nace door, which is double, to prevent inconvenient radiation. The external fire-box has sometimes a semi-cylindrical top, joined by turning over the sides like an arch, and sometimes a dome-shaped top.

The fire-bars have always been a source of much expense in the locomotive engine, as they burn out very rapidly, and have to be often renewed: from the rapid combustion going on over their upper surfaces, they become heated intensely throughout, causing them to throw off scale, and to bend under the weight of the fuel. The best remedy has been found to consist in making the bars very thin and deep, so as to keep their lower edges exposed to a cooling draught of air, and to diminish the area of metal conducting heat downwards from their heated upper edges. Thin fire-bars admit of being placed nearer together than thick ones, thus offering no increased impediment to free draught, while preventing the loss of small pieces of unburnt coke, which might otherwise drop through into the ashbox and be wasted. Fire-bars have given much satisfaction when made 4 inches deep, (parallel,) and full inch thick on the upper edge and inch on the lower edge. The frame carrying the fire-bars has often been made capable of being dropped on the instant, with its fire-bars and fire, into the ashbox, or upon the road, by means of catches drawn back by levers; but though the fire-bar frame is

thus left unsupported, very often it will not drop, and even cannot be forced down out of its place, owing to the clinkers and tarry products of combustion forming an adhesive binding between its edge and the fire-box: it has accordingly been found best to support the fire-box frame permanently, and when any cause requires the sudden withdrawal of the fire, to lift the fire-bars singly out of place, by means of the ordinary dart. It is necessary to place the fire-bars with their upper surface about 3 inches higher than the bottom of the water-spaces, which, by this means, will be allowed to contain quiescent water, ready to retain without injury any deposite that subsides from the water; and the water-spaces should be periodically cleansed, by means of the mud-holes placed opposite the edge of each waterspace in the lower part of the outer fire-box shell. These mud-holes are made water-tight by means of either a brass plug simply screwed in and with a slight taper, or by a door applied with a soft packing on its face, and screwed up with a bridge-piece and bolt, making the joint on the internal surface of the outer shell, the hole and door being made sufficiently oval to enable the door to be introduced into the water-space. The latter plan often gives rise to inconvenience, from the joint being found leaky when the steam is raised, rendering it necessary to drop the fire, and empty the boiler, before it can be renewed. In some very large square fire

boxes, such as those used on the Great Western Railway, a diaphragm, or divisional 4-inch waterspace, has been placed across the middle of the firebox, with the view of obtaining increased heating surface. This diaphragm has its lower edge (in which deposite takes place) made straight, and about 2 inches below the general surface of the fire-bars, but its upper edge is of the form of an inverted. arch, in order to promote the free delivery of the steam generated within it into the steam-dome. The sides of the fire-box, where the diaphragm is attached, are not cut away to form passages for the water and steam, but are pierced with a series of circular holes, 3 inches in diameter, to permit a due circulation without uselessly weakening the fire-box; but the uppermost hole of the series must be placed at the highest point of the diaphragm, otherwise an accumulation of steam, and consequent injury at that point, will ensue. The use of a diaphragm is found to be beneficial in the case of a very powerful engine, provided its upper edge be made sufficiently low to admit of the tubes being conveniently drifted over it, and to allow the dart to be used with facility in dropping the front set of fire-bars.

The ash-box consists of a plate-iron tray, placed below the fire-box, to receive the burning ashes that drop from between the fire-bars. In the earlier locomotives, no ash-boxes being used, the red-hot ashes were dispersed to a considerable distance by

coming in contact with the wheels, and conflagrations were often thereby originated. The ash-box should be as large as convenient, and not less than 10 inches deep, otherwise it will materially impede the draught; but if of ample dimensions, and closed at the sides and back, it will increase the draught, particularly when running against a head wind, at which time a strong draught is required. A hanging shutter to open or close the front of the ash-box forms a good damper. The bottom of the ash-box is placed about 9 inches above the level of the rails, and should on no account be nearer than 6 inches, otherwise the engine cannot pass safely over stones or similar objects lying accidentally between the rails.

Tubes.—The tubes are generally formed of brass; the ferules by which they are secured are for the most part made of steel at the furnace end, and of malleable iron at the smoke-box end, and the holes. in the tube-plates are tapered, so that the tubes bind them together. Great care should be taken in securing the tubes, as any neglect will be productive of much inconvenience. The ferules are found to be very injurious to freedom of draught, particularly in very small tubes; and to overcome this objection the methods we have mentioned, and many others, have been tried for fastening the tubes in by riveting over or screwing into the tube-plates; but hitherto no method, except that of internal tube

rings, has been found to answer in the case of brass tubes; but we think it likely that, with wrought-iron tubes, internal tube-rings will be ultimately abandoned. Stephenson has frequently adopted iron tubes of late, in preference to brass, on the score of their greater cheapness and durability; and in some cases, where unusual attention has been paid to them, and pure water used, they have been found to answer very well. A common internal diameter of tubes is 1ğ in. If made very small, the tubes are liable to be choked by pieces of coke, and the sectional area will be inconveniently contracted, while, if made much larger, the heating surface will be unduly diminished. The number of tubes varies considerably in different boilers; in one species of locomotive in extensive use the number is 134, and the pitch 24 in. Sufficient space is left below the tubes for deposite, that it may not be in contact with the tubes and cause them to be burned: the extreme tube of the widest row is about the diameter of a tube from the boiler shell. In the long-boiler engines of Stephenson, from the volume of water contained in them, considerable time is required to get up the steam, even so much as three and a half hours where the ordinary engines take two hours, and they require great care in firing and feeding to prevent the steam running low.

Smoke-box and chimney.-The smoke-box door of many engines is hinged at the bottom, and is kept