f this valve is to
prevent a vacuum from being formed accidentally in the boiler, which might
collapse it; for if the pressure in the boiler subsides to a point
materially below the pressure of the atmosphere, the valve will open and
allow air to get in. A bent pipe, which rises up from the top of the
boiler, immediately behind the position of the manhole, is the steam pipe
for conducting the steam to the engine; and a bent pipe which ascends from
the top of the boiler, at the back end, is the waste-steam pipe for
conducting away the steam, which escapes through the safety valve. This
valve is set in a chest, standing on the top of the boiler, at the foot of
the waste-steam pipe, and it is loaded with iron or leaden weights to a
point answerable to the intended pressure of the steam.

88. _Q._--How is the proper level of the water in the boiler maintained?

_A._--By means of a balanced buoy or float. This float is attached to a
rod, which in its turn is attached to a lever set on the top of a large
upright pipe. The upper part of the pipe is widened out into a small
cistern, through a short pipe in the middle of which a chain passes to the
damper; but any water emptied into this small cistern cannot pass into the
pipe, except through a small valve fixed to the lever to which the rod is
attached. The water for replenishing the boiler is pumped into the small
cistern on the top of the pipe; and it follows from these arrangements that
when the buoy falls, the rod opens the small valve and allows the feed
water to enter the pipe, which communicates with the water in the boiler;
whereas, when the buoy rises, the feed cannot enter the pipe, and it has,
therefore, to run to waste through an overflow pipe provided for the
purpose.

89. _Q._--How is the strength of the fire regulated?

_A._--The draught through the furnaces of land boilers is regulated by a
plate of metal or a damper, as it is called, which slides like a sluice up
and down in the flue, and this damper is closed more or less when the
intensity of the fire has to be moderated. In wagon boilers this is
generally accomplished by self-acting mechanism. In the small cistern pipe,
which is called a stand pipe, the water rises up to a height proportional
to the pressure of the steam, and the surface of the water in this pipe
will rise or fall with the fluctuations in the pressure of the steam. In
this pipe a float is placed, which communicates by means of a chain with