vacuum be produced within it, the glass gauges become
of essential service, as the gauge cocks will not operate in such a case,
for though opened, instead of steam and water escaping from them, the air
will rush into the boiler. It is expedient to carry a pipe from the lower
end of the glass tube downward into the water of the boiler, and a pipe
from the upper end upward into the steam in the boiler, so as to prevent
the water from boiling down through the tube, as it might otherwise do, and
prevent the level of the water from being ascertainable. The average level
of water in the boiler should be above the centre of the tube; and the
lowest of the gauge cocks should always run water, and the highest should
always blow steam.

248. _Q._--Is not a float sometimes employed to indicate the level of the
water in the boiler?

_A._--A float for telling the height of water in the boiler is employed
only in the case of land boilers, and its action is like that of a buoy
floating on the surface, which, by means of a light rod passing vertically
through the boiler, shows at what height the water stands. The float is
usually formed of stone or iron, and is so counterbalanced as to make its
operation the same as if it were a buoy of timber; and it is generally put
in connection with the feed valve, so that in proportion as the float
rises, the supply of feed water is diminished. The feed water in land
boilers is admitted from a small open cistern, situated at the top of an
upright or stand pipe set upon the boiler, and in which there is a column
of water sufficiently high to balance the pressure of the steam.

249. _Q._--What is the cataract which is employed to regulate the speed of
pumping engines?

[Illustration: Fig. 38. ]

_A._--The cataract consists of a small pump-plunger _b_ and barrel, set in
a cistern of water, the barrel being furnished on the one side with a
valve, _c_, opening inwards, through which the water obtains admission to
the pump chamber from the cistern, and on the other by a plug, _d_, through
which, if the plunger be forced down, the water must pass out of the pump
chamber. The engine in the upward stroke of the piston, which is
accomplished by the preponderance of weight at the pump end of the beam,
raises up the plunger of the cataract by means of a small rod,--the water
entering readily through the valve already referred to; and when the engine
reaches the top of the stroke, it liberates the rod by