2111. Can-flues (fig. 616.), long since used by the Dutch, embedded in sand, and for the last fifty years occasionally in England, are sometimes employed. They consist of earthen pipes, straight (a), or rounded at the ends for returns (b), and joined together by cement, placed on bricks (c). They are rapidly heated, and as soon cooled. None of the heat, however, which passes through them, can be said to be absorbed and lost in the mass of enclosing matter, as Knight and Sir Joseph Banks (Hort. Trans.) assert to be the case with common flues. They are only adapted for moderate fires, but, judiciously chosen, may frequently be more suitable and profitable than common flues; as, for example, where there are only slight fires wanted occasionally, or where there is a regular system of watching the fires; in which case, but not otherwise, the temperature can be regulated with sufficient certainty. These can-flues are now very seldom met with.
