on Steam- Boats. 53 
opened, and the steam is allowed to escape into the atmosphere, 
and the operation is reversed; the piston of the cylinder is made 
to ascend by the same kind of force. In the condensing engine, 
or the low pressure engines, the steam having been once per- 
mitted to fill the cylinder, a communication is then made be- 
tween the cylinder in which the piston works and the vessel in 
which the steam is condensed :—that is the distinguishing feature 
of the two engines. I will describe a further difference, which 
contingently arises out of the use of the two: that is, in the high 
pressure engines the engineer has it at his option to use what 
degree of expansive force he pleases, to convert an engine 
adapted for the power of five horses, or producing the power of 
five horses, to that of ten horses, or to any other extent which 
he may think his materials capable of sustaining. In the low 
pressure or condensing engines, the steam can never be advan- 
tangeously employed above from two and a half to six pounds 
upon a square inch, 
Whatever power there is in what you call a high pressure en- 
gine, the pressure in that engine may increase the power beyond 
what it is calculated for, and by means of that may render it 
danger ous ?—Certainly. 
Is it your opinion, that a boiler could be made of proper ma- 
terials, with safety-valves, and under proper guard and direction, 
to make that high pressure perfectly safe?—-That would depend 
upon the quantity of pressure to be used ; a safety-valve might 
be carried to three hundred, or to any assignable force. I think 
that a high pressure engine may be made safe to a certain ex- 
tent, but where they are left ad libitum they never can be per- 
fectly safe. 
Do you mean to convey the idea, that it is impossible or diffi- 
cult to adapt to a high pressure engine one or two safety-valves 
joined with a mercurial gauge, acting at the same point of pres- 
sure, so as to make it equally safe with that upon any other con- 
struction ?—In answer to the first part of the question, relative 
to the safety-valve, I think I have answered that already; that 
we can apply a safety-valve to any degree of pressure without 
any difficulty, but that the safety of the engine does not tptally 
depend upon the safety-valve. 
State upon what other circumstances the safety of the engine 
depends ?—My idea of the difficulty of obtaining a proper de- 
gree of strength at all times in the materials of which boilers 
may be made, arises from the constant deterioration which those 
boilers must be suffering from the action of the fire, and from 
the various degrees of expansion and contraction operating on 
different parts of the-boiler, 
Is it then your opinion, that in high pressure engines carried 
D: to 
