Several instances have occurred of explosion of Portable Crane Boilers. Their small size has led to their condition being disregarded, under the idea that scarcely any pressure could burst them. In practice it is found however that they are often exposed to greater pressure than other boilers, because the fire is large and quick in proportion to their size; and they often have to stand for a considerable time with the steam up, and their exposed position and long intervals of rest add to the chances of corrosion, as shown by the example in Fig. 28, No. 14, 1869. The large manholes without strengthening rings, that are so often put in these boilers, have been the cause of explosions such as that shown in Fig 29, No. 57, 1866. [Illustration: _Fig. 31._] The same remark applies to some of the portable or agricultural boilers which have exploded, such as those shown in Figs. 30 and 31, No. 43, 1868, and No. 12, 1869. Much mischief is often caused by bad imitation of well planned boilers. Thus in boilers of the Cornish form, the ends are made sometimes so rigid as to give no allowance for the expansion of the tube, and the result is such continued strain as to cause constant leaking and the consequent risk of fracture. In furnace boilers the tops of the crowns of the inside tubes are often made flat, as in Fig. 37, instead of being domed; or the inside tube is of undue size, as in Fig. 21, No. 23, 1870, see page 73. Furnace boilers have been made with the omission of the stays that are so peculiarly necessary in that form, whereby both ends have been left free to bulge outwards with the pressure, as in Fig. 32. [Illustration: _Fig. 37._] [Illustration: _Fig. 32._] [Illustration: _Fig. 33._] [Illustration: _Fig. 34._] [Illustration: _Fig. 35._] Cornish boilers are often altered to the plain cylindrical form, without compensation being made for the loss of strength caused by the removal of the tube; this has led to such explosions as shown in Fig. 33, No. 47, 1869, where two tubes where taken out, and Fig. 34, No. 42, 1867, where one tube was taken out. One of the most frequent and serious causes of loss of strength is the repairing of externally fired boilers. Not only are the patches sometimes only bolted on in a temporary manner, as in Fig. 35, No. 29, 1869, but even where they are rivetted on there is an entire want of bond or crossed joint, as in the case of the exploded boilers shown in Figs. 15, 16, and 17, see page 70 and 71, No. 10, 1869, and Fig. 36. [Illustration: _Fig. 36._] [Illustration: _Fig. 38._]