water, which would else be accumulated in the condenser, is continually
being pumped out by a pump worked by the engine. This pump is called the
air pump, because it also discharges any air which may have entered with
the water.

8. _Q._--If a vacuum be an empty space, and there be water in the
condenser, how can there be a vacuum there?

_A._--There is a vacuum above the water, the water being only like so much
iron or lead lying at the bottom.

9. _Q._--Is the vacuum in the condenser a perfect vacuum?

_A._--Not quite perfect; for the cold water entering for the purpose of
condensation is heated by the steam, and emits a vapor of a tension
represented by about three inches of mercury; that is, when the common
barometer stands at 30 inches, a barometer with the space above the mercury
communicating with the condenser, will stand at about 27 inches.

10. _Q._--Is this imperfection of the vacuum wholly attributable to the
vapor in the condenser?

_A._--No; it is partly attributable to the presence of a small quantity of
air which enters with the water, and which would accumulate until it
destroyed the vacuum altogether but for the action of the air pump, which
expels it with the water, as already explained. All common water contains a
certain quantity of air in solution, and this air recovers its elasticity
when the pressure of the atmosphere is taken off, just as the gas in soda
water flies up so soon as the cork of the bottle is withdrawn.