lish a free communication between the water within the
conical vessel and the water outside it. The particles of stony matter
which are ballooned to the surface by the steam in every other part of the
boiler, subside within the cone, where, no steam being generated, the water
is consequently tranquil; and the deposit is discharged overboard by means
of a pipe communicating with the sea. By blowing off from the surface of
the water, the requisite cleansing action is obtained with less waste of
heat; and where the water is muddy, the foam upon the surface of the water
is ejected from the boiler--thereby removing one of the chief causes of
priming.

400. _Q._--What is the cause of the rapid corrosion of marine boilers?

_A._--Marine boilers are corroded externally in the region of the steam
chest by the dripping of water from the deck; the bottom of the boiler is
corroded by the action of the bilge water, and the ash pits by the practice
of quenching the ashes with, salt water. These sources of injury, however,
admit of easy remedy; the top of the boiler may be preserved from external
corrosion by covering it with felt upon which is laid sheet lead soldered
at every joint so as to be impenetrable to water; the ash pits may be
shielded by guard plates which are plates fitting into the ash pits and
attached to the boiler by a few bolts, so that when worn they may be
removed and new ones substituted, whereby any wear upon the boiler in that
part will be prevented; and there will be very little wear upon the bottom
of a boiler if it be imbedded in mastic cement laid upon a suitable
platform.

401. _Q._--Are not marine boilers subject to internal corrosion?

_A._--Yes; the greatest part of the corrosion of a boiler takes place in
the inside of the steam chest, and the origin of this corrosion is one of
the obscurest subjects in the whole range of engineering. It cannot be from
the chemical action of the salt water upon the iron, for the flues and
other parts of the boiler beneath the water suffer very little from
corrosion, and in steam vessels provided with Hall's condensers, which
supply the boiler with fresh water, not much increased durability of the
boiler has been experienced. Nevertheless, marine boilers seldom last more
than for 5 or 6 years, whereas land boilers made of the same quality of
iron often last 18 or 20 years, and it does not appear probable that land
boilers would last a very much shorter time if salt wat