LOCOMOTIVE ENGINE. 
411 
The fire-grate D D is fixed 3 feet 2 inches below the roof of the fire-box, and 
9 inches above the bottom, and is composed of separate loose bars of wrought iron, 2-J 
inches deep in the middle, and 1 inch thick at the upper side, tapering downwards 
to allow more free ingress for the air ; the fire-bars are bent down at the ends and 
drop into holes in a square ring of iron, h h, which runs round the fire-box at a little 
distance from the side, and is supported by a piece of angle iron, i i, bolted to the 
front and back plates of the fire-box. The fire-bars are made separate and move- 
able in order that they may be easily replaced when worn out, which happens very 
frequently from the great heat to which they are exposed; and also for the facility 
with which the fire can be extinguished in case danger should be apprehended from 
any accident; for by lifting them out of the holes with a bent rod, kept for the pur 
pose, they can be dropped, together with the whole of the fire, upon the road : this 
is indeed the manner in which the fire is extinguished when the engine has done 
working. An ash-pan is fixed under the fire-box in some engines, made of sheet- 
iron, of the same shape as the external fire-box, and open only in front; it serves to 
catch the cinders and prevent their falling on the road, but is rather inconvenient 
when the fire has to be let out. 
The boiler being cylindrical has an equal strain in all directions from the pressure 
of the steam; but as the fire-boxes are flat on all sides except the top of the external 
one, they would have a tendency to be separated from each other by the pressure of 
the steam if they had not some support. For this purpose they are connected to 
gether by a number of three-quarter inch copper bolts, k k, which are screwed along 
their whole length and are passed through holes in both plates, tapped to receive 
them, and then riveted over at the ends for additional security; these copper bolts 
are screwed in about 4 inches apart all over the sides and back of the internal fire 
box and that portion of the front that is below the boiler. As the roof of the in 
ternal fire-box only requires support, that of the external one being cylindrical, it is 
strengthened by six wrought iron ribs, 11, (Plates XC. and XCII.,) placed parallel to 
each other and longitudinally upon the roof, and fastened to it by bolts screwed 
through the roof-plate, and having, in addition, a nut screwed on at the under side; 
the ribs are lj inch thick, and increased at the bolt holes, and 2-J inches deep at the 
middle, where the strain is greatest. The ribs //, are cut away on the underside 
between each of the bolts, so as to touch the roof-plate only where the bolts pass 
through them, in order that there may be as little mass of metal as possible exposed 
to the immediate action of the fire; for when a considerable thickness of metal is 
interposed between the water in the boiler and the fire, the heat cannot be absorbed 
by the water with such rapidity as it is supplied, and the metal becomes in conse 
quence greatly heated, and is rapidly destroyed. The durability of the internal