There are two distinct types of connexion between the furnaces and the combustion chambers. In one, shown in fig. 8, the furnace is flanged at the crown portion for riveting to the tube plate, and the lower part of the furnace is riveted to the "wrapper" or side plate of the combustion chamber. In the other type, shown in fig. 7, and known generally as the "Gourlay back end," the end of the furnace is contracted into an oval conical form, and is then flanged outwards round the whole of its circumference. The tube plate is made to extend to the bottom of the combustion chamber, and the furnace is riveted to the tube plate. The advantage of the Gourlay back end is that in case of accident to the furnace it can be removed from the boiler and be replaced by one of the same design without disturbing the end plates, which is not possible with the other design. The Gourlay back end, however, is not so stiff as the other, and more longitudinal stays are required in the boiler. [Illustration: FIG. 6.--Vertical Boiler with internal combustion chamber (the "Victoria," by Messrs Clarke, Chapman & Co.).] [Illustration: FIG. 7.--Single-ended Marine Boiler.] The flat sides and backs of the combustion chambers are stayed either to one another or to the shell of the boiler by numerous screw stays which are screwed through the two plates they connect, and which are nearly always fitted with nuts inside the combustion chambers. The tops of the chambers are usually stayed by strong girders resting upon the tube plates and chamber back plates. In a few cases, however, they are stayed by vertical stays attached to T bars riveted to the boiler shell. A few boilers are made in which the chamber tops are strengthened by heavy transverse girder plates. The end plates of the boiler in the steam space and below the combustion chambers are stayed by longitudinal stays passing through the whole length of the boiler and secured by double nuts at each end. The tube plates are strengthened by stay tubes screwed into them. Where natural or chimney draught is used the tubes are generally made 3 or 3-1/4 in. outside diameter and are rarely more than 7 ft. long, but where "forced" draught is employed they are usually made 2-1/2 in. diameter and 8 to 8-1/2 ft. long. A clear space of 1-1/4 in. between the tubes is almost always arranged for, irrespective of size of tubes. [Illustration: FIG. 8.--Double-ended Marine Boiler.] Stay tubes are screwed at both ends, the threads of the two ends being continuous so that they can be screwed into both tube plates; occasionally nuts are fitted to the front ends. The stay tubes are expanded into the plates and then beaded over. Locomotive.