2171. Pits, as applied to the interior parts of houses, are excavations, or rather enclosures, for holding bark or other fermentable substances; or sand, or ashes, or sometimes even common garden soil. They should be so formed as that the plants may stand at a moderate distance from the glass, which of course depends on whether they are dwarf bushy plants, as the pine-apple, &c., or taller, as palms and hothouse trees. The pits are generally surrounded by walls of brick, 4 or 9 inches thick; or, to save room, by plates of cast iron, stone, or slate. Sometimes the slope of the surface of the pit has a similar inclination to that of the roof; but as, in this case, the tan or leaves, in the course of fermentation, do not settle or compress regularly, the pots are thrown off their level, and therefore the more common way is to adopt a slope not exceeding 5ï¾¦, or to form a level surface. Tan will ferment with all the rapidity necessary for bottom heat, if in a layer of 2.5 or 3 feet thick, and therefore no tan-pits need exceed that depth. Those for leaves may be somewhat deeper. Heat from fire, or steam, or water, is sometimes substituted for that afforded by fermentable substances, and in these cases various forms of construction are adopted. Pits may be heated by steam, and hot water, by various modes which have been already pointed out (ï¾º 2124. to ï¾º 2145.).