726. _Q._--You have already stated that the formation of salt or scale in
marine boilers is to be prevented by blowing out into the sea at frequent
intervals a portion of the concentrated water. Will you now explain how the
proper quantity of water to be blown out is determined?

_A._--By means of the salinometer, which is an instrument for determining
the density of the water, constructed on the principle of the hydrometer
for telling the strength of spirits. Some of the water is drawn off from
the boiler from time to time, and the salinometer is immersed in it after
it has been cooled. By the graduations of the salinometer the saltness of
this water is at once discovered; and if the saltness exceeds 8 ounces of
salt in the gallon, more water should be blown out of the boiler to be
replenished with fresher water from the sea, until the prescribed limit of
freshness is attained. Should the salinometer be accidentally broken, a
temporary one may be constructed of a phial weighted with a few grains of
shot or other convenient weight. The weighted phial is first to be floated
in fresh water, and its line of floatation marked; then to be floated in
salt water, and its line of floatation marked; and another mark of an equal
height above the salt water mark will be the blow off point.

727. _Q._--HOW often should boilers be blown off in order to keep them free
from incrustation?

_A._--Flue boilers generally require to be blown off about twice every
watch, or about twice in the four hours; but tubular boilers may require to
be blown off once every twenty minutes, and such an amount of blowing off
should in every case be adopted, as will effectually prevent any injurious
amount of incrustation.

728. _Q._--In the event of scale accumulating on the flues of a boiler,
what is the best way of removing it?

_A._--If the boilers require to be scaled, the best method of performing
the operation appears to be the following:--Lay a train of shavings along
the flues, open the safety valve to prevent the existence of any pressure
within the boiler, and light the train of shavings, which, by expanding
rapidly the metal of the flues, while the scale, from its imperfect
conducting power, can only expand slowly, will crack off the scale; by
washing down the flues with a hose, the scale will be carried to the bottom
of the boiler, or issue, with the water, from the mud-hole doors. This
method of scaling must be practised only by the en