116 GREENHOUSE CONSTRUCTION AND HEATING. 
gratings, supported by suitable bearing-bars of T-iron, 
is fixed at about the ground level, or rather more than a 
foot below the top of the front wall. A lower chamber is 
thus formed, in which the manure or other fermenting 
material is contained ; this is brought in, and again removed 
when necessary through doors in the back wall, opening on 
to a sunk path behind, as shown in Fig. 74. The warmth 
from the manure, etc., rises through the perforated false 
bottom into the upper chamber, where the plants are 
placed, and when exhausted the material is easily removed 
and replaced by fresh without disturbing the plants in- 
Fig. 74. 
the least. This form of pit is an excellent and most 
useful one for the culture of melons, cucumbers, and other 
plants in particular. 
The principal difference between a pit and a frame 
proper is that the former is a fixed structure, built on 
brick in concrete walls, and consequently a landlord’s 
fixture, while a frame is a moveable affair, constructed 
entirely of wood. 
A frame consists, as a rule, of a wooden ‘‘box’”’ formed 
of tongued and grooved boards, which may be from 3in. 
