care, no confidence can be placed in a boiler which has worked so long, unless it is examined in every part. [Illustration: _Fig. 4._] [Illustration: _Fig. 5._] [Illustration: _Fig. 6._] [Illustration: _Fig. 7._] The opinion is more general than many are aware of, that explosions as a rule are caused by shortness of water and the sudden turning on of the feed water upon red-hot plates; and the appearances of injury in the plates from fire, arising in the ordinary course of working, have been frequently mistaken for signs of overheating from shortness of water at the time of explosion, as illustrated in Figs. 6 and 7, No. 24, 1867, and No. 59, 1866. Although boilers do explode from the softening of the plates by overheating in consequence of shortness of water, yet it is very doubtful whether the turning on of the cold water at such a time is ever the cause of explosion. The feed water being always introduced at the bottom of the boiler, as in Figs. 8, 9, and 10, cannot be scattered suddenly near the overheated parts, but must rise gradually up the sides; and the boiler would have gone to pieces from the giving way of the softened parts long before the water reached them, as was the case in the explosions shown in Figs. 11 and 12, end of 1868. The experiment of injecting cold water into red-hot boilers has been carefully tried more than once, without producing any explosion. [Illustration: _Fig. 8._] [Illustration: _Fig. 9._] [Illustration: _Fig. 10._] Although it may be too much to suppose that boiler explosions will ever be entirely prevented, it is important that those who have the care of boilers should understand better what are the true causes of explosion, in order that they may know what to guard against in addition to shortness of water. This better understanding of the subject has been much retarded by the supposition that the causes of boiler explosions are beyond the comprehension of the boiler minders; and still more by the important differences of opinion among those under whom they work. Much evil has resulted from the promulgation of strongly expressed views, which have been founded upon facts but of too limited extent, and such as must become modified by consideration of the facts of a large number and variety of explosions. Mysterious theories to account for explosions have been resorted to only from want of clearer explanations. [Illustration: _Fig. 11._] [Illustration: _Fig. 12._] [Illustration: _Fig. 13._] [Illustration: _Fig. 14._]