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The fire-grate D D is fixed 3 feet 2 inches below the roof of the fire-box, and9 inches above the bottom, and is composed of separate loose bars of wrought iron, 2^inches deep in the middle, and 1 inch thick at the upper side, tapering downwardsto allow more free ingress for the air ; the fire-bars are bent down at the ends anddrop into holes in a square ring of iron, h h, which runs round the fire-box at a littledistance from the side, and is supported by a piece of angle iron, i i, bolted to thefront 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 veryfrequently from the great heat to which they are exposed; and also for the facilitywith which the fire can be extinguished in case danger should be apprehended fromany 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 : thisis indeed the manner in which the fire is extinguished when the engine has doneworking. 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 tocatch the cinders and prevent their falling on the road, but is rather inconvenientwhen the fire has to be let out.
The boiler being cylindrical has an equal strain in all directions from the pressureof the steam; but as the fire-boxes are flat on all sides except the top of the externalone, they would have a tendency to be separated from each other by the pressure ofthe 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 alongtheir whole length and are passed through holes in both plates, tapped to receivethem, and then riveted over at the ends for additional security; these copper boltsare 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 isstrengthened by six wrought iron ribs, 11, (Plates XC. and XCII.,) placed parallel toeach other and longitudinally upon the roof, and fastened to it by bolts screwedthrough 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-| inches deep at themiddle, where the strain is greatest. The ribs 11, are cut away on the undersidebetween each of the bolts, so as to touch the roof-plate only where the bolts passthrough them, in order that there may be as little mass of metal as possible exposedto the immediate action of the fire; for when a considerable thickness of metal isinterposed between the water in the boiler and the fire, the heat cannot be absorbedby 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