ble to have the supply of air to the furnace as abundant as possible, it should be made as large as can be done without causing waste, by allowing the coal to fall through into the ash-pit. A greater number of thin bars is thus to be preferred to a smaller number of broad or thick bars; indeed, to such an extent is this carried in France, where coal is more valuable than in this country, and the chemistry of the subject perhaps more generally understood, that the bars are made not more than half an inch thick, the necessary strength being obtained by making them four inches deep. With coke or wood, which cannot fall through the bars and be wasted, in the same way as coal, the space is always made much wider, and with great advantage; so much so with coke, as to have led to the opinion that a given quantity of coke would produce as much heat by its combustion as the coal from which it was made. Any grounds for such an opinion could only have arisen from the combustion of the coal having been so imperfect, that not only had the whole of the gases passed off unconsumed, but even a large portion of the solid carbon must have been allowed to escape in the form of carbonic oxide, without having generated its due amount of heat, and been converted into carbonic acid gas.

In the combustion of coke, or of the solid portion of coal, as left in an incandescent state on the fire-bars of a common furnace, after the volatile gases have passed off, the amount of heat generated by the whole of the carbon, uniting at once with its full amount of oxygen, will be the same as what would be generated by its combination, first, with a smaller quantity of oxygen, forming carbonic oxide; and subsequently, by the ignition of this gas, by its combination with the further quantity of oxygen required to turn it into carbonic acid gas.

As some portion of the carbon is always converted into carbonic acid gas in the furnace, it follows, that the air for the ignition of any carbonic acid there formed, and allowed to pass into the flues, must be greatly in excess of the quantity chemically required; and the whole of this excess must be raised to the temperature of the other gases, with which it will be mingled. The superior economy, therefore, of at once converting the whole of the carbon into carbonic acid gas, is apparent; and there is no doubt but that this very desirable. result may be obtained nearly to the full extent, by due care in the formation and subsequent management of the furnace.

The best mode of supplying air to the other inflammable gases resulting from the combustion of bituminous coal, which are composed of hydrogen and carbon, and which will be treated of under the common name of carburetted hydrogen, has been a matter of much controversy, and been the subject of many patents. The mode proposed by the greater number of the patentees is, to admit the air immediately behind the furnace, at the back of what is termed the bridge. A bridge does not exist in every case; but where it does exist, it is generally in the form of a wall or obstruction right across the back of the furnace; often placed there for no other purpose than to prevent the fire from being pushed back into the flue, The whole

of the products of combustion, as formed in the furnace, necessarily pass over this bridge, before entering the flue. The additional air is sometimes heated, previously to its being admitted to the gases, after they have left the furnace, and the manner in which it is supplied varies exceedingly; one party advocating its admission in a long thin film, another through a great number of small orifices, and others again attach less importance to the manner of its admission, so that it is only admitted in sufficient quantity. All these plans proceed upon the supposition that large quantities of inflammable gas pass off from the furnace, and as none of them directly affect the operations going on within the furnace itself, the gases which are actually given off would be the same until they pass over the bridge, whichever plan might be adopted.

These plans must all cease to be necessary or useful, if a furnace can be so constructed, and the combustion of the coal in it so inanaged, that a very small proportion only of uncombined inflammable gases would pass off, as in this case no economy would result from their combustion, owing to the large excess of air which must be supplied and heated as before explained.

The admission of a large quantity of air into the flue, at a distance from the furnace, though advocated by some authorities, cannot be advantageous, unless in extreme cases, when the temperature in the flue is very high, and where the combustion in the furnace has been more than usually imperfect.

As the carburetted hydrogen gases are generated rapidly, on the application of heat to the coal, and are in themselves much lighter than the carbonic acid gas, or the nitrogen gas, formed at the same time, it is sometimes assumed that they rise nearly unmixed to the top of the space over the furnace, and thence it is considered more advantageous to supply the air at this place than in the flue. The cooling effect of air, if admitted into the furnace, has been stated to be more injurious than if admitted into the flue; but the correctness of this statement may be doubted, especially if the gases be unmixed, as this would render a much less quantity of air sufficient.

The bars in this case should be placed at least two and a half feet or three feet below the boiler, or the crown of the furnace, to allow the principle to be more fully carried out. An increase of space over the bars to this extent has always been found to be advantageous, and ought to be particularly attended to. The system of admitting the air to the gases in a subdivided form, in whatever part of the boiler the admission of it may take place, is very efficacious in procuring a thorough and speedy mixture of the particles. It has been very extensively and successfully introduced by Mr. C. W. Williams in supplying air behind the bridge of the furnace.

An opinion is entertained that a sufficient supply of air for the gases may be obtained through the fire-bars; and it is obvious that a partial supply, at least, may be obtained in this manner, by a judicious management of the fire. This may be effected by keeping the fires thin and open, feeding by small quantities at a time, or by a system of coking the coal, allowing the combustion of it to be slow at first,

by which means the coal is formed into masses of coke, between which the air has a passage. The air which passes through is not vitiated further than in being mechanically mixed with the carbonic acid and nitrogen gases, caused by the combustion of the coal on the bars.

The perfect combustion of the whole ingredients of coal being entirely dependent, chemically considered, on the supply of the due quantity of atmospheric air, it is evident that the velocity with which the air flows into the fire will materially affect the result. According as this velocity is greater or less, so in proportion must the quantity of coal that is to be consumed on a given area of grate be increased or diminished, and there is no limit to the quantity that may be so consumed, beyond the difficulty of supplying the air sufficiently rapid. The various circumstances which affect the velocity of the entering air, have placed this question, as yet, completely beyond the reach of theory, so that practical experiments must be taken as the only guide, in determining what quantity of air can be made to enter into a given furnace, and, consequently, what amount of coal can be properly consumed in a given time.

Mr. Parkes has stated, as the result of a long series of experiments made by him, (vide Trans. Inst. C. E., vol. iii.,) that the rate of combustion should not exceed seven pounds per superficial foot of grate bar per hour, and that this quantity may with advantage be reduced as low as four pounds, or even three pounds. General experience would tend to prove that these latter quantities are unnecessarily low, and can only be advantageous when the arrangements for supplying the air, or for carrying off the products of combustion, are defective or inefficient. It is evident that if the area of any part of the passage, for either of these currents, be too limited, the velocity at this contracted spot cannot rise higher than is due to the weight of the ascending column of heated gases in the chimney. The quantity passing through is therefore diminished in proportion as the area is limited; and a good draught at a particular place, as at the bridge of a boiler, may here be quite compatible with an insufficient supply of air, and imperfect combustion of the coal. The draught in every other part of the flues is, at the same time, rendered slow and languid, and deposition of soot takes place in them. This fault is apparent in a great number of boilers at present in use, and in some cases, especially in tubular boilers, it is attended with very injurious results, by stopping up the tubes, and decreasing the amount of heating surface. to such an extent, as to render the boilers incapable of generating the required amount of steam.

The furnaces of the boilers in general use in Cornwall, are upon the common principle of construction, and as in them it is not usual to apply any of the peculiar patented arrangements for the supply of air to the gases, behind the bridge, it follows, either that these gases are not consumed, or that they are consumed by air admitted through the bars. In the Cornish system of raising steam, slow combustion is adopted in its fullest extent; the fires are kept thin and open, the fuel is supplied in small quantities and frequently, and it is well

spread over the whole surface. As the result is highly favorable in the economy of fuel, it may be presumed that the combustion of the gases, as well as of the solid carbon, is comparatively perfect. When more air is admitted into the furnace than can be made to enter through the bars, it is generally done by apertures in the furnace doors.

The average rate of combustion throughout the country is far above even the largest quantity named by Mr. Parkes, and may be stated to be about thirteen pounds per superficial foot of grate per hour. With due care in the construction of the furnaces and flues, there seems to be no reason why this quantity may not be as perfectly consumed, and the heat as thoroughly extracted from the products of combustion, before they leave the boiler, as with the smaller quantity. Whether this be so or not, it is necessary in practice to prepare for many cases, as on board of steam-vessels, where it is impossible to allow a larger amount of fire grate, or boiler room, and when it would cease to be ultimately economical to obtain a small saving of fuel, by great additional expense in boilers and their fittings, and in space for them.

To determine the velocity with which the products of combustion pass off from the furnace, or from the boiler, is attended with much difficulty, on account of the great number of extraneous circumstances which do so easily and so constantly affect it. Some experiments on this subject were made by Dr. Ure, and an account of them was read before the Royal Society, (read June 16, 1836,) when he stated, that he considered the velocity might be taken at about thirty-six feet per second, and as this result has been corroborated by others, it may be assumed, in the absence of better data, as nearly correct.

The subject, in a theoretical point of view, is surrounded by many difficulties-in discovering the allowance which must be made for friction, and other circumstances, similar to those affecting the flow of water through pipes; and though this latter has engaged much more of the attention of scientific men, no very definite results, to bear accurately upon practice, can yet, even in this case, be obtained by calculation.

The practical question of the proper proportions of the different parts of boilers is then proceeded with in the paper, the leading chemical and physical features connected with the combustion of coal in their furnaces having been considered.

The supply of the requisite quantity of air to the fuel on the bars being of the utmost importance, it is usual to make the ash-pit, and the entrance to it, as large and as free as the situation will allow. In marine boilers, or wherever it is necessary to limit the size of the ashpit, the area for the entrance of the air into it should never be less than one-fourth part of the area of the grate; and in order to facilitate the supply to the back part of the grate, the bars should be made to incline downwards to the extent of about one inch in a foot. No advantageous results will be obtained from increasing the ash-pit, as is sometimes done in land boilers to a very great extent, by making it five or six feet deep; about two and a half feet is sufficiently deep,

even supposing that the ashes are not cleared out oftener than once a day.

The extent of "dead plate" in front of the furnace is not material, as respects combustion; in marine boilers it is generally not more than about six inches broad, which is the width of the water space between the fire and the front of the boiler; but in land boilers it is frequently required to be very broad, to support the brickwork, especially in those cases where the flue is carried across the front.

The amount of the opening between the bars should be about seven-sixteenths of an inch, but this must be regulated by the kind of coal to be burnt upon them; but for any kind of coal, it should not be less than three-eighths of an inch, nor more than half an inch. If the space were made larger, the waste from the amount of cinders, or of small pieces of coke, which would fall through in a state of incandescence, would be considerable; otherwise it would be preferable to have a larger space. In order to facilitate the supply of air, each bar should be as thin as is consistent with the strength required. The bars in general use in this country are one inch or one and oneeighth inch in thickness, but it would be much more advantageous to use them thinner, as in France, where they are frequently used not more than half an inch thick.

The advantage of a considerable amount of space in the furnace, over the fire-bars, has been already mentioned, but no very decisive experiments have been made on this subject. Three cubic feet of space to each superficial foot of grate bar surface, may be stated as a good proportion, where there is nothing to prevent this amount being obtained. When the space is reduced below one foot and a half of grate, it will be found to be attended with a marked disadvantage.

The area of the flue, and subsequently of the chimney, through which the products of combustion must pass off, must be regulated by their bulk and their velocity. The quantity of air chemically required for the combustion of one pound, of coal, has been shown to be 150.35 cubic feet, of which 44.64 enter into combination with the gases, and 105.71 with the solid portion of the coal. From the chemical changes which take place in the combination of the hydrogen with oxygen, the bulk of the products is found to be to the bulk of the atmospheric air required to furnish the oxygen, as 10 is to 11. The amount is therefore 49.104. This is without taking into account the augmentation of the bulk, due to increase of the temperature. In the combination which takes place between the carbon and the oxygen, the resultant gases (carbonic acid gas and nitrogen gas) are of exactly the same bulk as the amount of air, that is, 105.71 cubic feet, exclusive, as before, of the augmentation of bulk from the increase of temperature. The total amount of the products of combustion in a cool state would therefore be 49.104+105.71=154.814 cubic feet.

The general temperature of a furnace has not been very satisfactorily ascertained, but it may be stated at about 1000° Fah., and at this temperature the products of combustion would be increased, according to the laws of the expansion of aëriform bodies, to about