at c. This wall, if 
swell built, is no doubt as warm as a I2- 
inch solid brick wall.* 
° 
flatter than 30 
=O 
Ere) 
or 
— 
ao 
Y 8 
> 2 
ZS 
1) 
= & 
fs 
= 
=) 
w 
Ventilators—The house should have 
ample provision for ventilating it, yet it is 
easy to get the openings so large that the 
18. A plain sash-temperature of the interior may be lowered 
oak. 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- 
——_{5,- > 
* Experiments with greenhouse walls have been made at the Minne- 
sota Station (Bull. 7) and the Massachusetts Station (Bull. 4). 
4 FORC. 
