Flues are pipes formed of brick or slate, for conducting heated air through stoves or other buildings where a high artificial temperature is desired. It is a mode of heating nearly banished by the much more manageable and effectual modes of heating by hot water; and flues have the additional disadvantages, that they require frequent sweeping, and that they emit a sulphurous fume that is injurious to plants and disagreeable to the frequenters of the structures so heated. This has been obviated by using Valencia slates in the place of bricks, yet flues under no circumstances can compare with either the pipe or tank system of hot water heating. When flues are employed they are constructed inside and near the walls of the building; each flue eight or nine inches wide in the clear, by two or three bricks on edge deep, ranged horizontally one over the other the whole length of the back wall, in three or four returns communicating with each other, continued also along the end and front walls in one or two ranges, to be used occasionally; furnished with a regulator to slide open and shut as required, the whole proceeding from the first lowermost flue, which communicates immediately from the furnace or fire-place behind either the back wall at one end, or in the back part of the end walls; or if very long stoves, of more than forty feet length, two fire-places are requisite, one at each end; each having its set of flues ranging halfway; each set of flues terminating in an upright chimney at the end of the back outside. - Hood on Warming, &-c.

Morris, Tasker and Morris of the Pascal Works near Philadelphia, have paid considerable attention to the construction of heating apparatus, whether for air or water. Those who desire such structures for green-houses, conservatories, etc, may safely rely on their experience and probity.