on Steam-Boats. 251 
Mr. Joun Sreew’s Evidence. 
Where do you reside ?—At Dartford. 
What is your profession >—An engineer. 
Are you acquainted with the construction of steam boilers ?>— 
Perfectly so. 
Will you give your opinion as to the comparative merits of 
wrought and cast iron?—I cannot conceive as to the safety of 
the two, that there is any differeuce whatever, when the steam 
is used, as it generally is for high pressure engines, to forty 
pounds to the inch. If it was required to make the strongest 
boiler imaginable, J should consider cast iron preferable, because 
there you can get to an unlimited strength of resistance; wrought 
iron you can only have of a certain thickness. 
Are you of opinion, that a boiler can be made of cast metal, 
free from all imperfections in the substance of the metal itself? 
—No; I do not imagine that it can exactly, but at the same 
time it ean be ascertained whether it is so or not before it is 
used. 
Do you mean to say by that, that you can by any pressure say 
that it is free from imperfections; or do you mean to state, that 
it will only sustain the pressure that it is calculated for ?—When 
boilers are proved, they are generally proved to four or five or 
six or eight times the pressure intended to be put on them. 
But still, though they bear that pressure, they might have 
those imperfections ?—Certainly; but without those imperfec- 
tions, they would sustain, perhaps, fifty times what is wanted. 
Are you then of opinion, that the proof arising from the pres- 
sure of cold water, is sufficient to ascertain the safety of a boiler 
which shall afterwards be exposed to the operation of fire or of 
highly heated steam ?—Perfectly so; because I imagine it is a 
great deal stronger when heated to the extent steam will heat i it; 
cast or wrought 1 iron is at its greatest strength when it is at 300 
degrees of heat, which | believe has never been arrived at yet by 
steam. 
Supposing the interior of the cast iron to contain cavities, by 
which the thickness of the extenal coat is very much diminished 
in those parts, and that those parts shall be afterwards exposed 
to the action of the fire, do you apprehend then, that the ap- 
parent thickness of the boiler would be any sufficient saints 
No; by no means, 
Have the boilers which you have been accustomed to use been 
furnished with two safety-valves or one only?—T wo, universally, 
Has either of those been locked up from the engineer ?>—The 
sometimes have and sometimes they have not; I should imagine 
two- 
