FORCING AND FORCING STRUCTURES 75 



high (see figure 23) is popular. In still other sections the type of 

 building in common use is an even-span house with the ridge 

 running north and south, or a three-quarters-span house with a 

 long slope to the south. The tendency at present is toward the 

 large even-span house (see figure 23). Since all types of construc- 

 tion give good results it is impossible to say which is best. 



Pitch of the roof. The proper angle of the roof is a very impor- 

 tant factor in the construction of a forcing house. At the time of 

 year when it is desirable to use these artificial structures for the 

 growth of plants, the sun is at its lowest point in the horizon ; that 

 is, it is farthest south and least effective in the northern hemisphere. 



FIG. 23. Ridge-and-furrow type of greenhouse construction 



The pitch of the roof of the house should be such as to allow the 

 rays of the sun to strike the roof as nearly as possible at right angles 

 when the sun is at its zenith during these short days. In the vicin- 

 ity of New York and Boston a roof with a pitch of 34 is near the 

 ideal. Farther south, where the sun's rays fall more nearly verti- 

 cally, the angle of the roof should be increased rather than lessened. 

 It is quite as desirable to have a large percentage of the sun's rays 

 reflected when the power of the sun is more intense as it is to have 

 a large percentage of them pass through the glass during mid- 

 winter. The slope of the roof for a forcing house will therefore 

 depend upon the latitude in which it is erected, the position of the 

 house, and the purpose for which it is intended. 



