to the chimney, should descend over the first
casing, so as to prevent the rain or spray which may beat against the
chimney from being poured down within the casing upon the top of the
boiler. The pipe for conducting away the waste water from the top of the
safety valve should lead overboard, and not into the bilge of the ship, as
inconvenience arises from the steam occasionally passing through it, if it
has its termination in the engine room.

387. _Q._--Are not the chimneys of some vessels made so that they may be
lowered when required?

_A._--The chimneys of small river vessels which have to pass under bridges
are generally formed with a hinge, so that they may be lowered backward
when passing under a bridge; and the chimneys of some screw vessels are
made so as to shut up like a spyglass when the fires are put out and the
vessel is navigated under sails. In smaller vessels, however, two lengths
of chimney suffice; and in that case there is a standing piece on deck,
which, however, does not project above the bulwarks.

388. _Q._--Will you explain any further details in the construction of
marine boilers which occur to you as important?

_A._--The man-hole and mud-hole doors, unless put on from the outside, like
a cylinder cover, with a great number of bolts, should be put on from the
inside with cross bars on the outside, and the bolts should be strong, and
have coarse threads and square nuts, so that the threads may not be
overrun, nor the nuts become round, by the unskilful manipulations of the
firemen, by whom these doors are removed or replaced. It is very expedient
that sufficient space should be left between the furnace and the tubes in
all tubular boilers to permit a boy to go in to clear away any scale that
may have formed, and to hold on the rivets in the event of repair being
wanted; and it is also expedient that a vertical row of tubes should be
left out opposite to each water space to allow the ascent of the steam and
descent of the water, as it has been found that the removal of the tubes in
that position, even in a boiler with deficient heating surface, has
increased the production of steam, and diminished the consumption of fuel.
The tubes should all be kept in the same vertical line, so as to permit the
introduction of an instrument to scrape them; but they may be zig-zagged in
the horizontal line, whereby a greater strength of metal will be obtained
around the holes in the tube plates, and the tube