on Steum-Boats.' 59 
What expansive force of steam is generally employed in those 
high pressure engines?—lI fancy that is very variable, from 
30 pounds to 120 upon the square inch, or even perhaps higher 
than that. 
Instances have been known in which a boiler bas been worked 
at 160 and 180; have there not ?>—TI have heard of such things, 
but I never knew of them. 
What is the proof to which high pressure boilers are generally 
exposed previous to their being used ?—The most eligible. proof 
they ought to be exposed to is by water. 
To what pressure ?>—J should think double the pressure to 
which they are intended afterwards to be subjected. 
What is the estimated force in your opinion, which would 
burst a high pressure boiler of the best construction ?——That is 
very different, because it depends upon the strength and con- 
struction of the materials; I never entered into the calculation, , 
Have not the greatest advantages been derived to the mines 
and manufactures from the use of the high pressure boilers ?— 
I believe inestimable advantages. 
Have you any doubt that Cornwall has derived an advantage 
which may be considered as incalculable from them ?—None in 
the world. 
Do you think, from the few accidents which have occurred in 
the use of gies; there is any better argument to be brought 
against the permitting them to be employed, than could be de- 
rived from the accidents which have arisen in the explosion-of 
gunpowder in the clearing passages in the mines i ib not as 
applied to the mines, certainly. 
You have mentioued that security is afforded to the engine by 
feeding the boiler by a column of water; from what does that 
security arise ?—The pressure from the steam in the boiler could 
never rise to a force greater than that which would be equal to 
the pressure of the column of water; whenever it did arrive at 
that pressure, or beyond that pressure, the water would be blown 
out and the steam would follow. 
You have stated, that in the steam-vessels used upon rivers, 
this precaution is not resorted to?—TI never saw one, and it 
would be extremely inconvenient. 
For what reason ?—On account of the undulations the water 
would be subject to; it would be thrown out of the pipe from 
the motion of the vessel; and other inconveniences would arise, 
‘such as bringing the pipe through the deck of the vessel. 
Do you apprehend that a mercurial gauge would be exposed 
‘to the same inconveniences?—Certainly, I do; the altitude 
would be lessened by every new assumed position of the vessel ; 
that is, if a tube placed vertically at first, should by the action 
of 
