154 Mr Stevenson on the Explosion of a Steam-Boiler. 
state of the end of the boiler next the feeding-pipe. This is also 
rendered likely, from the body of one of the unfortunate suf- 
ferers having been found near the position of this pipe after 
the accident, who is supposed to have been in the act of turn- 
ing the cock when the explosion took place. It is, indeed, 
hardly possible upon any other hypothesis, to account for the 
production of a pressure of upwards of 200 pounds to the 
square inch, required for tearing the boiler asunder, and pro- 
jecting about two-thirds of it into the atmosphere to the height 
of 70 feet. 
Still it must be allowed, that a considerable degree of uncertainty 
hangs over the cause of this explosion. If the valves were not 
strangely mismanaged, how is it possible that the steam could be 
raised gradually to such a pressure as was necessary to produce 
effects equal to the most violent results from the explosion 
of gunpowder ? On the other hand, if we suppose that the end 
of the boiler next to the pipe for supplying it with water had 
got into a state of incandescence, and that the fireman had sud- 
denly let a quantity of water into it, we can thus account for 
the extrication of gases capable of producing all the phenome- 
na which followed. The boiler, in the first or experimental trials, 
had vibrated in its place with a pressure of 60 pounds to the 
square inch, as before noticed ; but the valves were now loaded 
only with 40 pounds to the square inch. The probability there- 
fore is, that at the moment of the explosion, the weakest part 
of the boiler had given way, before the safety-valves, compara- 
tively small, when considered in reference to a boiler 37 feet 
in length, could operate for its relief, from the sudden and im- 
mense pressure applied. The safety-valves, indeed, appear 
to have performed their office, though they were inadequate to 
the intended purpose. One of them was thrown out to a great 
height, and described a large arch, in a direction somewhat dif- 
ferent from that of the great mass of the boiler : this iron-valve, 
in its fail, passed through the roof of a distant house, one of 
the inmates of which narrowly escaped being killed. 
Edinburgh, ) 
June 1. 1821. J 
