2005. The pit with a rising frame (fig. 564.) contains a basement-wall of brickwork, of the height of the dung or bark (a a); and in this is a perpendicular vacuity (b b), in which a common frame (c c) is placed, and, by a spindle, pinion, &c. (d d), may be raised or lowered at pleasure. Its object is the same as that of Weeks's frame, already described; and which it attains with less risk to the plants, but at a great comparative expense. This variety of pit is the invention of John Nairn (Hort. Trans., vol. iii.), who has had it executed, and heated by surrounding tubes (e e) filled with steam.