on Steam-Boatis. 55. 
4 
You think that all cast-iron boilers are dangerous ?—Certainly, 
when used for steam of high expansive force. 
In your former answer, where you spoke of the extreme diffi- 
culty of so regulating high pressure engines as to insure their 
safety, did you mean to speak of those which had cast-iron 
boilers, or of both cast and wrought metal ones ?—Chiefly as to 
the cast iron; it is more practicable to make a boiler of the mal- 
leable metals to resist a high pressure, as far as the tenacity of 
the metals is concerned; but another difficulty occurs, which 
prevents the application of the malleable metals to boilers for 
high pressure engines, which is that of rendering the joining of 
the plates secure. 
Do you mean then to say, that wrought-iron boilers are not 
in frequent use to high pressure engines, in point of fact ?—I 
believe they are in much less frequent use than the cast-iron 
boilers; and in Woolf’s engine they are scarcely used at all. 
Is not the cast-iron boiler much cheaper than the wrought ?— 
1 can scarcely tell that; I should think the cast iron would be 
cheaper, if made of equal strength. 
In ease of the explosion of a cast- or a wrought-iron boiler, 
which is attended with the greatest danger ?—Cast iron, un- 
questionably. 
Why ?—From the frangible nature of the metal. 
What do you apprehend to be the common effect, in case of 
the explosion of a cast-iron boiler?—The metal is broken into 
fragments, and driven off with great violence.in various direc- 
tions. 
What is the effect when a wrought-iron boiler gives way ?— 
Generally a rent; but I have seen one instance of a wrought- 
iron boiler, where the whole of the upper part was rent from the 
bottom, driven, through the house in which it was placed, and 
carried to a considerable distance; I believe several yards. 
Do you apprehend, that speaking generally, and unless by 
some extraordinary circumstance, such as the wilful shutting of 
the steam-valve, there would be any reason to apprehend such 
an effect as you have just now mentioned, to arise from the rent- 
ing of a wrought-iron boiler ?--- No, I searcely think it possible. 
Supposing the boiler to be made of wrought iron, or coppér 
riveted, and safety-valves properly adjusted, with a mercurial 
gauge also adapted in its diameter with due regard to the size 
of the boiler, do you conceive that any reasonable apprehension 
could arise respecting the safety of a high pressure engine ?—I 
think there might; but with Jess apprehension as to the extent 
of the destructive effect to be produced. 
You speak of less apprehension as to the destructive’ effect ; 
have the goodness to explain that ?—On account that in ‘the 
D4’ malleable 
