FORCING AND FORCING STRUCTURES 71 



such vegetables as lettuce and cucumbers. Here the benches are 

 usually made solid ; that is, they rest upon the soil and, if elevated, 

 are only 1 8 or 20 inches above the general surface of the surround- 

 ing soil. The heating pipes are distributed 18 or 20 inches above 

 these benches or are bracketed to the side walls and roof supports. 



Many of the best modern houses for forcing lettuce, cucumbers, 

 and tomatoes have no benches. The soil, if suitable, is enriched 

 and used without further modification. If the natural soil is not 

 suitable it is replaced by the desired compost, which is renewed 

 only once in several years. This long-continued use of the soil is 

 made possible by sterilizing it by means of steam heat at least once* 

 each year. The fertility is kept up by the use of stable manure and 

 mineral fertilizers. The piping in these houses is usually accom- 

 plished by using coils suspended from the side walls, or posts 

 under the gutters when ridge and furrow construction is employed. 



Both of these systems of heating are well adapted to steam plants 

 but are not so well suited to the use of hot water. Steam, because 

 it can be conducted in small pipes and is more efficient, can be 

 utilized for overhead heating as well as for securing bottom heat. 

 Hot water, however, is most advantageously used as bottom heat 

 only, for in order to secure a given amount of radiation much 

 larger pipes are necessary with hot water than with steam. The 

 larger-sized pipes require more space, and since large pipes dis- 

 tributed above the surface of a bed obstruct a considerable amount 

 of light, it is more advantageous to the grower to place them 

 under raised benches. 



TABLE IV. EFFECTS OF WIND IN COOLING GLASS. (LEUCHARS) 



24-54 ; -. -91 



27-27 81 



