The complicated action and re-action met with in burning coal may be divided into three processes. First, the conversion of coal into gases; second, the burning of the gases; and third, the separation and disposal of the ash and refuse. None of these processes are easily carried out in the ordinary locomotive firebox where bituminous coal is used.

The conversion of coal into gases can be managed without difficulty if it were possible to hold all of the coal on the firebars until gasification was completed; but under ordinary working conditions from 5 to 15 per cent, of the coal is picked up by the draught (or blast) and ejected at the smoke stack as cinders or sparks. Under bad conditions as much as 25 per cent, can be lost in this manner.

The loss of solid coal through the firebars and drop-grates generally averages from 1 to 2 per cent., but my observations lead me to believe it is nearer the vicinity of 10 per cent.—not through a badly designed fire-grate, but owing to careless and indifferent firing.

Under the most favourable conditions and at low rates of combustion, about 94 per cent, of the solid combustible matter is converted to gases. At medium rates about 85 per cent, is so converted, and at high rates only 75 per cent.

The burning of the combustible gases in the spaces above the fire is also attended with difficulties—due to lack of air, imperfect mixing of the gases, and insufficient combustion chamber volume (that is, the firebox isn't big enough).

The amount of air supplied through the grates depends upon the air opening through the ashpan and finger bars, the thickness of the fuel bed, the nature of the coal (as to size and ash and clinker forming contents), and above all, to the skill of the fireman. But even if all these conditions are favourable there is a gradual decrease in the air supplied as the rate of combustion increases. At high rates of combustion the supply of air is generally deficient.

Under ordinary conditions, with the firebox fitted with a deflector plate and a brick arch, the loss due to the escape of unburned gases varies from 2 to 10 per cent. The losses are much greater without the brick arch, as the saving of from 10 to 16 per cent, effected by the arch is largely due to the decrease in the amount of combustible matter that escapes unburned in the form of gases, coal dust, sparks and cinders.

The separation and disposal of the ash and clinker forming impurities is an unsolved problem which is largely responsible for the low average daily mileage of our locomotives. The frequent necessity for cleaning fires, dropping fires through the drop-grate (which often causes the tubes to leak) is due largely to imperfection in firebox design and to methods of burning coal that our great designers are always endeavouring to overcome.

The foregoing remarks will convey some idea, of the complicated structure of coal and the complexities of the problems yet to be solved in this direction. I trust, too, that firemen may be encouraged to study the problem of combustion. The firebox with which they are working becomes vastly more efficient for its purpose when an intelligent hand guides the coal and air supply.