especially when exposed to the action of the fire; and that the effect of explosion was very disastrous, because the boiler burst at once into many pieces, each of which was driven out with great velocity, and the danger was not mitigated by the circumstance of large masses holding together, as is found to be the case with wrought-iron boilers when exploded.

Fig. 11.

Fig. 12.

Fig. 13.

Fig. 14.

When wrought-iron boilers came into use the shapes were most varied, and the dimensions much larger than before. One of the earliest was the Wagon boiler, shown in Fig. 11., with round top and plain flat sides, which could only be made to bear even the smallest pressure by being strengthened with numerous stays. In most cases of explosion of this class of boiler the bottom was torn off, owing to the angle iron round it being weakened by the alternate bending backwards and forwards under each variation of pressure, as all the sides and the bottom must be constantly springing when at work. Such was the explosion at Chester in 1822, and many others. This shape was soon improved in its steam generating powers by making the sides concave instead of flat, as shown in Fig. 12., so that the heating surface was greater and also in a better position to receive the heat from the flame in the flues. This shape was further elaborated by rounding the ends as in Fig. 13., and in some cases making the bottom convex to correspond with the top, as in Fig. 14. All these forms however still required numerous stays to retain them in shape, the safety of the boiler being dependent upon the stays; and numerous explosions show the weakness of these boilers. They generally gave way at the bottom, as in an explosion that occurred at Manchester in 1842, where the boiler had been weakened by frequent patching; they also sometimes exploded through the failure of the stays.

Fig. 15.

Fig. 16.

Fig. 17.

Fig. 18.

A very early improvement in the right direction consisted in making the shell circular; and some few large boilers still exist that were made completely spherical, as show in Fig. 15., so that the whole of the iron was exposed to tension only, and required no assistance from stays, and the boiler had no tendency to alter its shape under varying pressure. This shape however had the great disadvantage of possessing the least amount of heating surface for its size or cubic contents; and also it was very liable to injury from sediment on the bottom, which accumulated on the most central spot. The spherical form was therefore soon modified into the shape shown in Fig. 16, by making the bottom more shallow, although still convex; and afterwards by putting flat or concave sides and a flat or concave bottom, with the angle constructed either of bent plates or angle iron, as in Fig. 17 and Fig. 18, which represent the forms known so well in the Staffordshire district as the common Balloon or Haystack boiler. Many of these have been made of very great size, measuring as much as 20 feet diameter, and containing so much water and steam as to be most formidable magazines for explosion. Perhaps no form of boiler has exploded more than this, partly because of the great number that have been used, but chiefly because of the inherent weakness of the shape. The records however have not been obtained of the great majority of these explosions, because they seldom caused sufficient damage or loss of life to attract much attention, as these boilers generally worked in isolated positions at collieries. The bottom is only prevented from blowing down into the fireplace by numerous stays from the top, and the angle iron round the bottom of the sides is much tried by the constant springing of the plates under every alternation of pressure; and the weakness thus occasioned is increased by the angle resting on the brickwork and being exposed to corrosion. The effect of this continued alternation of strain is well shown by the elastic model exhibited.

Fig. 19.

Fig. 20.

Fig. 21.

Fig. 22.

Notwithstanding the dependence of these boilers upon stays for their strength, many have been made as large as 12 and 15 feet diameter without