arch (made from fire brick) attached to the front wall (boiler throat plate) of the firebox immediately beneath the firetubes. This extends backwards over the front third to half of the firebed. It is supported on arch tubes, thermic syphons, or circulators. The brick arch directs heat, flames, and smoke back over the fire towards the rear of the firebox. Visible smoke contains unburned combustible carbon particles and combustible gasses. The purpose of this redirection is to cause more complete combustion of these particles and gasses which make the locomotive more efficient and causes less visible smoke to be emitted from the stack. Without the arch, flames and visible smoke would be sucked straight into the firetubes without having been fully burned, causing visible smoke to be emitted at the stack. The invention of the brick arch allowed locomotives to burn cheaper coal (which contains volatiles) instead of coke, it having been a legal requirement from the outset for engines to "consume their own smoke". The brick arch and its supports (arch tubes, thermic syphons, and circulators) require periodic replacement due to the extreme heat they endure. The brick arch also deflects "cold" air being drawn in through the firebox door from directly hitting the rear tube plate of the boiler and cooling it when the boiler is hot. Firetubes Firetubes are attached to one wall of the firebox (the front wall for a longitudinal boiler, the top for a vertical boiler) and carry the hot gaseous products of combustion through the boiler water, heating it, before they escape to the atmosphere. Firetubes serve the additional purpose of staying the flat tube (flue) sheets (front and rear) so that only the top of the front flue sheet and the bottom of the rear flue sheet must be separately braced. Sheets and stays The metal walls of the firebox are normally called sheets, which are separated and supported by stays. The stays support and brace the "sheets" (plates) against pressure. Ideally, they should be located at right angles to the sheets, but since the outer sheet (wrapper sheet) is often rounded and the top of the firebox (crown sheet) is relatively flat by comparison, such a relationship to both sheets is impossible. The actual location of the stays is a compromise. Since stay breakage is hidden, the stays have longitudinal holes, called tell-tales, drilled in them which will blow water and