172. A very convenient covering for horizontal flues, which can be removed and replaced easily, is shown in the annexed diagram. It consists of a kind of arched tile, made by bindingtogether, with an iron clamp, as many bricks as may be required by the width of the flue, each tile, or section, having the length of one brick. The flue is covered with these sections, and the interstices are filled with mortar, or a mixture of clay and sand.