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 incom'patible 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

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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 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

the

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

History of a Refractory Furnace.

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area for the discharge of the products. . . . . The area should have been 450 square inches; instead of these proportions, however, the actual area was found to be but 189 square inches-equal to ten and a-half square inches for each square foot of grate surface, instead of twenty-five. This area was manifestly so small that it was impossible that even one-half the volume of heated products could be discharged through it with any ordinary draft.

The consequence of this restriction might easily have been predicted, since, when the due quantity of air was admitted to the gases in the chamber, a proportionate abstraction from the supply by the ash-pit must be the inevitable consequence, thus causing a diminished action in the furnace, less fuel to be consumed, and less steam generated. In this case there were three modes of relief; either adequately to enlarge the throat areas-to reduce the area of the grate surface-or, to diminish the quantity of products passing over the bridge. The first was impracticable, from the smallness of the furnace and its semi-circular form. The second would have caused too serious a reduction in the quantity of steam produced. The third plan was adopted by admitting the supply of air to the gases through a perforated distributor placed in the bridge, thus in effect relieving the throat-area from the products arising from the combustion of the gas.

"This mode of relief being applied, the advantage, however, was scarcely perceptible, and the evil of a reduced pressure of steam still continued..... On further examination, a still greater source of error was discovered. This was a remarkably contracted area of exit in the flue leading from the boiler to the main chimney-stack. Although each of the two furnaces had eighteen square feet of grate surface, and which, at twenty-five square inches for each, would have required an area of exit of 450 square inches, this area was actually contracted to 126 square inches.

This area was then, but with some difficulty, enlarged, when it equalled 240 inches. This, however, was still too small for such large furnaces. Although considerable relief to the exit of the products was thus obtained, its effect was nevertheless unsatisfactory, and even intermittent,—a circumstance which still remained to be accounted for. Not deterred by these difficulties, a further examination was made. The proprietor had observed that occasionally the hot products from some one of the furnaces, instead of ascending the stack, which was eighty feet high, appeared to influence the draft of some others of them, forcing back, as it were, the hot products out of their doors. . . . Having with difficulty obtained access to the base of the stack, the evil was at once manifested. The interior area of the base was but five feet diameter, into which four apertures were made for the exit of the products of the two steam-boiler furnaces and two large charcoal heating stoves. These openings being opposite each other, it was evident that the products from each would be projected directly against those of the one opposite, thus acting the part of a damper on its issue, and necessarily diminishing the draft, the stronger overpowering

the weaker. The remedy was that suggested by Peclet, and well understood by engineers in this country, namely, the interposing diaphragms or cross walls, thus giving to each outlet an independent vertical action.

This being effected, a sufficient increase of draft was produced, and the gases were consumed without the recurrence of the diminution of steam, although the area of exit still remained manifestly too small to do justice to such large furnaces.'

There is one aspect of this question which, as far as we know, has never received that attention to which it is entitled. Carbon will combine with only half the quantity of oxygen requisite to form carbonic acid, and produce another invisible gas, well known to chemists by the name of carbonic oxide. Now we do not know whether the cloud of smoke issuing from a refractory furnace contains any of this gas, but we believe it is produced in locomotives, and we have some recollection of seeing the blue flame indicative of it playing about some blast-furnaces up the Swansea valley. Besides, where carbonic acid passes over carbon at a red heat, there are precisely the conditions for its production. Now this carbonic oxide used to be considered a very gentle and harmless member of the community of gases, but lately it has been convicted of serious delinquencies, and its character has sunk in chemical estimation. Nay, so insidious has been this wicked gas, that many of its evil doings have been attributed to its brother carbonic acid. Thus we strongly suspect that many of the cases of suffocation from sleeping in rooms heated by charcoal-braziers are due to this cause. Take the following anecdote as an illustration of its deleterious effects.

On the 18th June, 1851, M. Dupuis-Delcourt made a scientific ascent in a balloon with several newly-invented philosophical instruments. Having only a small balloon at his disposal, he proposed filling it with hydrogen, from the works of M. Selligue. It was intended to prepare this gas by passing steam over iron, but the workpeople during the night, to save themselves trouble, substituted charcoal in place of the iron. M. Delcourt ascended, though he found the balloon not so buoyant as he anticipated. At the height of about 5000 feet, the rarefaction of the air and the influence of the sun dilating the gas, he let some of it out by the lower part of the balloon. The aeronaut instantly felt too ill to continue his observations, and speedily became insensible, while the balloon, continuing to empty, descended, and he was roused from his stupor by the people who received him on the ground. More than three hundred persons having assembled, and the wind beginning to beat the balloon down, four or five became entangled in the ropes; M. Delcourt not wishing the