312 FORCING DEPARTMENT. 



roofs are necessarily fixed, and are, in consequence, 

 often very deficient in power of ventilation ; air 

 being, in most cases, only admitted by having venti- 

 lators in the back and front walls, which has 

 frequently proved inefficient for modification of the 

 temperature in hot weather. This defect is now, 

 however, principally removed, by having parts of the 

 roof and ends so constructed, as to admit a free cir- 

 culation of air, which will certainly obviate that 

 difficulty. 



But the principal objection that I have against 

 using the wrought iron sash bar in forcing-houses, is 

 its attractive qualities, which allow the heat and cold 

 to pass rapidly through it. Wrought iron is, also, 

 very liable to corrode, much more so than cast metal, 

 of which all who have had any experience of the 

 two must be fully aware. 



In the construction of the Forcing-House Roofs, 

 cast iron rafters, wall plates, &c. wrought iron sash 

 frames, and copper bars, have been extensively used; 

 they are considered by many the most durable 

 materials that can possibly be introduced; and when 

 properly executed, I should imagine that a roof of 

 this description cannot be surpassed by any other 

 for durability, the admission of sun and light, and 

 elegance of appearance. The annexed Plate, No. 18, 

 will illustrate the materials of which the Forcing- 

 Houses, at Woburn, are constructed. They are 

 raised on somewhat a different principle from 

 any that I have yet treated of; that is, with a com- 

 bination of the several materials. In this range, the 

 rafters, standards, spouts, and sills, where strength 



