the course of the flues, is given in fig. 74. The corresponding parts in all the figures are marked by the same letters.

The door by which fuel is introduced upon the grate is represented at A, and the door leading to the ash-pit at B. The fire bars at c slope downwards from the front at an angle of about 25°, giving a tendency to the fuel to move from the front towards the back of the grate. The ash-pit D is constructed of such a magnitude, form, and depth, as to admit a current of atmospheric air to the grate-bars, sufficient to sustain the combustion. The form of the ash-pit is usually wide below, contracting towards the top.

The fuel when introduced at the fire-door A, should be laid on that part of the grate nearest to the fire-door, called the dead plates: there it is submitted to the process of coking, by which the gases and volatile matter which it contains are expelled, and being carried by a current of air, admitted

through small apertures in the fire-door over the burning fuel in the hinder part of the grate, they are burnt. When the

fuel in front of the grate has been thus coked, it is pushed back, and a fresh feed introduced in front. The coal thus pushed back soon becomes vividly ignited, and by continuing this process, the fuel spread over the grate is maintained in the most active state of combustion at the hinder part of the grate. By such an arrangement, the smoke produced by the combustion of the fuel may be burnt before it enters the flues. The flame and heated air proceeding from the burning fuel arising from the grate, and rushing towards the back of the furnace, passes over the fire-bridge E, and is carried through the flue F which passes under the boiler. This flue (the cross section of which is shown in fig. 73., by the dark shade put under the boiler) is very nearly equal in width to the bottom of the boiler, the space at the bottom of the boiler, near the corners, being only what is sufficient to give the weight of the boiler support on the masonry forming the

S

sides of the flue. The bottom of the boiler being concave, the flame and heated air as they pass along the flue rise to the upper part by the effects of their high temperature, and lick the bottom of the boiler from the fire-bridge at E to the further end G.

At & the flue rises to H, and turning to the side of the boiler at II, conducts the flame in contact with the side from the back to the front; it then passes through the flue K Fig. 74. 50 yrs 00 Da

fue L. The side flue is represented, stripped of the masonry, in fig.71., and also appears in the plan in fig. 74., and in the cross section in fig. 73. The course of the air is represented in fig. 74. by the arrows. From the flue L the air is con

ducted into the chimney at M.

By such an arrangement, the flame and heated air proceeding from the grate are made to circulate round the boiler, and the length and magnitude of the flues through which it is conducted should be such, that when it shall arrive at the chimney its temperature shall be reduced, as nearly as is consistent with the maintenance of draught in the chimney, to the temperature of the water with which it is in contact.

The method of feeding the furnace, which has been described above, is one which, if conducted with skill and care, would produce a much more perfect combustion of the fuel than would attend the common method of filling the grate from the back to the front with fresh fuel, whenever the furnace is fed. This method, however, is rarely observed in the management of the furnace. It requires the constant attention of the stokers (such is the name given to those who feed the furnaces). The fuel must be supplied, not in large quantities, and at distant intervals, but in small quantities and more frequently. On the other hand, the more common practice is to allow the fuel on the grate to be in a great degree burned away, and then to heap on a large quantity of fresh fuel, covering over with it the burning fuel from the back to the front of the grate. When this is done, the heat of the ignited coal acting upon the fresh fuel introduced, expels the gases combined with it and, mixed with these, a quantity of carbon, in a state of minute division, forming an opaque black smoke. This is carried through the flues and drawn up the chimney. The consequence is, that not only a quantity of solid fuel is sent out of the chimney unconsumed, but the hydrogen and other gases also escape unburned, and a proportional waste of the combustible is produced; besides which, the nuisance of an atmosphere filled with smoke Such effects are visible to all who observe the chimneys of steam-vessels, while the engine is in operation. When the furnaces are thus filled with fresh fuel, a large volume of

ensues.

dense black smoke is observed to issue from the chimney, This gradually subsides as the fuel on the grate is ignited, and does not reappear until a fresh feed is introduced.

This method of feeding, by which the furnace would be made to consume its own smoke, and the combustion of the fuel be rendered complete, is not however free from counteracting effects. In ordinary furnaces the feed can only be introduced by opening the fire-doors, and during the time the fire-doors are opened a volume of cold air rushes in, which passing through the furnace is carried through the flues to the chimney. Such is the effect of this in lowering the temperature of the flues, that in many cases the loss of heat occasioned is greater than any economy of fuel obtained by the complete consumption of smoke. Various methods, however, may be adopted by which fuel may be supplied to the grate without opening the fire-doors, and without disturbing the supply of air to the fire. A hopper built into the front of the furnace, with a moveable bottom or valve, by which coals may be allowed to drop in from time to time upon the front of the grate, would accomplish this.

(148.) In order to secure the combustion of the gases evolved from the coals placed in the front of the grate, it is necessary that a supply of atmospheric air should be admitted with them over the burning fuel. This is effected by small apertures or regulators, provided in the fire-doors, governed by sliding-plates, by which they may be opened or closed to any required extent,

A patent has recently been granted to Mr. Williams, one of the directors of the City of Dublin Steam Navigation Company, for a method of consuming the unburned gases which escape from the grate, and are carried through the flues. This method consists in introducing into the flue tubes placed in a vertical position, the lower ends of which being inserted in the bottom of the flue are made to communicate with the ash-pit, and the upper ends of which are closed. The sides and tops of these tubes are pierced with small holes, through which atmospheric air drawn from the ash-pit issues in jets. The oxygen supplied by this air immediately combines with the carburetted hydrogen, which