better than cure, however, we do not doubt, and we think Mr. Williams has done great service in fixing attention on this fact, even though his determination to maintain his favourite theory, that smoke cannot be consumed, has led him into several unguarded statements in Section VII. of his Treatise.

It was stated above that a furnace is a fireplace on a large scale and more confined, and it is just upon this confinement that the great difficulty hinges. The engineer's object is of course to heat the boiler as much as possible; thus he must leave as little space as he can for the heat to radiate away into and be lost. Besides, in many places, especially in steam-boats, the smallness of the whole machinery is a great desideratum. It must be remembered also, that if a very large amount of cold air sweeps through the burning fuel, or between it and the bottom of the boiler, it will most materially reduce the temperature of the surrounding iron, and will also tend to carry the hot gases up into the chimney. The fear of this seems to be continually before the minds of our engine-makers, and they sometimes will make a furnace of a cylindrical form seven or eight feet long, and only about two feet in diameter; and this long box the stoker will almost fill with coal, and then shut the door as tight as possible, just as though the object were to imitate a gas retort. If they could succeed perfectly in this endeavour, unignited gas would assuredly stream up the chimney, generating no heat, and eventually the fire itself would go out for want of the great supporter of combustion. But this of course is never attained in practice; air does find access, the hydrogen of the gas is consumed, but the widowed carbon has little chance of meeting with another partner, and so in a sad black stream the sooty particles are borne up the flue and into the surrounding air. Disgust of neighbours, prosecution, fine, follow; but in vain, the furnace does not feel the punishment, the tall chimney smokes ruthlessly as before; renewed disgust, and further prosecution and heavier fine threaten its unhappy owner, and he turns doubtfully to those who with clamorous voices profess their ability to cure the giant evil.

Sir Henry de la Beche and Dr. Lyon Playfair presented, in March, 1845, to the House of Commons a Report, in which they ascribed the nuisance complained of to the three following

causes:

1st. The want of proper construction and adjustment between the fireplaces and the boiler, and the disproportionate size of the latter to the amount of work which they are expected to perform.

'2nd. The deficiency of draft and imperfect construction of the flues leading to a chimney of inadequate height or capacity.

Opposite Ideas of a Stoker's Duty.

55

3rd. The carelessness of stoking and management by those intrusted with the charge of the fireplaces and boilers.'

To take the last of these first into consideration, an improvement in the mode of 'firing the furnaces' might be often easily effected; and on that subject Mr. Williams has many judicious remarks. It is impossible for a stoker to do credit to himself if he have to feed a furnace which is radically defective in its construction; but a man, by his inefficiency, will often impede the proper working of a good furnace. There are stokers who seem to think that their whole duty consists in shovelling coals into the flue as rapidly as they can, and heaping them up inside, just where it will give them least trouble; that is, immediately within the door, and midway between the sides. Now, this is just the wrong place to put the coals; the bars ought to be equally covered; and as the combustion goes on most rapidly (in ordinary cases) at the sides, and at the further end, attention should be especially paid to throw fresh fuel into these places. Then, again, no heaping up ought ever to be allowed, for it narrows the passage which the air can traverse, and so produces imperfect combustion. The throwing of much coal on at a time, or in great lumps, is another vicious practice. In Cornwall they manage these things well, as the following quotation from Tredgold will show.*

The mode of firing adopted in Cornwall is spreading the charge of fuel equally and thinly over the fire, and feeding the fire frequently with small quantities at a time, and with coal broken into small pieces. It is, in fact, merely a return to the method recommended by Smeaton and Watt. The former in his direction for working the York Waterworks engine, August 29, 1785, says:- Break every coal that is bigger than a goose's egg, and the oftener you fire, and the thinner, the better. The fire should be kept an equal thickness, and free from open places or holes, which are extremely prejudicial, and should be filled up as soon as they appear."'

The following narrative by Mr. Lowe, as given in the Society of Arts' Journal, may be interesting in illustration of this point:

'He had often occasion to cross in the steamer from Holyhead to Kingstown, and he had remarked the splendid bow of carbon which the Columbia left in her wake from port to port. He had pointed it out one day to the captain of the vessel, and he had been permitted to try the experiment of stoking three of the furnaces every ten minutes instead of every twenty minutes, and by leaving the furnace-door on the latch instead of closed tight; the result was that the smoke all

* As quoted in Mr. Williams's Essay.

disappeared; and from that day to this the engines of the Columbia were enabled to make twenty-one revolutions per minute instead of nineteen, and no black smoke was ever seen from her funnel to pave the sky with carbon from port to port.'

Several mechanical contrivances have been devised for feeding the fires regularly without the aid of the human stoker, who may be careless or negligent, even if he be thoroughly instructed. We may mention Brunton's revolving grate, and Stanley's selffeeding apparatus, and Juckes's, which is thus described:

'An arrangement of endless chains which, being caused to revolve upon cylinders, stretch the chains as it were from one end of the furnace space to the other; such chains being formed of links a few inches long, constitute in themselves the fire bar surface of the furnace. Over the outward end is fixed a contrivance for feeding the apparatus with small coal, and motion being given to the machine, the fuel is carried gradually onward.'

This plan has been adopted at Messrs. Truman and Hanbury's with success and economy, although large brewers' vats are much more difficult to deal with than ordinary boilers. The annealing furnaces of the Royal Mint have also been improved by Juckes's chain-apparatus. We remember seeing this arrangement, or something very similar, at the Printing-office of Messrs. Chambers in Edinburgh, and as we gazed at the small pieces of coal on the endlessly revolving chains, we saw the appearance, which is poetically described in the Prize Essay as that of a bed of crocus flowers-the flame rising in numerous ' detached vertical jets over the whole surface of the thin bed of fuel, by reason of the air passing upwards through it in small streams.' 'All these systems, however, are incompatible with the requirements of heavy charges and more active firing.'

But the mischief which arises from bad stoking is insignificant as compared with the far less remediable mischief caused by faultiness of construction in the furnace and flue. It will be readily seen that, for the complete combustion of a certain quantity of coal, a certain large quantity of air is required. Thus in the blast furnaces for the reduction of iron ore, the air that enters by the twyers is computed as equal in weight to all the iron-stone coal and limestone taken together that is thrown into the fiery cavern, equal in weight we say, we leave our readers to imagine how many times as much in bulk. Now, we have already seen that, where the gases from the coal are not mixed with a sufficiency of oxygen, they are only partially burnt, and a deposit of black carbon results. There must, indeed, be enough

Air and Hydrocarbons.

57

air to do something more than ignite the gases, for, to use Mr. Williams's expression

Flame is not the combustion of the gas. Flame itself has to undergo a further process of combustion, being but a mass of carbon atoms still unconsumed, though at the temperature of incandescence and high luminosity. Flame is, then, but one of the stages of the process of combustion. Its existence marks the moment, as regards each atom, of its separation from and the combustion of its accompanying hydrogen, by which so intense a heat is produced as instantaneously to raise the solid carbon atom, then in contact, to that high temperature: thus preparing it the more rapidly to combine with oxygen so soon as it shall have obtained contact with the air, but not a moment sooner.'

Now, it would not be very difficult to calculate the precise amount of air theoretically necessary for the combustion of a certain coal; but in practice a much larger quantity is indispensable, because the air never gets absolutely intermixed with the gases. The following statement of the late Professor Daniel will illustrate this point:

'With regard to the different forms of hydrocarbon, it is well known that the whole of the carbon is never combined with oxygen in the process of detonation, or silent combustion, unless a large excess of oxygen be present. For the complete combustion of olefiant gas, it is necessary to mix the gas with five times its volume of oxygen, though three only are consumed. If less be used, part of the carbon escapes combination, and is deposited as a black powder. Even subcarburetted hydrogen (our common coal-gas), it is necessary to mix with more than twice its bulk of oxygen, or the same precipitation will occur. It is clear, therefore, that the whole of the hydrogen of any of the compounds of carbon may be combined with oxygen, while a part of their carbon may escape combustion, and that even when enough of oxygen is present for its saturation.'

'That which takes place when the mixture is designedly made in the most perfect manner, must undoubtedly arise in the common process of combustion (in a furnace), where the mixture is fortuitous and much less intimate.'

It is absolutely necessary, therefore, that while a charge of coal is burning, there should pass through the furnace not merely the amount of air theoretically necessary for its combustion—namely, about 150 cubic feet for each pound, but a considerably larger quantity. A good deal will depend also on the way in which the air is introduced, so that it may become as well mixed as possible with the gaseous fuel. Now, instead of this being attended to, the door of the furnace is often shut directly the charge of coal is shovelled in, the only access of air

is through the bars of the grate, which are covered with an almost impermeable layer of clinkers, ashes, and fuel, and there is little space allowed between the heap of coal and the boiler, the bridge at the end of the furnace will not allow any large quantity of gaseous products of combustion to pass, and the chimney itself is not large enough for the whole expanded volume of carbonic acid gas, steam, and nitrogen, nor is it sufficiently tall or sufficiently heated to produce the necessary draft. Now, any one of these errors of construction is enough to impede the proper combustion and produce smoke. As to the exact dimensions requisite for all these different parts of a furnace, we are not sufficiently acquainted with practical engineering to form any correct opinion, or to offer any adequate advice; nor, indeed, would it be possible for any man to lay down precise rules applicable in every instance. There are, however, a series of very valuable remarks on the subject in the Prize Essay, arranged under the following heads:

'1st. Of the chamber of the furnace and the area above the fuel. 2nd. Of the ash-pit and the area below the fuel.

3rd. Of the means and mode by which the air should be admitted to the gas in the chamber.

4th. Of the required quantity of air in reference to the draft.

5th. Of the passages through which the products of combustion are carried away.

6th. Of the distance, length of flue, or run, along which the products have to travel.'

From this part of the work we shall merely quote, in a somewhat condensed form, and omitting the wood-cuts, a case given by the author, which will illustrate at once some of the causes of the disease, their diagnosis, and means of cure.

'Messrs. Crossfield and Company, large sugar-refiners, having been convicted for a smoke nuisance, employed a person who had previously been successful in the application of the perforated air-distributors. In this case, however, he was at fault. On the admission of the

air through the perforated box in the door, . . . the gases were effectually consumed, and the generation of smoke prevented. This was, however, accompanied by a diminished supply of steam. Here was a mystery which could not then be solved. In this state of things, the active partner, Mr. Barrow, applied to the writer of this Essay. . . . On examining the furnace, one cause of failure was apparent. The boiler was cylindrical, twenty-four feet long by six feet six inches in diameter, containing two cylindrical flues of two feet seven inches. In each of these was a furnace of seven feet long, thus giving a grate surface of eighteen square feet. With so large a furnace the evolution of gas from each charge of coal was necessarily large, and requiring a large quantity of air for its combustion, with a commensurate throat