the act of drawing them out, as to be too short for
reinsertion: this result might be to a considerable extent obviated by
setting the tube plates at different angles, so that the several horizontal
rows of tubes would not be originally of the same length, and the damaged
tubes of the long rows would serve to replace the short ones; but the
practice would be attended with other inconveniences.

732. _Q._--Is there no other means of keeping boilers free from scale than
by blowing off?

_A._--Muriatic acid, or muriate of ammonia, commonly called sal-ammoniac,
introduced into a boiler, prevents scale to a great extent; but it is
liable to corrode the boiler internally, and also to damage the engine, by
being carried over with the steam; and the use of such intermixtures does
not appear to be necessary, if blowing off from the surface of the water is
largely practised. In old boilers, however, already incrusted with scale,
the use of muriate of ammonia may sometimes be advantageous.

733. _Q._--Are not the tubes of tubular boilers liable to be choked up by
deposits of soot?

_A._--The soot which collects in the inside of the tubes of tubular boilers
is removed by means of a brush, like a large bottle brush; and the
carbonaceous scale, which remains adhering to the interior of the tubes, is
removed by a circular scraper. Ferules in the tubes interfere with the
action of this scraper, and in the case of iron tubes ferules are now
generally discarded; but it will sometimes be necessary to use ferules for
iron tubes, where the tubes have been drawn and reinserted, as it may be
difficult to refix the tubes without such an auxiliary. Tubes one tenth of
an inch in thickness are too thin: one eighth of an inch is a better
thickness, and such tubes will better dispense with the use of ferules, and
will not so soon wear into holes.

734. _Q._--If the furnace or flue of a boiler be injured, how do you
proceed to repair it?

_A._--If from any imperfection in the roof of a furnace or flue a patch
requires to be put upon it, it will be better to let the patch be applied
upon the upper, rather than upon the lower, surface of the plate; as if
applied within the furnace a recess will be formed for the lodgment of
deposit, which will prevent the rapid transmission of the heat in that
part; and the iron will be very liable to be again burned away. A crack in
a plate may be closed by boring holes in the direction of the crack, and
inse