nomy of fuel, all
the engines are made with steam jackets, and in some cases a flue winds
spirally round the cylinder, for keeping the steam hot. Mr. Watt, in his
early practice, discarded the steam jacket for a time, but resumed it
again, as he found its discontinuance occasioned a perceptible waste of
fuel; and in modern engines it has been found that where a jacket is used
less coal is consumed than where the use of a jacket is rejected. The cause
of this diminished effect is not of very easy perception, for the jacket
exposes a larger radiating surface for the escape of the heat than the
cylinder; nevertheless, the fact has been established beyond doubt by
repeated trials, that engines provided with a jacket are more economical
than engines without one. The exterior of the cylinder, or jacket, should
be covered with several plies of felt, and then be cased in timber, which
must be very narrow, the boards being first dried in a stove, and then
bound round the cylinder with hoops, like the staves of a cask. In many of
the Cornish engines the steam is let into casings formed in the cylinder
cover and cylinder bottom, for the further economisation of the heat, and
the cylinder stuffing box is made very deep, and a lantern or hollow brass
is introduced into the centre of the packing, into which brass the steam
gains admission by a pipe provided for the purpose; so that in the event of
the packing becoming leaky, it will be steam that will be leaked into the
cylinder instead of air, which, being incondensable, would impair the
efficiency of the engine. A lantern brass, of a similar kind, is sometimes
introduced into the stuffing boxes of oscillating engines, but its use
there is to receive the lateral pressure of the piston rod, and thus take
any strain off the packing.

444. _Q._--Will you explain the proper course to pursue in the production
of cylinders?

_A._--In all engines the valve casing, if made in a separate piece from the
cylinder, should be attached by means of a metallic joint, as such a
barbarism as a rust joint in such situations is no longer permissible. In
the case of large engines with valve casings suitable for long slides, an
expansion joint in the valve casing should invariably be inserted,
otherwise the steam, by gaining admission to the valve casing before it can
enter the cylinder, expands the casing while the cylinder remains unaltered
in its dimensions, and the joints are damaged, and in some cas