Single-furnace boilers have been much used in the form of a single-tube boiler standing on end, as shown in Fig. 47, with the flame passing up the tube, which is continued in the form of a chimney on the top of the boiler. The tube passes through the steam at the top, so that the plate is not protected from overheating by contact with water; and this has caused explosion in some instances, although the tube has been lined on the inside with firebrick to shield the plate from the flame. Another great disadvantage of this Chimney boiler is that the space between the tube and the shell is so narrow that it is almost impossible to examine or clean it internally. * * * * * A further arrangement for a single-furnace boiler is the Elbow boiler, shown in Fig. 48, where the two difficulties mentioned in the previous boiler are avoided. * * * * * Many internally fired upright boilers of various shapes have been constructed to suit various purposes. One of a large size that has been at work many years is shown in Fig. 49, with an internal fireplace and a suspended cone and cross tube for increasing the heating surface. This boiler is set in brickwork in such a way that the heat passes through the side tubes and round the exterior shell before going off to the chimney. [Illustration: _Fig. 48._] [Illustration: _Fig. 49._] A very fatal explosion at Stoke-upon-Trent, in 1863, resulted from an attempt to work a boiler of somewhat the same general form, but without the same careful attention to the details of construction. This boiler is shown in Fig. 50; the internal fireplace is of conical shape, 4 ft. 6 ins. diameter on the top and 6 ft. 10 ins. at bottom, and was joined to the external shell by a flat annular bottom. Almost the first time it was worked at high pressure the conical fireplace collapsed, breaking off at the seam at the top of the cone, and blowing down upon the grate, as shown in Fig. 51. The flat bottom was then left without the support of the cone and side tubes, and gave way all round the outside angle iron; and the top flew up a great height into the air, and fell a crumpled heap, as shown in the sketch. In this case the only wonder is that a boiler of such weak construction worked at all without explosion. [Illustration: _Fig. 50._] [Illustration: _Fig. 51._] * * * * *