proportion, and as
there will be a greater waste of heat, if from the existence of scale upon
the flues the heat can be only imperfectly transmitted to the water, there
cannot be even an economy of fuel in niggard blowing off, while it involves
the introduction of other evils. The proportion of 4/33rds of saltness,
however, or 16 oz. to the gallon, is larger than is advisable, especially
as it is difficult to keep the saltness at a perfectly uniform point, and
the working point should, therefore, be 2/33rds as before prescribed.

396. _Q._--Have no means been devised for turning to account the heat
contained in the brine which is expelled from the boiler?

_A._--To save a part of the heat lost by the operation of blowing off, the
hot brine is sometimes passed through a number of small tubes surrounded by
the feed water; but there is no very great gain from the use of such
apparatus, and the tubes are apt to become choked up, whereby the safety of
the boiler may be endangered by the injurious concentration of its
contents. Pumps, worked by the engine for the extraction of the brine, are
generally used in connection with the small tubes for the extraction of the
heat from the supersalted water; and if the tubes become choked the pumps
will cease to eject the water, while the engineer may consider them to be
all the while in operation.

397._Q._--What is the usual mode of blowing off the supersalted water from
the boiler?

_A._--The general mode of blowing off the boiler is to allow the water to
rise gradually for an hour or two above the lowest Working level, and then
to open the cock communicating with the sea, and keep it open until the
surface of the water within the boiler has fallen several inches; but in
some cases a cock of smaller size is allowed to run water continuously, and
in other cases brine pumps are used as already mentioned. In every case in
which the supersalted water is discharged from the boiler in a continuous
stream, a hydrometer or salt gauge of some convenient construction should
be applied to the boiler, so that the density of the water may at all times
be visible, and immediate notice be given of any interruption of the
operation. Various contrivances have been devised for this purpose, the
most of which operate on the principle of a hydrometer; but perhaps a more
satisfactory principle would be that of a differential steam gauge, which
would indicate the difference of pressure between the stea