330 Report of the Select Committee 
its upper side, which is inclosed within the iron case, and may he 
drawn by the engine-man or any person on board, and which 
does not allow him to keep it down or to confine it. We have 
also found it necessary to prevent the accumulation of water 
upon the top of this valve, arising from the condensed steam | 
when escaping; this is done by a small waste-pipe descending 
from the bottom of the pipe which conveys away the waste 
steam; it is a waste-pipe for water. I have thought it advisa- 
ble to make the steam-valves large, and that the weight which 
is laid on being of itself large may admit easily of addition. I 
have one or two more precautions to suggest for safety: In this 
vessel there are two boilers communicating, and two safety- 
valves; there is also a mercurial gauge provided with receivers, 
so as to prevent the loss of the mercury in case of any sudden 
collapsation or disengagement of steam, also a tube of glass at- 
tached to the boiler, which exhibits the level of the water in the 
boiler, and precludes any idea of danger in the minds of the 
passengers ; these boilers are made of wrought iron, but I do 
not consider them as being better on that account. 
Do you think equal mischief is likely to arise from the explo- 
sion of the wrought-iron boiler, as from the explosion of the 
cast-iron boiler ?—That depends upon construction. 
Put construction entirely out of the question; suppose the 
form exactly similar, do you conceive that equal mischief is 
likely to attend the explosion of the wrought-iron boiler, as the 
cast-iron boiler ?—If the construction of the cast-iron boiler ad- 
mits of its being made of wrought iron with equal strength, then 
_the explosion of the cast iron one would be more dangerous, as 
it will fly in pieces, whereas the other would probably tear; but 
it is scarcely fair to stop at this hypothetical case, as we must 
consider what can be done in practice. It is scarcely possible 
to form cast iron every where equally strong, and if a part be 
weaker than the rest, either on purpose or by accident, that will 
not have the safety that would be obtained by a wrought-iron 
boiler ; for instance, in cast-iron boilers it is common to have 
holes, and if these be filled with some metal of different melting 
temperature from cast-iron, more fusible for instance than that, 
the juncture will part first, and it may be made to tear as a 
wrought-iron boiler would do; and again, the wrought iron is 
so much more liable to oxidation than cast iron, that although 
found very efficient at first, its strength and tenacity may be very 
speedily altered ; for these reasons cast-iron boilers have been 
preferred where high pressure engines have been used’, and in 
small tubes the tenacity of cast iron can be made greatly to ex- 
ceed that which can be given to wrought iron in the same form. 
[believe all large boilers have latterly been made of wrought 
iron, 
