gineer himself, and must
not be intrusted to the firemen who, in their ignorance, might damage the
boiler by overheating the plates. It is only where the incrustation upon
the flues is considerable that this method of removing it need be
practised; in partial cases the scale may be chipped off by a hatched faced
hammer, and the flues may then be washed down with the hose in the manner
before described.

729. _Q._--Should the steam be let out of the boiler, after it has blown
out the water, when the engine is stopped?

_A._--No; it is better to retain the steam in the boiler, as the heat and
moisture it occasions soften any scale adhering to the boiler, and cause it
to peel off. Care must, however, be taken not to form a vacuum in the
boiler; and the gauge cocks, if opened, will prevent this.

730. _Q._--Are tubular boilers liable to the formation of scale in certain
places, though generally free from it?

_A._--In tubular boilers a good deal of care is required to prevent the
ends of the tubes next the furnace from becoming coated with scale. Even
when the boiler is tolerably clean in other places the scale will collect
here; and in many cases where the amount of blowing off previously found to
suffice for flue boilers has been adopted, an incrustation five eighths of
an inch in thickness has formed in twelve months round the furnace ends of
the tubes, and the stony husks enveloping them have actually grown together
in some parts so as totally to exclude the water.

731. _Q._--When a tubular boiler gets incrusted in the manner you have
described, what is the best course to be adopted for the removal of the
scale?

_A._--When a boiler gets into this state the whole of the tubes must be
pulled out, which may be done by a Spanish windlass combined with a pair of
blocks; and three men, when thus provided, will be able to draw out from 50
to 70 tubes per day,--those tubes with the thickest and firmest
incrustations being, of course, the most difficult to remove. The act of
drawing out the tubes removes the incrustation; but the tubes should
afterward be scraped by drawing them backward and forward between the old
files, fixed in a vice, in the form of the letter V. The ends of the tube
should then be heated and dressed with the hammer, and plunged while at a
blood heat into a bed of sawdust to make them cool soft, so that they may
be riveted again with facility. A few of the tubes will be so far damaged
at the ends by