Some stokers take credit to
themselves for making the boiler "Hum"; when coals are thrown into the
fire indiscriminately--small and large--the air finds the least
resistance through the small coals, which soon burst into holes, while
the lumps remain solid; then the air rushes into the holes and the
humming commences; or, if the firebars are not equally separated, the
air enters the widest space and the boiler hums; or it may be that the
bars next the side of the fireplace are out of line and lets the air
rush up against the side and the boiler hums. If the stoker would only
drop a shovelful of coals dexterously into each hole the humming would
stop immediately, or level the fire with the rake or long poker, or open
the fire door if the rake is too heavy, and the noise will cease. The
chief point is to have a good set of firebars and well placed; if they
are too long they will hump in the middle or they will bulge sideways;
if they are too close together they become red-hot because there is not
room enough for the air to pass between them to keep them moderately
cool, and if they are too short they will drop down into the ash-pit.

11. _Question._--Why is it more difficult to keep steam-tight the
manhole cover of a portable boiler than the manhole cover of a
stationary boiler?

_Answer._--The portable cover is usually on the side of the boiler, and
about half the cover is immersed in the water and half in the steam; the
portion under water is about 212 deg. of heat, the portion of the same cover
in the steam is about 500 deg. of heat, the hottest part expanding much more
than the cooler part, and is constantly tending to tear itself away from
the lower portion of the cover, and the joint cannot stand the unequal
strain. The manhole cover of a stationary boiler is nearly always on top
of the boiler, and the heat is equal all over it and no contraction and
expansion to cause the joint to leak as in the portable cover.

12. _Question._--How would you prepare a boiler for the inspection of a
boiler inspector?

_Answer._--I would blow her right out, take off the manhole cover, take
out the safety valve, take out all firebars and the bridge, take down
flue-port brickwork, have the boiler and flues thoroughly cleaned and
swept, have a lamp or candle ready to light, a hand hammer and chisel,
or scraper, a pailful of clean water, and a wad of cotton waste. When
the inspector arrives, he quickly dons his overalls; I hand