CONSTRUCTION OF GREENHOUSES 
123 
short slopes to the north. They are inexpensive to con- 
struct, but inconvenient to operate, because the ground 
in the houses usually slopes decidedly — a condition which 
makes work very tiresome and satisfactory watering 
difficult. 
The even-span house (Figure 26) is used extensively. 
It secures an even distribution of light and gives the 
plants in all parts of the house the same chance, and 
FIG. 26. UNEVEN AND EVEN-SPAN GREENHOUSES 
for these reasons is the most popular form of construc- 
tion. 
Three-quarter span houses (Figure 27) have been 
used for many years near Boston. They usually run 
east and west, and plants grown in them get the full ben- 
efit of light and heat from the sun in the forenoon. The 
houses near Boston are usually 40 feet wide and 200 to 
500 feet long'. 
178. Walls. — Whatever material is used in their con- 
struction, the walls should be started below frost line 
