on Steam- Boats. 333 
than you can of wrought iron ?—TI can make it stronger and — 
more to be depended on for great pressure ; but where great 
pressure is not wanted, wrought iron can be made sufficiently 
strong to depend on. 
Supposing an accident happened, would not a wrought-iron 
boiler be attended with as great mischief as a cast-iron one ?— 
As great a number of accidents happen from the common boilers 
or wrought-iron boilers, as from the cast ones. 
Mr. ANpREW ViviaAN’s Evidence. 
What is your profession ?—Miner and engineer. 
Where do you reside ?—At Cambourne, in Cornwall. 
Have you been long acquainted with the construction of steam- 
engines ?—For thirty years and upwards. 
You are then capable of giving an opinion as to those cireum- 
stances by which danger is occasioned in the working them, and 
the means of preventing it ?—I am. . 
Be pleased to state them ?—The danger arises from making 
the steam-vessel of insufficient strength for the steam; every 
engineer ought to be well acquainted with the power of the steam, 
and make the steam-vessels in proportion to the strength of the 
steam required. 
What precautions do you use to prevent explosion ?—Safety- 
yalves ; not less than two on every boiler where a high pressure 
of steam is required, aud that the boilers be made of sufficient 
strength, and proved before used. 
To what proof are those boilers subjected, or to what proof 
ought they to be subjected >—By filling them with water, and 
loading the safety-valves with, perhaps, ten times the weight 
required for the engine, and then by injecting water into them, 
so as to lift those valves with ten times the weight required. 
When you say ten times, do you mean exactly ten times ?-— 
Perhaps, ten or twelve times the weight it is intended to work. 
You conceive that a boiler which has been so proved and fur- 
nished with safety-valves, properly adjusted to its contents, to be 
perfectly safe in working with steam, whether high or low pres- 
gure ?—Yes, I do. 
Is there any difficulty in so adjusting the apertures or valves ; 
that is, in calculating of what size the yalves ought to be, to pro- 
duce safety in a boiler of any given magnitude ?—No, no diffi- 
eulty at all; itisa plain and well-known thing to all engineers, 
or to every one who ought to pretend to be an engineer. 
Is it usual to work a high pressure engine at all near to that 
point to which the valves are thus adjusted ?—We load the en- 
gines in the mines under my directions, to about forty pounds 
en inch; aud the valves are then loaded to about forty-five, per- 
haps. ut 
