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. 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. 



