12 GREENHOUSE CONSTRUCTION AND HEATING. 
has to be heavily shaded the plants still receive the benefit 
of a good deal of light through the glass on the north 
side. 
A lean-to or three-quarter span-roofed house built 
against a wall with a south, south-west, or even south-east 
aspect is usually very warm—appreciably more so than 
on i 
y\_ 
_ eee 
Fig. 3. 
an isolated span-roofed structure would be under similar 
circumstances — receiving every ray of sun there is 
(especially during the winter season), while the solid wall 
at the back acts powerfully as a screen from cold north 
or easterly winds. Such houses are consequently well 
adapted for forcing purposes and the production of early 
