m in the boiler
and the steam of a small quantity of fresh water enclosed in a suitable
vessel, and immerged in the water of the boiler.

398. _Q._--What is the advantage of blowing off from the surface of the
water in the boiler?

_A._--Blowing off from a point near the surface of the water is more
beneficial than blowing off from the bottom of the boiler. Solid particles
of any kind, it is well known, if introduced into boiling water, will lower
the boiling point in a slight degree, and the steam will chiefly be
generated on the surface of the particles, and indeed will have the
appearance of coming out of them; if the particles be small the steam
generated beneath and around them will balloon them to the surface of the
water, where the steam will be liberated and the particles will descend;
and the impalpable particles in a marine boiler, which by their subsidence
upon the flues concrete into scale, are carried in the first instance to
the surface of the water, so that if they be caught there and ejected from
the boiler, the formation of scale will be prevented.

399. _Q._--Are there any plans in operation for taking advantage of this
property of particles rising to the surface?

_A._--Advantage is taken of this property in Lamb's Scale Preventer, which
is substantially a contrivance for blowing off from the surface of the
water that in practice is found to be very effectual; but a float in
connection with a valve at the mouth of the discharging pipe is there
introduced, so as to regulate the quantity of water blown out by the height
of the water level, or by the extent of opening given to the feed cock. The
operation, however, of the contrivance would be much the same if the float
were dispensed with; but the float acts advantageously in hindering the
water from rising too high in the boiler, should too much feed be admitted,
and thereby obviates the risk of the water running over into the cylinder.
In some boilers sheet iron vessels, called sediment collectors, are
employed, which collect into them the impalpable matter, which in Lamb's
apparatus is ejected from the boiler at once. One of these vessels, of
about the size and shape of a loaf of sugar, is put into each boiler with
the apex of the cone turned downwards into a pipe leading overboard, for
conducting the sediment away from the boiler. The base of the cone stands
some distance above the water line, and in its sides conical slits are cut,
so as to estab