GLASS STRUCTURES 55 



tion of the roof, the more nearly glass is brought at right 

 angles to the sun's rays, the greater light efficiency it 

 has. In so-called two-thirds and three-fourths span- 

 houses, more glass is so placed. In uneven-span houses, 

 therefore, there is less reflected light than in even-span. 

 Rose-growers in the East prefer this type of house. Com- 

 parative widths of the south span and the north span vary 

 with different builders, but the preference is to place the 

 ridge far enough to the north so that it casts no shade on 

 the north bench at any time of the year. 



In older houses it was customary to elevate the north 

 bench considerably to bring plants nearer the glass and to 

 give them better light conditions. It is now considered 

 better to raise each bench and to grade the soil in the 

 houses before the benches or beds are installed, so that, 

 while all benches are of the same height, each bench 

 towards the north is elevated about six inches above the 

 one on the south. This facilitates ease in working among 

 plants and eliminates unhealthful atmospheric conditions 

 which prevailed under high walks and benches in older 

 houses. 



51. Ridge-and-furrow houses. Ridge-and-furrow 

 houses are much in favor in the Central States. These 

 houses are modifications of even-span houses. They 

 are so built that extensive areas are practically under 

 one roof. 



This system permits of a lessening of the cost of construc- 

 tion, for there are fewer outside walls to build. As they 

 cover the entire area of land there is no unoccupied land 

 between them, and where land values are high, this is 

 quite an important factor. It is possible to heat these 

 houses economically because there is less exposed wall area 

 than in separated houses. They may also be worked easily, 



