. Cassell's popular gardening. Gardening. GLASS STRUCTUEES AND APPLIANCES. 223 frequent intervals, they are not suiiiciently strong- to bear the wear-and-tear of pit-management; and be- sides, four-inch cannot be trusted to exclude twenty or thirty degrees of frost. The lower portions of the walls of pits are also very generally pigeon-holed, that is, a certain number of the headers are left out, and thus any quantity of four-inch openings are left right through the walls. Other pits rest on pillars, fourteen inches or more square, and from two to four feet high; or wood or iron pillars are pl
. Cassell's popular gardening. Gardening. GLASS STRUCTUEES AND APPLIANCES. 223 frequent intervals, they are not suiiiciently strong- to bear the wear-and-tear of pit-management; and be- sides, four-inch cannot be trusted to exclude twenty or thirty degrees of frost. The lower portions of the walls of pits are also very generally pigeon-holed, that is, a certain number of the headers are left out, and thus any quantity of four-inch openings are left right through the walls. Other pits rest on pillars, fourteen inches or more square, and from two to four feet high; or wood or iron pillars are placed at in- tervals of four feet or so along the front and back walls, and York or other slab-stones, or slates, are carried along from one to the other. These may be a foot or more wide, and the upper wall of the pit is These may or may not differ greatly in size, form, or structure. They must differ in temperature, and with the object of enabling them so to-differ with greater facility and despatch, certain structural pecu- liarities are introduced. For example, the oldest plan of heating pits—some practitioners would still call it the best—viz., by the use of fermenting ma- terials, while possessing many elements of danger and uncertainty, had at least the merit of keeping them warm for a long time without attention, and of giving out moisture as well as heat. This, however, was associated with one very constant and consider- able drawback. In the further decomposition of the heating materials they lost bulk as well as caloric. Now in all pits this was an evil—the deeper the i^it the greater the evil. In pits a vard or four feet. Fig. 15.—Pit Built on Pillars. Showing wooden battens covered with brushwood, over hot- air chamber, a, chamber ; b, battening ; c, brushwood; d, space for soil; e, space for Melon or other plants. Fig. 16.—Pit with Stages. Showing how the stages are arranged near the glass for the storage of plants, or the culture of Kidnf'y Beans, Straw-
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