overheating: in another type of water tube boiler, the Thornycroft, the steam-generating tubes joined the top chamber above the water line, and their tops were "observed to get red-hot when the water was low." Overheated tubes were liable to fail. Large, external "down-comer" tubes transferred water from the top chamber to the two bottom ones. The down-comers thus promoted convection within the boiler, which needed to be rapid because of the small diameter of the tubes, and formed "a substantial part of [its] framework." Steam was collected inside a dome on top of the top chamber, from which it passed out of the boiler for use via engine room controls, and in the Reed boiler all but the dome and the ends of the three water chambers was enclosed in a double-layered casing with an air gap and asbestos lining that reduced the temperature of the outer layer. The casing rose at the top to form an outlet for hot gases into a funnel. The furnace was fed with coal by stokers through firebox doors at one end, and, whereas the Normand boiler required a fire of about 18 inches (460 mm) depth, the Reed boiler required a shallower one of between 8 and 12 inches (200–300 mm). Air was admitted to the furnace through the air gap in the boiler casing, thus providing a supply of heated air that was directed to the rear of the ashpan. This air entered the ashpan through three doors that, along with the firebox doors, closed automatically if a tube failed, the intention being to prevent flames, steam and debris escaping into the boiler room. A constant supply of pure water was essential for this type of boiler, as a shortage of water would rapidly result in an empty boiler liable to severe damage from the furnace, and the deposition of any contaminant, such as limescale, would result in a significant loss of efficiency and could block tubes. To surmount this problem, boiler feedwater circulated in a closed system from the boiler as steam to the engines and then to condensers, from which it returned as water to the boiler, thus completing a cycle. However, some incidental loss of water from the system was unavoidable, and the French naval engineer Louis-Émile Bertin regarded a 5% loss of water per cycle as the maximum that could be sustained in a water tube boiler installation.