192 PRACTICAL STATIONARY ENGINEERING
of the air pump. A perfect vacuum cannot exist, and in
the condenser there is always more or less pressure from im-
perfect condensation and air passing in with the condensing
water.
To produce a vacuum in a surface condenser, open the
injection valve shortly before starting the engine, so that
the circulating water may enter the condenser tubes and
cool them. Then, when the engine is started, the exhaust
steam comes in contact with the cooling surface of the tubes,
and is condensed, then a vacuum is formed.
How is vacuum measured?
The vacuum is measured by inches in the height of a col-
umn of mercury, 2 inches of mercury equalling 1 pound
pressure per square inch. Thus 10 inches of mercury
means 10 pounds' pressure per square inch. The state of the
vacuum is shown by the vacuum gauge attached to the con-
denser; and, if it be imperfect, the cause must be ascer-
tained and the fault corrected. If the water in the hot
well is above the ordinary temperature, more injection water
must be admitted; and, if the vacuum continues imper-
fect, the case may be due to an air leak, the location of which
the engineer must endeavor to discover. Very often the
fault will be found in the joint of the injection pipe, the
gland of which will require to be tightened. The joints of
the condenser may be tested by holding a candle to them,
the flame of which will be drawn in if the joints are leaky.
Is vacuum power?
A vacuum is not power, as all power in the steam engine
is derived from the pressure of steam on the piston. If
there is no resistance on one side of the piston, the entire
pressure on the other side is available. Whenever there is
resistance on one side of the piston, it must be deducted
from the pressure on the other side,
