on Steam-Boats. 99 
gine boilers of cast iron, arises from allowing them to get cool 
too suddenly, and raising the steam too suddenly, the metalcon- 
tracts and expands at a period when we cannot investigate its 
eccurrence. 
Wrought iron would not be attended with that danger ?—Not 
to the same extent; the rivets would go. 
But not with the same degree of explosion ?—No. 
Would it not be a great safeguard in the construction of a boiler, 
if a safety-valve was so made as to be put out of the power of the. 
engineer to get at it?—No doubt, it ought in all cases to be so. 
It could be so constructed ?—No doubt ; if the pressure, how- 
ever, is greater than what the safety-valve is intended to relieve, 
there might be an accident from the causes which I previously 
stated; that is, that a boiler might be defective without its being 
known. rdeya 
You apply that to cast-iron boilers ?—Yes, and in a small de- 
gree to wrought-iron boilers. 
’ Do you conceive it impossible, or even difficult, to construct 
a wrought-metal boiler, with safety-valves properly adjusted to 
its capacity, and a mercurial gauge, supposing that to be capa-. 
ble of being applied, which should render a high pressure en- 
gine on board a steam-boat what might be called perfectly 
safe ?—No, I do not think it impossible; and I hope some time 
or other it will be accomplished. 
Wherein do you apprehend that the difficulty of so construct- 
ing a boiler would consist ?—Because I have found that difficulty 
in making boilers myself steam-tight, even for condensing-en- 
gines, where the plates were of a thickness fit to undergo high 
pressure. ; 
Do you apprehend that any danger of a fatal accident could 
arise from that mere want of tightness in the riveting, which 
would permit some steam to escape ?—That danger would de- 
pend upon the degree of the pressure, and the extent of the 
aperture through which the steam escaped. ) 
What is the species of danger which you would expect to oc- 
cur in such a case ?—I am not able to answer the extent of it. 
If the safety-valves acted, of course the danger would be re- 
moved; supposing that the safety-valves are properly constructed, 
and their operation is secured, the danger would not be extremely 
great ; it is only from their defect of action that the danger is to 
be apprehended. : 
Then do you mean to say, that if the valves were really in 
point of fact performing their functions properly, in that event 
you would not consider there was any danger ?—Certainly not, 
if the boiler was adequate to the pressure. : 
What is the pressure per inch which you conceive to be ge- 
' tf anG! 2 nerally 
