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water flies up so soon as the cork of the bottle is withdrawn.

11. _Q._--Is a barometer sometimes applied to the condensers of steam
engines?

_A._--Yes; and it is called the vacuum gauge, because it shows the degree
of perfection the vacuum has attained. Another gauge, called the steam
gauge, is applied to the boiler, which indicates the pressure of the steam
by the height to which the steam forces mercury up a tube. Gauges are also
applied to the boiler to indicate the height of the water within it so that
it may not be burned out by the water becoming accidentally too low. In
some cases a succession of cocks placed a short distance above one another
are employed for this purpose, and in other cases a glass tube is placed
perpendicularly in the front of the boiler and communicating at each end
with its interior. The water rises in this tube to the same height as in
the boiler itself, and thus shows the actual water level. In most of the
modern boilers both of these contrivances are adopted.

12. _Q._--Can a condensing engine be worked with a pressure less than that
of the atmosphere?

_A._--Yes, if once it be started; but it will be a difficult thing to start
an engine, if the pressure of the steam be not greater than that of the
atmosphere. Before an engine can be started, it has to be blown through
with steam to displace the air within it, and this cannot be effectually
done if the p