on Steam- Boats. 93 
gulated by the height of that tube to admit of the efflux of the 
steam from the boiler as fast as it was generated by the fire, in 
that case the expansive force could not increase in the boiler, but 
the mercury would be blown out and the steam would escape: 
that [ consider one of the best securities to the boiler. Besides 
the common safety-valve, which may be at the discretion of the 
workman, I conceive it essential to have another safety-valve, 
which is under the control of the master or proprietor of the 
works. There is another small contrivance, which I consider 
very important to the safety of the boiler. Boilers have been 
weakened very much by the water having been evaporated tuo 
low, so that the bottom begins to be acted upon by the fire and 
weakened. A hole having been previously bored in the bottom 
where the fire acts, may be riveted by a piece of lead, so that 
that lead remains perfectly secure as long as it is cowered with 
water, but the moment the water leaves it the lead melts; the 
steam is blown out through the hole and puts out the fire; be- 
sides giving the signal of what is wanted, it at once puts an end 
to the cause of danger. 
Do you consider that the mercurial gauge acts in any other 
manner than as a safety-valve, which cannot be stopped or put 
out of order ?—It does not act only in that manner, but it has 
the advantage of exhibiting during all times of the boiler’s work- 
ing, the state of the steam within the boiler, by the fluctuation 
that takes place in that coiumn, as indicated by the index upon 
the surface of the mercury, and the state of that mercurial gauge 
is observable every moment. If the mercury becomes stationary, 
one would strongly suspect that that tube was stopped, therefore 
it would point out itself instantly that it had become not what it 
ought to be; the safety-valve has not that advantage, as it does 
not indicate any thing ‘till the steam is blown out by raising the 
weight. 
An observation of the mercurial gauge by an intelligent per- 
son, would tend to guard against mischief 7+Yes, by any per- 
son. 
What are the different effects produced by the explosion of 
cast and wrought iron ?—<As far as I have stated the fact with 
respect to Trevethick’s boiler, which was of wrought iron, a rent 
or fissure was produced, and the form of the boiler was disfigured, 
but no fragments were thrown about so as to produce any serious 
injury. 
Do you conceive that to be the usual effect ?—I conceive it 
would be the effect ; and I conceive further, that one might pre- 
dict with some degree of certainty where that fissure would take 
place ; it would take place in that part of the boiler that:is most 
exposed to the action of the fire, that growing thinnest. 
Ilave 
