Steam Engine. 77 
ensure its tightness : but here the air is discharged at first 
setting the engine to work ; and this valve is called the 
blowing valve. When the cylinder and other vessels are 
properly heated and the air discharged, which is well 
known by a very smart crackling noise at that valve, like 
a violent decrepitation of salt in a fire these valves /r 
and i are to be shut ; and after waiting a few seconds, 
gently open the valve i .• and if the engine does not move, 
the injection valve ^ must be opened a little y\ and if the 
engine does not move, then the operation of blowing must 
be performed again, though but for a few seconds, and 
the engine in general will go off smartly. 
In all engines on this principle, it is necessary that the 
parts appropriated to condensation of the steam, should be 
kept as cold as possible, and that those parts intended for 
the operation or passage of the steam be kept as liot as 
possible : hence the discharging pump and condenser are 
placed in a cistern of cold water kept constantly full, and 
a little running away ; and if the injectioru valve is placed 
low in this cistern, it will take the water in the coldest 
state. 
As the condenser is immersed in water to be kept cold, 
so the cylinder should if possible be immersed in steam 
to be kept hot: for which purpose Mr. Watt formerly 
used a casing round the cylinder, and at the top and bot» 
tom ; and this would have been attended with very bene- 
ficial effects if it did not enlarge the steam surface, and 
expose it to a more rapid condensation when it ought to 
* This noise is occasioned by the air being all gone, and the 
water producing a sudden and rapid condensation of the steam. 
t The valve i being opened? there is a passage made from the 
cylinder to the condenser ; but on account of long blowing, the 
sides of the condenser become hot, and the water in the cistern 
hot likewise % so that the condensation must needs be very slow, 
even at the first injection of the water into the condenser. 
