2110. Broad and deep flues, agreeably to the Dutch practice, have been recommended by Stevenson (Caled. Mem.); that of making them narrow and deep, agreeably to the practice in Russia, is recommended by Oldacre, gardener to Sir Joseph Banks; and that of using thin bricks (fig. 614.) with thick edges, by J. E. Gowen, Esq. (Hort. Trans., iii.) In Mr. Gowen's flues, the section (fig. 615. a) shows less materials than any other brick flue, the covers (b) and the side-wall bricks (c) being quite thin, the base requisite for building the latter on one another being obtained by the thickness of their edges (d, e), which is equal to that of common bricks. Glazed tiles are now frequently used for making flues.