or stiffening the crown of the furnace
in a locomotive?

_A._--The roof of the internal fire box, whether flat as in Stephenson's
engines, or dome shaped as in Bury's, requires to be stiffened with cross
stay bars, but the bars require to be stronger and more numerous when
applied to a flat surface. The ends of these stay bars rest above the
vertical sides of the fire box; and to the stay bars thus extending across
the crown, the crown is attached at intervals by means of stay bolts. There
are projecting bosses upon the stay bars encircling the bolts at every
point where a bolt goes through, but in the other parts they are kept clear
of the fire box crown so as to permit the access of water to the metal;
and, with the view of facilitating the ascent of the steam, the bottom of
each stay bar should be sharpened away in those parts where it does not
touch the boiler.

416. _Q._--Is any inconvenience experienced from the intense heat in a
locomotive furnace?

_A._--The fire bars in locomotives have always been a source of trouble, as
from the intensity of the heat in the furnace they become so hot as to
throw off a scale, and to bend under the weight of the fuel. The best
alleviation of these evils lies in making the bars deep and thin: 4 or 5
inches deep by five eighths of an inch thick on the upper side, and three
eighths of an inch on the under side, are found in practice to be good
dimensions. In some locomotives a frame carrying a number of fire bars is
made so that it may be dropped suddenly by loosening a catch; but it is
found that any such mechanism can rarely be long kept in working order, as
the molten clinker by running down between the frame and the boiler will
generally glue the frame into its place. It is therefore found preferable
to fix the frame, and to lift up the bars by the dart used by the stoker,
when any cause requires the fire to be withdrawn. The furnace bars of
locomotives are always made of malleable iron, and indeed for every species
of boiler malleable iron bars are to be preferred to bars of cast iron, as
they are more durable, and may if thin be set closer together, whereby the
small coal or coke is saved that would otherwise fall into the ash pit. The
ash box of locomotives is made of plate iron, a quarter thick: it should
not be less than 10 in. deep, and its bottom should be about 9 in. above
the level of the rails. The chimney of a locomotive is made of plate iron
one eighth of an inch