ressure of the steam be very low. After the engine is started,
however, the pressure in the boiler may be lowered, if the engine be
lightly loaded, until there is a partial vacuum in the boiler. Such a
practice, however, is not to be commended, as the gauge cocks become
useless when there is a partial vacuum in the boiler; inasmuch as, when
they are opened, the water will not rush out, but air will rush in. It is
impossible, also, under such circumstances, to blow out any of the sediment
collected within the boiler, which, in the case of the boilers of steam
vessels, requires to be done every two hours or oftener. This is
accomplished by opening a large cock which permits some of the supersalted
water to be forced overboard by the pressure of the steam. In some cases,
in which the boiler applied to an engine is of inadequate size, the
pressure within the boiler will fall spontaneously to a point considerably
beneath the pressure of the atmosphere; but it is preferable, in such
cases, partially to close the throttle valve in the steam pipe, whereby the
issue of steam to the engine is diminished; and the pressure in the boiler
is thus maintained, while the cylinder receives its former supply.

13. _Q._--If a hole be opened into a condenser of a steam engine, will air
rush into it?

_A._--If the hole communicates with the atmosphere, the air will be drawn
in.

14. _Q._--With what Velocity does air rush into a vacuum?

_A._--With the velocity which a body would acquire by falling from the
height of a homogeneous atmosphere, which is an atmosphere of the same
density throughout as at the earth's surface; and although such an
atmosphere does not exist in nature, its existence is supposed, in order to
facilitate the computation. It is well known that the velocity with which
water issues from a cistern is the same that would be acquired by a body
falling from the level of the head to the level of the issuing point; which
indeed is an obvious law, since every particle of water descends and issues
by virtue of its gravity, and is in its descent subject to the ordinary
laws of falling bodies. Air rushing into a vacuum is only another example
of the same general principle: the velocity of each particle will be that
due to the height of the column of air which would produce the pressure
sustained; and the weight of air being known, as well as the pressure it
exerts on the earth's surface, it becomes easy to tell what height a colu