2009. Thompson's forcing pits (figs. 568, 569, and 570.) differ from those of M'Phail by substituting stone lintels in place of pigeon-holes to the outside walls. The suspended insulated position of the pits admits a circulation of warm air, both under and all around the pit of each light, whereby a greater degree of surface temperature is obtained, in the absence of solar rays, in the early forcing season. Fig. 568.: in the ground-plan, a a a show the open-work end and the support for the north and south lintels; and b b b are bricks on edge, to support the bottom of the pits and the surface hot-air flues round each. Fig. 569. is the longitudinal section: c c c c c are hot-air flues round and under the bottom of the pit, which are covered with a single tile. When the bottom of the pit is laid, the brick on edge is continued up to a convenient height for the surface hot-air flues, which are also covered with a single tile, laid the reverse way to those at the bottom of the pit. Fig. 570. is the transverse section, showing the pits and the position of the lintels, which admit the fermenting body of manure to act under the north and south flues. The internal part of the pits is covered with a thin coat of hair mortar, made rough by finishing it with a wood float and brush; and the tiles are the flat draining ones, without knobs. These pits are generally from 5 ft. to 6 ft. 10 in. wide.
