110 
Steam Engine, 
is let in to fill the cylinder for the next stroke ; and that 
steam will be condensed into water as fast as it enters, un- 
til all the matter it comes in contact with, is nearly as hot 
as itself. 
Every attempt to make the vacuum more perfect by the 
addition of injection- water, will cool the cylinder more ef- 
fectually, and cause a greater destruction of steam in the 
next filling ; and if the engine has already a proper load, 
the destruction of steam will proceed in a greater ratio 
than the increase of power by the amendment of the va- 
cuum. 
Though it appears that the constructors of steam-en- 
gines have never investigated these causes, yet they have 
been so sensible of the effects, that a judicious engineer 
does not attempt to load his engine with a column of water 
heavier than seven pounds for each square inch of the area 
of the piston. 
Mr. Watt’s improvements arc founded upon these, and 
some other collateral observations. He preserves an uni- 
form heat in the cylinder of his engines, by suffering no 
cold water to touch it ; and by protecting it from the air 
or other cold bodies, by a surrounding case filled with i 
the steam, or with hot air or water, and by coating it over ‘ 
with substances that transmit heat slowly. He makes his i 
vacuum to approach nearly to that of the barometer, by 
condensing the steam in a separate vessel, called the con- 
denser ; which may be cooled at pleasure without cooling ; 
the cylinder, either by injection of cold water, or by sur- 
rounding the condenser with it ; and generally by both. 
He extracts the injection- water and detached air from the 
cylinder or condenser, by pumps, which are wrought by 
the engine itself ; or he blows it out by the steam. 
As the inside of the cylinder was in the old engine ex- 
posed to the air at every stroke when the piston descended, ' 
and was considerably cooled thereby, he incloses the top 
