in by ferules. The whole of the iron plates of a locomotive boiler
Which are subjected to the pressure of steam, should be Lowmoor or Bowling
plates of the best quality; and the copper should be coarse grained, rather
than rich or soft, and be perfectly free from irregularities of structure
and lamination.

413. _Q._--What are the usual dimensions of the barrel?

_A._--The thickness of the plates composing the barrel of the boiler varies
generally from 5/16ths to 3/8ths of an inch, and the plates should run in
the direction of the circumference, so that the fibres of the iron may be
in the direction of the strain. The diameter of the barrel commonly varies
from 3 ft. to 3 ft. 6 inches; the diameter of the rivets should be from
11/16ths to 3/4ths of an inch, and the pitch of the rivets or distance
between their centres should be from 17/8th to 2 inches.

414. _Q._--How are the fire boxes of a locomotive constructed?

_A._--The space between the external and internal fire boxes forms a water
space, which must be stayed every 4-1/2 or 5 inches by means of copper or
iron stay bolts, screwed through the outer fire box into the metal of the
inner fire box, and securely riveted within it: iron stay bolts are as
durable as copper, and their superior tenacity gives them an advantage.
Sometimes tubes are employed as stays. The internal and external fire boxes
are joined together at the bottom by a N-shaped iron, and round the fire
door they are connected by means of a copper ring 1-1/4 in. thick, and 2
in. broad,--the inner fire box being dished sufficiently outward at that
point, and the outer fire box sufficiently inward, to enable a circle of
rivets 3/4 of an inch in diameter passing through the copper ring and the
two thicknesses of iron, to make a water-tight joint. The thickness of the
plates composing the external fire box is in general 3/8ths of an inch if
the fire box is circular, and from 3/8ths to 1/2 inch if the fire box is
square; and the thickness of the internal fire box is in most cases 7/16ths
if copper, and from 3/8ths to 7/16ths of an inch if of iron. Circular
internal fire boxes, if made of iron, should be welded rather than riveted,
as the rivet heads are liable to be burnt away by the action of the fire;
and when the fire boxes are square each side should consist of a single
plate, turned over at the edges with a radius of 3 inches, for the
introduction of the rivets.

415. _Q._--Is there any provision f