stays; and explosion sooner or later has been the consequence. Such was an explosion that took place at Smethwick in 1862, which is shown in Fig. 19. As the force of the explosion was only slight, the effect of the bottom giving way, and the consequent rolling over caused by the reaction of the issuing steam and water, is clearly seen. Another example that occurred at Wednesbury in 1862 is shown in Fig. 20, where the explosion was rather more violent, the bottom of the boiler being torn off all round and left upon the firegrate, and rent nearly into two pieces; while the top and sides were thrown some height in one mass, and were only put out of shape by the fall. The weakness of this boiler had been further increased by making the bottom angle of angle iron, as shown enlarged in Fig. 21, with a ring of flat plate A interposed between the angle-iron ring and the concave bottom of the boiler; so that all the effect of the springing of the bottom, as shown by the dotted lines, was thrown upon the angle iron, which was accordingly found cut off all round. Had the concave bottom been made to rise direct from the angle iron, as in Fig. 22, the springing could not have been so great, and the angle iron would only have had to stand the shearing strain of retaining in its place the rigid bottom; but as about one foot all round the bottom was flat, and the concavity was only in the central part, the angle-iron ring had to bear an up and down strain, as shown by the dotted lines in Fig. 21, and the bending action was more severe than it would have been if the bottom had even been made quite flat all over.

Fig. 23.

A further form of the Balloon boiler is shown in Fig. 23, where the heating surface of the bottom is increased by an internal central dome-shaped fireplace, with an arched and curved flue conducting the flame through one revolution within the boiler before passing again round the outside. This construction however must necessarily have diminished the strength of the boiler greatly. In the drawing the top of the boiler, as indicated by the dotted lines, is removed to show the interior.

Fig. 24.

The desire to add to the strength of boilers by lessening the diameter of the shell led to the construction of the Plain Cylindrical boilers. They were made first with flat ends of cast iron, which frequently cracked and gave way when exposed to the fire, as described in many of the early American explosions. The flat ends when made of wrought iron, as shown in Fig. 24, are exposed to the same strain as the bottom of the balloon and wagon boilers, and are constantly springing with variation of pressure like drum heads, causing injury to the angle-iron joint. They also require long stays through them to hold in the ends, and these are subject to so much vibration that they seldom continue sound for long together, especially when joined with forked ends and cotters.

Fig. 25.

As the flat ends of such boilers are always being sprung by each alternation of pressure into a more or less spherical shape, as shown by the elastic model exhibited, this consideration no doubt led to the ends being made hemispherical, as shown in Fig. 25; and plain cylindrical boilers with these hemispherical ends are now so commonly used that they far outnumber any other form of boiler. Their shape renders them very strong, as the whole of the iron is in simple tension, and internal pressure has no tendency to alter the shape, as is shown by the elastic model exhibited. There is one circumstance very much in favour of the plain cylindrical boilers, and that is that they can be so easily cleaned and repaired, as a man can stand properly at his work at every part and the whole of the interior surface is exposed equally to view. They are of course exposed to all the evils of boilers externally fired, the part under greatest strain being weakened by the action of the fire; and the bottom is also exposed to injury from accumulation of mud and chips of scurf, which cannot be prevented from falling there, and lying upon the part exposed to the direct action of the fire. When made of great length, such as 70 or 80 feet, as is the practice for applying the waste gas from blast furnaces, these boilers are also liable to seam-rips or "broken backs," owing to the greater expansion of the bottom exposed to the fierce flame for its whole length, than of the top which is kept cooler by exposure to the air; and it would therefore be better to have a succession of short boilers, rather than only a single one, where great length is required.

Fig. 26.

One boiler has been seen by the writer where extreme length was avoided by curling the boiler round until the ends met forming a Ring or Annular boiler. This boiler is shown in Fig. 26, and is 5 feet diameter with 25 feet external diameter of the ring, or a mean length of about 63 feet; it has been found to work well for some years, although exposed to the heat of six puddling furnaces.

Fig. 27.

Explosions of plain cylindrical boilers have been very frequent indeed, although they have not caused a proportionate number of deaths, because they work usually in isolated positions at colliery and mine engines. The sketch shown in Fig. 27, represents an