on Steam- Boats. 37 
You have never heard of low pressure boilers blowing up ?— 
No; I have never known of any, explosion with injurious con- 
sequences ; they give way repeatedly; but do no injury. 
Is there any thing in low pressure boilers which may be de- 
pended upon absolutely, for preventing the steam sn an 
expansive force beyond what is intended >—Well regulated salety- 
valves; mercurial gauges or water gauges will at all times se- 
cure it. 
Do those means of limiting the expansive force in low pressure 
boilers continue perfectly efficacious under all circumstances of 
misconstruction and mismanagement ?—That entirely depends 
upon the construction ; I have known safety-valves fail in their 
action from bad construction. 
Can such or similar means be applied to high pressure boilers? 
Yes, certainly. 
Is not the feeding of low pressure boilers usually done by a 
column of water; and is not this column the great reason of their 
safety?—~That is one reason, but they ought to have a safety- 
valve besides. 
Is it not the principal reason of their safety?—It is the most 
secure one. 
If the feeding column of water be taken away, is not the se- 
curity left to depend upon the safety- valve? Unquestionably. 
Are low pressure boilers employed in boats always or com- 
monly fed by a column of water ?—I never saw an instanee of it. 
If the mechanical means which are used to render the low 
pressure boilers secure succeed, will not similar means render 
high pressure boilers secure?—As far as the expansive force is 
not permitted to arrive beyond ¢ertain limits, so far it will af- 
ford security. 
At what expansive force are low pressure boilers safe aeoord- 
ing to their usual construction ?—I have seen very few boilers 
constructed for the purpose of a low pressure engine, or a con- 
densing engine that would sustain a pressure of ten pounds, with- 
out occasioning considerable leakage, or without forcing the 
joints. 
Are they not very often used with a force to render them un- 
safe?—I never knew an instance of it. 
Is not the explosion of them likely to do mischief ?—Not un- 
der the pressure they are capable of sustaining. 
Not even if they are made of cast iron ?—Certainly. 
Are they uniformly made of wrought iron?—No; several of 
them are made of cast iron. 
Are not the greater number of them made of cast iron ?>—No ; 
J pprehend not, 
You’ 
