on Steam- Boats, 329 
zpproaches to a sphere is the strongest, but a cylinder with semi- 
circular ends is best; I mean hemispherical ends. 
Is it not very possible to burst a low pressure engine, if the 
engine-man is careless, or rash enough wholly to negleet his 
steam-valve ?—Certainly ; I think that they are equally hable 
to burst, only the one bursts with greater danger and risk than 
the other. 
Mr. ALexanpEr Nimmo’s Evidence. 
What are you?—A civil engineer, and generally residing in 
Dubiin. 
Have vou any experience of the construction of steam-engines 
for packets or passage vessels ?—I have seen the steam-vessels 
in the Clyde, on the Thames, and vessels in Ireland, and those 
vessels lately constructed for passage between Dublin and Holy- 
head; and I have studied the subject with a good deal of care, 
in a professional point of view. I have lately been employed by 
the Dublin Steam Packet Company, to alter one of their vessels, 
which was not found completely fitted for crossing the sea; { 
have altered that vessel, and she is now plying in the Bristol 
Channel preparatory to going to Ireland. 
Have you, in consequence of your experience, any suggestions 
to make as to the safe construction of the engine boiler con- 
nected with such packets ?—A great part of the alterations that 
I made upon this’ vessel, were intended to fit her for going 
through the waves, and to alter her machinery; and another 
portion of them was likewise directed to make her safe as to the 
engine. You are aware that it is necessary for all engines of 
that description to have safety-valves. The defects of the safety- 
valve which I altered, were, that it is not now in the power of 
the engine-man to keep it shut; it is in his power, or that of 
any passenger, to open it, however, at all times so as to discover 
whether it be in good order, by a small chain and a weight being 
within the boiler: it is not in his power to add to it while in action: 
and lastly, this vessel being intended to go to sea, and to work 
_ as she has done, in very rough weather, the safety-valve is made 
equally effectual in everv position of the ship, whether she heel, 
pitch, or roll. The weight preserves the valve in motion, so as 
to keep it from sticking, and it has always the same effort to 
overcome. I will thus describe the nature of the valve: It is a 
hemispherical cup with its convex surface downwards, resting 
upon a collar, and to the bottom of the cup a weight is hung 
which has previously been adjusted ; by this means the valve is 
always steam-tight in every position, yet without danger of ad- 
hering, and must be lifted by the steam when it exceeds a given 
pressure; but the valve may also be lifted by a chain attached to 
its 
