ge, and when discovered,
the blow through valves of the engine and blow off cocks of the boiler
should at once be opened, and the fires raked out. A cone in the ball of
the waste steam pipe to send back the water carried upward by the steam,
should never be inserted; as in some cases this cone has become loose, and
closed up the mouth of the waste steam pipe, whereby the safety valves
being rendered inoperative, the boiler was in danger of bursting.

309. _Q._--May not danger arise from excessive priming?

_A._--If the water be carried out of the boiler so rapidly by priming that
the level of the water cannot be maintained, and the flues or furnaces are
in danger of becoming red hot, the best plan is to open every furnace door
and throw in a few buckets full of water upon the fire, taking care to
stand sufficiently to the one side to avoid being scalded by the rush of
steam from the furnace. There is no time to begin drawing the fires in such
an emergency, and by this treatment the fires, though not altogether
extinguished, will be rendered incapable of doing harm. If the flues be
already red hot, on no account must cold water be suffered to enter the
boiler, but the heat should be maintained in the furnaces, and the blow off
cocks be opened, or the mud hole doors loosened, so as to let all the water
escape; but at the same time the pressure must be kept quite low in the
boiler, so that there will be no danger of the hot flues collapsing with
the pressure of the steam.

310. _Q._--Are plugs of fusible metal useful in preventing explosions?

_A._--Plugs of fusible metal were at one time in much repute as a
precaution against explosion, the metal being so compounded that it melted
with the heat of high pressure steam; but the device, though ingenious, has
not been found of any utility in practice. The basis of fusible metal is
mercury, and it is found that the compound is not homogeneous, and that the
mercury is forced by the pressure of the steam out of the interstices of
the metal combined with it, leaving a porous metal which is not easily
fusible, and which is, therefore, unable to perform its intended function.
In locomotives, however, and also in some other boilers, a lead rivet is
inserted with advantage in the crown of the fire box, which is melted out
if the water becomes too low, and thus gives notice of the danger.

311. _Q._--May not explosion occur in marine boilers from the accumulation
of salt on the flu