2019. The flued wall, or hot wall (figs. 579. and 580.), is generally built entirely of brick; though, where stone is abundant and more economical, the back or north side may be of that material. A flued wall may be termed a hollow wall, in which the vacuity is thrown into compartments (a a a a) to facilitate the circulation of smoke and heat, from the base or surface of the ground to within 1 or 2 feet of the coping. They are generally arranged with hooks inserted under the coping, to admit of fastening some description of protecting covers (1971.), and sometimes for temporary glass frames. A length of 40 ft., and from 10 to 15 ft. high, may be heated by one fire, the furnace of which (b), being placed 1 or 2 feet below the surface of the ground, the first course or flue (c) will commence 1 ft. above it, and be 2 ft. 6 in. or 3 ft. high; and the second, third, and fourth courses (d, c, f) narrower as they ascend. The thickness of that side of the flue next the south or preferable side should, for the first course, be 4 in., or brick and bed; and, for the other courses, it were desirable to have bricks cast in a smaller mould; say, for the second course, 3 in.; for the third, 2.75 in.; and for the fourth, 2.5 in. in breadth. This will give an opportunity of bevelling the wall; and the bricks, being all of the same thickness, though of different widths, the external appearance will be everywhere the same. Sometimes a vacuity is formed between the flue and the south or valuable side of the wall (Hort. Trans., vol. iv. p. 139.); but this, we think, may be considered an extravagant refinement. It cannot be carried into execution without employing a great quantity of materials and much labour. A wooden or wire trellis is also occasionally placed before flued walls: but both modes suppose a degree of forcing which does not appear advisable, unless the wall is kept constantly covered with glass; in which case, without this precaution, constant fires might injure, by occasioning the partial growth of the trees, or even burning those parts of them immediately opposite the furnace. To prevent accidents of this kind, the furnace must always be placed at some distance, say from 18 in. to 3 ft. from the back of the wall. An improvement has been made in flued walls by Mr. Shiells, of Erskine House, Renfrewshire, by the introduction of a register, to admit, at pleasure, a part of the smoke and heat from the fire directly into the second course of flues.