78 
Steam Engine. 
be preserved ; for to have the vacuum as perfect as pos^ 
sible, it is necessary that the cylinder be kept up to such 
a temperature as to prevent the least condensation on the 
internal surface either above or below the piston : because, 
if the sides of the cylinder were to be wet, as in the com- 
mon atmospherical engine, the vacuum would be vitiated, 
as it is there occasioned by this wetness or moisture gra- 
dually forming to steam, which the outside casing pre- 
vents, being filled with steam from the boiler. But if it 
were possible to cover this outward case with any sort of 
substance which would entirely prevent the transmission 
of lieat for that casings it would supercede the use of the 
easing altogether, and would apply with more advantage to 
the cylinder itself. But we do not know of any substance 
which will not admit this transmission more or less. They 
who wish for information on this subject may find it in 
eount Rumford’s Essays. 
The first circumstance of importance to the pro- 
portion and disposition of the several parts, is the soli- 
dity of the vessels and the perfection of the joints. Cop- 
per tubes are apt to be unsound at the seams, and 
other parts which are required to be bent out of a right 
line ; and iron castings, which require any particular sort of 
stay in the moulding to keep the core from the outside, as 
these stays are made mostly of wrought iron, they con- 
tract more in cooling than the cast-iron does about them, 
even so as to become loose sometimes ; in such cases, it 
is unutterably perplexing to find out the places or cause 
of this defection; the joints are suspected for the most 
part, but even remaking them sometimes proves no 
amendment ; and this must be the cause in general why 
one engine from the same patterns is better or worse than 
Others. And we have reason to fear that this matter of 
complaint is on the increase ; for self-interest has so pow- 
erful a preponderancy, especially in tlie metropolis, that 
