a dense elastic vapour arising from burning bodies. As this vapour is extremely disagreeable, and often prejudicial to health, several contrivances have been devised for the purpose of enabling us to enjoy the benefit of a fire without being annoyed by smoke; hence the use of chimneys, which, when properly constructed, carry it off entirely, but when improperly constructed, allow a part of it to escape into the room.

Chimneys operate in carrying off smoke by the rarity of the gaseous fluids which escape from the fire, and which, owing to their being expanded, are lighter than the adjoining air, and are therefore pressed up by it, carrying the smoke along with them. So long, then, as the gaseous products of the fire are lighter than the air, they ascend, while at the same time cold air must rush in from below to supply their place, which air, being heated in its turn, consumed, and mixed with the smoke, must also rise. The more, therefore, that the air in the chimney can be kept warm, so much the better; not only will the draught become greater, but there will also be less chance of any of the smoke being thrown back. The worse the power of the material of which the chimney is constructed for transmitting heat, the longer will the air within it be kept warm, and therefore the chimney will work better; hence the superior efficacy of bricks.

It is evident from what has been said, that there is a certain height to which chimneys may be carried, beyond which any further lengthening, instead of being beneficial, will prove injurious; because, when the air within the chimney and without become of the same weight, which must be at a particular height in each, then if that chimney be made higher, the air within does not escape, or escapes slowly; air must therefore enter in slowly from below, and consequently the draught must be diminished. There are many circumstances which prevent chimneys from working well, or in other words, which make them return a part of the smoke into the room.

The causes of the smoking of chimneys may be reduced to nine, differing from each other, and therefore requiring different remedies.

1. Smoky chimneys in a new house are such frequently from mere want of air. The workmanship of the rooms being all good, and just out of the workman's hands, the joints of the boards of the flooring, and of the panels of wainscoting, are all true and tight; the more so as the walls, perhaps not yet thoroughly dry, preserve a dampness in the air of the room which keeps the wood-work swelled and close. The doors and the sashes too, being worked with truth, shut with exactness, so that the room is as tight as a snuff-box, no passage being left open for air to enter except the key-hole, and even that is sometimes covered by a little dropping shutter. Now if smoke cannot rise but as connected with rarefied air, and a column of such air, suppose it filling the funnel, cannot rise unless other air be admitted to supply its place; and if therefore no current of air enter the opening of the chimney, there is nothing to prevent the smoke from being diffused in the room. If the motion upwards of the air in a chimney that is freely supplied be observed by the rising of the smoke or a feather in it, and it be considered that in the time such feather takes in rising from the fire to the top of the chimney, a column of air equal to the content of the funnel must be discharged, and an equal quantity supplied from the room below, it will appear absolutely impossible that this operation should go on if the tight room is kept shut; for were there any force capable of drawing constantly so much air out of it, it must soon be exhausted like the receiver of an air-pump, and no animal could live in it. Those therefore who stop every crevice in a room to prevent the admission of fresh air, and yet would have their chimney carry up the smoke, require inconsistencies, and expect impossibilities. Yet under this situation it is not uncommon to see the owner of a new house in despair, and ready to sell it for much less than it cost; conceiving it uninhabitable because not a chimney in any one of its rooms will carry off the smoke unless a door or window be left open. Much expense has also been incurred to alter and amend new chimneys which had really no fault. In one house particularly which Dr. Franklin knew that belonged to a nobleman in Westminster, that expense amounted to no less than L300, after his house had been, as he thought, finished, and all charges paid. And after all, several of the alterations were ineffectual, for want of understanding the true principles.

Remedies. When you find on trial that opening the door or a window enables the chimney to carry up all the smoke, you may be sure that want of air from without is the cause of its smoking. "I say from without," adds Dr. Franklin, "to guard you against a common mistake of those who may tell you the room is large, contains abundance of air sufficient to supply any chimney, and therefore it cannot be that the chimney wants air. These reasoners are ignorant that the largeness of a room, if tight, is in this case of small importance, since it cannot part with a chimneyfull of its air without occasioning so much vacuum; which it requires a great force to effect, and could not be borne if effected."

It appearing plainly then, that some of the outward air must be admitted, the question will be, how much is absolutely necessary? for you would avoid admitting more, as being contrary to one of your intentions in having a fire, namely, that of warming your room. To discover this quantity, shut the door gradually while a middling fire is burning, till you find that before it is quite shut the smoke begins to come out into the room; then open it a little till you perceive the smoke comes out no longer. There hold the door, and observe the width of the open crevice between the edge of the door and the rabbet into which it should shut. Suppose the distance to be half an inch, and the door eight feet high; you thence find that your room requires an entrance for air equal in area to ninety-six half inches, or forty-eight square inches, or a passage of six inches by eight. This, however, is a large supposition, there being few chimneys that, having a moderate opening and a tolerable height of funnel, will not be satisfied with such a crevice of a quarter of an inch. Dr. Franklin found a square of six by six, or thirty-six square inches, to be a pretty good medium that will serve for most chimneys. High funnels with small and low openings may indeed be supplied through a less space; because, for reasons that will hereafter appear, the force of levity, if one may so speak, being greater in such funnels, the cool air enters the room with greater velocity, and consequently more enters in the same time. This, however, has its limits; for experience shews, that no increased velocity so occasioned has made the admission of air through the key-hole equal in quantity to that through an open door, though through the door the current moves slowly, and through the key-hole with great rapidity.

It remains then to be considered how and where this necessary quantity of air from without is to be admitted so as to be least inconvenient; for if at the door, left so much open, the air thence proceeds directly to the chimney, and in its way comes cold to your back and heels as you sit before your fire. If you keep the door shut, and raise a little the sash of your window, you feel a similar inconvenience. Various have been the contrivances to avoid this, such as bringing in fresh air through pipes in the jams of the chimney, which pointing upwards should blow the smoke up the funnel; opening passages in the funnel above, to admit air for the same purpose. But these produce an effect contrary to that intended; for as it is the constant current of air passing from the room through the opening of the chimney into the funnel which prevents the smoke from coming out into the room, if you supply the funnel by other means or in other ways with the air which it requires, and especially if that air be cold, you diminish the force of that current, and the smoke in its efforts to enter the room finds less resistance.

The required air must then indispensably be admitted into the room, to supply what goes off through the opening of the chimney. M. Gauger, a very ingenious and intelligent French writer on the subject, proposes with judgment to admit it above the opening of the chimney; and to prevent inconvenience from its coldness, he directs that it may be so made, that it shall pass in its entrance through winding cavities made behind the iron back and sides of the fire-place, and under the iron hearth-plate; in which cavities it will be warmed, and even heated, so as to contribute much, instead of cooling, to the warming of the room. This invention is excellent in itself, and may be used with advantage in building new houses, because the chimneys may then be so disposed as to admit conveniently the cold air to enter such passages. But in houses built without such views, the chimneys are often so situated as not to afford that convenience without great and expensive alterations. Easy and cheap methods, though not quite so perfect in themselves, are of more general utility; and such are the following.

In all rooms where there is a fire, the body of air warmed and rarefied before the chimney is continually changing place, and making room for other air that is to be warmed in its turn. Part of it enters and goes up the chimney, and the rest rises and takes place near the ceiling. If the room be lofty, that warm air remains above our heads as long as it continues warm, and we are little benefited by it, because it does not descend till it is cooler. Few can imagine the difference of climate between the upper and lower parts of such a room, who have not tried it by the thermometer, or by going up a ladder till their heads are near the ceiling. It is then among this warm air that the requisite quantity of outward air is best admitted, with which being mixed, its coldness is abated, and its inconvenience diminished so as to become scarcely observable. This may be easily done by drawing down about an inch the upper sash of a window; or, if not moveable, by cutting such a crevice through its frame; in both which cases it will be well to place a thin shelf of the length to conceal the opening, and sloping upwards, to direct the entering air horizontally along and under the ceiling. In some houses the air may be admitted by such a crevice made in the wainscot, cornice, or plastering, near the ceiling and over the opening of the chimney. This, if practicable, is to be chosen, because the entering cold air will there meet with the warmest rising air from before the fire, and be soonest tempered by the mixture. The same kind of shelf should also be placed here. Another way, and not a very difficult one, is to take out an upper pane of glass in one of your sashes, set it in a tin frame, giving it two springing angular sides, and then replacing it, with hinges below, on which it may be turned to open more or less above. It will then have the appearance of an internal sky-light. By drawing this pane in, more or less, you may admit what air you find necessary. Its position will naturally throw that air up and along the ceiling. In England some have of late years cut a round hole about five inches diameter in a pane of the sash, and placed against it a circular plate of tin hung on an axis, and cut into vanes; which, being separately bent a little obliquely, are acted upon by the entering air, so as to force the plate continually round like the vanes of a windmill. This admits the outward air, and by the continual whirling of the vanes, does in some degree disperse it. The noise only is a little inconvenient.

2. A second cause of the smoking of chimneys is, their openings in the room being too large; that is, too wide, too high, or both. Architects in general have no other ideas of proportion in the opening of a chimney than what relate to symmetry and beauty respecting the dimensions of the room; while its true proportion respecting its function and utility depends on quite other principles; and they might as properly proportion the step in a staircase to the height of the storey, instead of the natural elevation of men's legs in mounting. The proportion then to be regarded, is what relates to the height of the funnel. For as the funnels in the different storeys of a house are necessarily of different heights or lengths, that from the lowest floor being the highest or longest, and those of the other floors shorter and shorter, till we come to those in the garrets, which are of course the shortest; and the force of draft being, as already said, in proportion to the height of funnel filled with rarefied air, and a current of air from the room into the chimney, sufficient to fill the opening, being necessary to oppose and prevent the smoke from coming out into the room; it follows, that the openings of the longest funnels may be larger, and that those of the shorter funnels should be smaller. For if there be a larger opening to a chimney that does not draw strongly, the funnel may happen to be furnished with the air which it demands by a partial current entering on one side of the opening, and leaving the other side free of any opposing current, may permit the smoke to issue there into the room. Much, too, of the force of draft in a funnel depends on the degree of rarefaction in the air it contains, and that depends on the nearness to the fire of its passage in entering the funnel. If it can enter far from the fire on each side, or far above the fire, in a wide or high opening, it receives little heat in passing by the fire, and the contents of the funnel are by those means less different in levity from the surrounding atmosphere, and its force in drawing consequently weaker. Hence, if too large an opening be given to chimneys in upper rooms, those rooms will be smoky. On the other hand, if too small openings be given to chimneys in the lower rooms, the entering air operating too directly and violently on the fire, and afterwards strengthening the draft as it ascends the funnel, will consume the fuel too rapidly.

Remedy. As different circumstances frequently mix themselves in these matters, it is difficult to give precise dimensions for the openings of all chimneys. Our fathers made them generally much too large. We have lessened them; but still they are often of greater dimensions than they should be, the human eye not being easily reconciled to sudden and great changes. If you suspect that your chimney smokes from the too great dimensions of its opening, contract it by placing moveable boards, so as to lower and narrow it gradually, till you find the smoke no longer issues into the room. The proportion so found will be that which is proper for that chimney, and you may employ the bricklayer or mason to reduce it accordingly. However, as in building new houses something must be sometimes hazarded, Dr. Franklin proposes to make the openings in the lower rooms about thirty inches square, and eighteen deep, and those in the upper only eighteen inches square, and not quite so deep; the intermediate ones diminishing in proportion as the height of the funnel is diminished. In the larger openings, billets of two feet long, or half the common length of cordwood, may be burned conveniently; and for the smaller, such wood may be sawed into thirds. Where coals are the fuel, the grates will be proportioned to the openings. The same depth is nearly necessary to all, the funnels being all made of a size proper to admit a chimney-sweeper. If in large and elegant rooms custom or fancy should require the appearance of a larger chimney, it may be formed of expensive marginal decorations, in marble, or in any thing else. But in time, perhaps, that which is fittest in the nature of things may come to be thought handsomest.

3. Another cause of smoky chimneys is too short a funnel. This necessarily happens in some cases, as where a chimney is required in a low building; for, if the funnel be raised high above the roof, in order to strengthen its draft, it is then in danger of being blown down, and crushing the roof in its fall.

Remedies. Contract the opening of the chimney, so as to oblige all the entering air to pass through or very near the fire; whereby it will be more heated and rarefied, the funnel itself be more warmed, and its contents have more of what may be called the force of levity, so as to rise strongly and maintain a good draft at the opening.

Or you may in some cases, to advantage, raise additional storeys over the low building, which will support a high funnel. If the low building be used as a kitchen, and a contraction of the opening therefore inconvenient, a large one being necessary, at least when there are great dinners, for the free management of so many cooking utensils; in such cases the best expedient perhaps would be to build two more funnels joining to the first, and having three moderate openings, one to each funnel, instead of one large one. When there is occasion to use but one, the other two may be kept shut by sliding plates, hereafter to be described; and two or all of them may be used together when wanted. This will indeed be an expense, but not an useless one, since your cooks will work with more comfort, see better than in a smoky kitchen what they are about, your victuals will be cleaner dressed, and not taste of smoke, as is often the case; and to render the effect more certain, a stack of three funnels may be safely built higher above the roof than a single funnel.

The case of too short a funnel is more general than would be imagined, and often found where one would not expect. For it is not uncommon, in ill-contrived buildings, instead of having a funnel for each room or fire-place, to bend and turn the funnel of an upper room, so as to make it enter the side of another funnel that comes from below. By these means the upper room funnel is made short of course, since its length can only be reckoned from the place where it enters the lower room funnel; and that funnel is also shortened by all the distance between the entrance of the second funnel and the top of the stack; for all that part being readily supplied with air through the second funnel, adds no strength to the draft, especially as that air is cold when there is no fire in the second chimney. The only easy remedy here is, to keep the opening of that funnel shut in which there is no fire.

4. Another very common cause of the smoking of chimneys is, their overpowering one another. For instance, if there be two chimneys in one large room, and you make fires in both of them, with doors and windows close shut, you will find that the greater and stronger fire shall overpower the weaker, from the funnel of which it will draw down air to supply its own demand; which air descending in the weaker funnel, will drive down its smoke, and force it into the room. If, instead of being in one room, the two chimneys are in two different rooms, communicating by a door, the case is the same whenever that door is open. In a very tight house, a kitchen chimney on the lowest floor, when it had a great fire in it, has been known to overpower any other chimney in the house, and draw air and smoke into its room as often as the door communicating with the staircase was opened.

Remedy. Take care that every room have the means of supplying itself from without with the air which its chimney may require, so that no one of them may be obliged to borrow from another, nor under the necessity of lending. A variety of these means has been already described.

5. Another cause of smoking is, when the tops of chimneys are commanded by higher buildings, or by a hill, so that the wind blowing over such eminences falls like water over a dam, sometimes almost perpendicularly on the tops of the chimneys that lie in its way, and beats down the smoke contained in them.

Remedy. That commonly applied in this case is a turn-cap made of tin or plate iron, covering the chimney above and on three sides, open on one side, turning on a spindle; and which, being guided or governed by a vane, always presents its back to the current. This may be generally effectual, though not certain, as there may be cases in which it will not succeed. Raising your funnels, if practicable, so as their tops may be higher, or at least equal, with the commanding eminence, is more to be depended on. But the turn-cap, being easier and cheaper, should first be tried. "If obliged to build in such a situation, I would choose," says Dr. Franklin, "to place my doors on the side next the hill, and the backs of my chimneys on the farthest side; for then the column of air falling over the eminence, and of course pressing on that below, and forcing it to enter the doors on that side, would tend to balance the pressure down the chimneys, and leave the funnels more free in the exercise of their functions."

6. There is another case which is the reverse of that last mentioned. It is where the commanding eminence is farther from the wind than the chimney commanded.

Remedy. There is but one remedy, which is to raise such a funnel higher than the roof, supporting it if necessary by iron bars. For a turn-cap in this case has no effect, the dammed-up air pressing down through it in whatever position the wind may have placed its opening.

Dr. Franklin mentions a city in which many houses are rendered smoky by this operation. For their kitchens being built behind, and connected by a passage with the houses, and the tops of the kitchen chimneys lower than the tops of the houses, the whole side of a street when the wind blows against its back, forms such a dam as above described; and the wind so obstructed forces down those kitchen chimneys, especially when they have but weak fires in them, to pass Smoke through the passage and house into the street. Kitchen chimneys so formed and situated have another inconvenience. In summer, if you open your upper-room windows for air, a light breeze blowing over your kitchen chimney towards the house, though not strong enough to force down its smoke as aforesaid, is sufficient to waft it into your windows, and fill the rooms with it, which, besides the disagreeableness, damages your furniture.

7. Chimneys, otherwise drawing well, are sometimes made to smoke by the improper and inconvenient situation of a door. When the door and chimney are on the same side of the room, if the door being in the corner is made to open against the wall, which is common, as being there, when open, more out of the way, it follows, that when the door is only opened in part, a current of air rushing in passes along the wall into and across the opening of the chimney, and flirts some of the smoke out into the room. This happens more certainly when the door is shutting, for then the force of the current is augmented, and becomes very inconvenient to those who, warming themselves by the fire, happen to sit in its way.

The remedies are obvious and easy. Either put an intervening screen, from the wall round great part of the fireplace; or, which is perhaps preferable, shift the hinges of your door, so that it may open the other way, and when open, throw the air along the other wall.

8. A room that has no fire in its chimney is sometimes filled with smoke which is received at the top of its funnel and descends into the room. Funnels without fires have an effect according to their degree of coldness or warmth on the air that happens to be contained in them. The surrounding atmosphere is frequently changing its temperature; but stacks of funnels covered from winds and sun by the house that contains them, retain a more equal temperature. If, after a warm season, the outward air suddenly grows cold, the empty warm funnels begin to draw strongly upwards; that is, they rarely the air contained in them, which of course rises, cooler air enters below to supply its place, is rarefied in its turn, and rises; and this operation continues till the funnel grows cooler, or the outward air warmer, or both, when the motion ceases. On the other hand, if after a cold season the outward air suddenly grows warm, and of course lighter, the air contained in the cool funnels, being heavier, descends into the room; and the warmer air which enters their tops being cooled in its turn, and made heavier, continues to descend; and this operation goes on till the funnels are warmed by the passing of warm air through them, or the air itself grows cooler. When the temperature of the air and of the funnels is nearly equal, the difference of warmth in the air between day and night is sufficient to produce these currents. The air will begin to ascend the funnels as the cool of the evening comes on, and this current will continue till perhaps nine or ten o'clock next morning, when it begins to hesitate; and as the heat of the day approaches, it sets downwards, and continues so till towards evening, when it again hesitates for some time, and then goes upwards constantly during the night, as before mentioned. Now when smoke issuing from the tops of neighbouring funnels passes over the tops of funnels which are at the time drawing downwards, as they often are in the middle part of the day, such smoke is of necessity drawn into these funnels, and descends with the air into the chamber.

The remedy is to have a sliding plate that will perfectly shut the offending funnel. Dr. Franklin has thus described it. "The opening of the chimney is contracted by brickwork faced with marble slabs to about two feet between the jams, and the breast brought down to within about three feet of the hearth. An iron frame is placed just under the breast, and extending quite to the back of the chimney, so that a plate of the same metal may slide horizontally backwards and forwards in the grooves on each side of the frame. This plate is just so large as to fill the whole space, and shut the chimney entirely when thrust quite in, which is convenient when there is no fire. Draw it out, so as to leave between its further edge and the back a space of about two inches; this space is sufficient for the smoke to pass; and so large a part of the funnel being stopped by the rest of the plate, the passage of warm air out of the room, up the chimney, is obstructed and retarded; and by those means much cold air is prevented from coming in through crevices, to supply its place. This effect is made manifest three ways. 1. When the fire burns briskly in cold weather, the howling or whistling noise made by the wind, as it enters the room through the crevices, when the chimney is open as usual, ceases as soon as the plate is slid in to its proper distance. 2. Opening the door of the room about half an inch, and holding your hand against the opening, near the top of the door, you feel the cold air coming in against your hand, but weakly, if the plate be in. Let another person suddenly draw it out, so as to let the air of the room go up the chimney, with its usual freedom where chimneys are open, and you immediately feel the cold air rushing in strongly. 3. If something be set against the door, just sufficient, when the plate is in, to keep the door nearly shut, by resisting the pressure of the air that would force it open; then, when the plate is drawn out, the door will be forced open by the increased pressure of the outward cold air endeavouring to get in to supply the place of the warm air that now passes out of the room to go up the chimney. In our common open chimneys, half the fuel is wasted, and its effect lost; the air it has warmed being immediately drawn off."

9. Chimneys which generally draw well, do nevertheless sometimes give smoke into the rooms, it being driven down by strong winds passing over the tops of their funnels, though not descending from any commanding eminence. This case is most frequent where the funnel is short, and the opening turned from the wind. It is the more grievous, when it happens to be a cold wind that produces the effect, because when you most want your fire you are sometimes obliged to extinguish it. To understand this, it may be considered that the rising light air, to obtain a free issue from the funnel, must push out of its way or oblige the air that is over it to rise. In a time of calm or of little wind this is done visibly; for we see the smoke that is brought up by that air rise in a column above the chimney. But when a violent current of air, that is, a strong wind, passes over the top of a chimney, its particles have received so much force which keeps them in a horizontal direction, and follow each other so rapidly, that the rising light air has not strength sufficient to oblige them to quit that direction and move upwards to permit its issue.

Remedies. In Venice, the custom is to open or widen the top of the flue, rounding it in the true form of a funnel. In other places the contrary is practised; the tops of the flues being narrowed inwards, so as to form a slit for the issue of the smoke, as long as the breadth of the funnel, and only four inches wide. This seems to have been contrived on a supposition that the entry of the wind would thereby be obstructed; and perhaps it might have been imagined, that the whole force of the rising warm air being condensed, as it were, in the narrow opening, would thereby be strengthened, so as to overcome the resistance of wind. This, however, did not always succeed; for when the wind was at north-east and blew fresh, the smoke was forced down by fits into the room where Dr. Franklin commonly sat, so as to oblige him to shift the fire into another. The position of the slit of this funnel was indeed north-east and southwest. Perhaps if it had lain across the wind, the effect might have been different. But on this we can give no certainty. It seems a matter proper to be referred to experi- ment. Possibly a turncap might have been serviceable, but it was not tried.

With all the science, however, that a man shall suppose himself possessed of in this article, he may sometimes meet with cases that shall puzzle him. "I once lodged," says Dr. Franklin, "in a house at London, which in a little room had a single chimney and funnel. The opening was very small, yet it did not keep in the smoke, and all attempts to have a fire in this room were fruitless. I could not imagine the reason, till at length observing that the chamber over it, which had no fireplace in it, was always filled with smoke when a fire was kindled below, and that the smoke came through the cracks and crevices of the wainscoat, I had the wainscoat taken down, and discovered that the funnel which went up behind it had a crack many feet in length, and wide enough to admit my arm; a breach very dangerous with regard to fire, and occasioned probably by an apparent irregular settling of one side of the house. The air entering this breach freely, destroyed the drawing force of the funnel. The remedy would have been, filling up the breach, or rather rebuilding the funnel; but the landlord rather chose to stop up the chimney.

Another puzzling case I met with at a friend's country-house near London. His best room had a chimney in which, he told me, he never could have a fire, for all the smoke came out into the room. I flattered myself I could easily find the cause and prescribe the cure. I opened the door, and perceived it was not want of air. I made a temporary contraction of the opening of the chimney, and found that it was not its being too large that caused the smoke to issue. I went out and looked up at the top of the chimney: Its funnel was joined in the same stack with others, some of them shorter, that drew very well, and I saw nothing to prevent its doing the same. In fine, after every other examination I could think of, I was obliged to own the insufficiency of my skill. But my friend, who made no pretension to such kind of knowledge, afterwards discovered the cause himself. He got to the top of the funnel by a ladder, and looking down found it filled with twigs and straw cemented by earth and lined with feathers. It seems the house, after being built, had stood empty some years before he occupied it; and he concluded that some large birds had taken the advantage of its retired situation to make their nests there. The rubbish, considerable in quantity, being removed, and the funnel cleared, the chimney drew well, and gave satisfaction."

Chimneys whose funnels go up in the north wall of a house, and are exposed to the north winds, are not so apt to draw well as those in a south wall; because, when rendered cold by those winds, they draw downwards. Chimneys enclosed in the body of a house are better than those whose funnels are exposed in cold walls. Chimneys in stacks are apt to draw better than separate funnels, because the funnels that have constant fires in them warm the others in some degree that have none.

Smoke-consuming. In manufacturing towns, where a great deal of fuel is used, it is of the utmost consequence to prevent, if possible, the production of smoke, so as not only to get quit of a great source of annoyance, but also to prevent the injury which it occasions to buildings; indeed to prevent the deterioration of property which in general occurs. Hence the clauses in acts of Parliament and in many police bills, compelling proprietors of steam engines, &c., to consume their smoke, if practicable; as also the numerous attempts which have been made to accomplish this object. Of course, if it could be done without an extra expenditure of fuel, so much the better; if with a saving of fuel, then the advantages to be gained are immense.

Numerous methods have been recommended, and many patents have been secured for accomplishing this desirable object. The principle of most of them depends on causing the smoke, immediately when given off from the coal, when fresh put on the fire, to pass over the half-burned or charred fuel, and thus be consumed.

Smoke is merely carbonaceous matter evolved from fuel; for when the whole of the inflammable gaseous product is not brought into contact with air at a proper degree of heat, it is not burned, and therefore escapes in the state of smoke. Now it is evident, that if that smoke or carbonaceous particles, as it flows over the burning fuel, be supplied with air, it also will be burned, provided they are brought together at a high temperature; but this is not done in the usual way of combustion, because the air passing up from the ash pit through the fuel, is almost entirely deprived of its oxygen before it reaches the upper part of the furnace. In addition to the air, therefore, which is requisite for the combustion of the fuel, air must also be admitted above the fire for the consumption of the smoke.

The most common method in practice for consuming smoke is, instead of throwing the fuel back into the furnace, to place it always on the dead plate or charring plate at the mouth, gradually pushing it on upon the bars, as it is charred, or deprived of its bituminous principle, and again supplying more on the dead plate, which is in general inclined, so as to allow the fuel placed on it to fall, and come into contact with that on the bars. For the success of this process, air must be admitted into the furnace, above the fuel, so as to mix with the flame and smoke; but this is attended with disadvantage, for being cold, it not only acts injuriously, as in lessening the production of steam, but by bringing down the temperature, prevents the action on the smoke itself, which is consequently not all destroyed. Hence the practice of heating the air to be thus admitted. When this is done by an additional fire, though the smoke may to a certain extent be consumed, yet it is attended with expense; but this is obviated in a great measure by the introduction of tubes into the flues or chimney, one end communicating with the atmosphere, the other terminating in the furnace, over the fuel, by which the air that passes through them derives a part of that heat which is otherwise carried up the vent.

This method of preventing smoke has not been attended with the success at first expected from it, as is proved by its not having come into general use. Perhaps one source of failure is the difficulty of getting the firemen to supply the coal carefully on the front of the furnace; and hence it has been recommended to employ hoppers, by which the coal is always thrown on the dead-plate; but even when these are used, the consumption of smoke is not complete. As to the heating of the air, by passing it through tubes in the chimney or vent, one great objection to them is the powerful influence they have over the draught. It is well known, that when cold air is allowed to rush into the chimney, the draught is instantly diminished. Now these tubes must operate in the same way; for though the air does not mingle with that in the vent, yet by withdrawing heat from it, it must reduce its temperature, and thus prevent it from being forced up.

By far the most effectual method of consuming smoke is that lately recommended and patented by Ivison. It consists in throwing steam into the fore-part of the furnace, above the fuel, by means of a fan-shaped distributor, in the front of which are small apertures, varying in size and number, according to the width of the furnace and the pressure of the steam. In general they are from five to eight or ten in number, and about an eighth or a twelfth of an inch in diameter. The quantity of steam thus admitted is very trifling; and by its passage through the flame and smoke given off from the coals on the charring-plate, it instantly causes the disappearance of the smoke, and accordingly none whatever escapes from the chimney-top, except just when fresh coal is thrown in, and then it is barely perceptible, and only for a few seconds.

When applied to furnaces with boilers, steam is easily procured, by taking from the top of the boiler a small tube, which is carried into the furnace, and connected with the distributor, and by means of a stop-cock the supply is regulated. In cases where there is no boiler, a very small one must be erected, which can be placed on some part of the furnace, and will thus give off steam without any extra expenditure of fuel.

The most remarkable circumstance attending the use of this patent process, is the almost total absence of draught through the ash-pit, consequently little or no air passes up through the fire, while there is a rush of air over the surface of the fuel. Hence the necessity of allowing air to flow in by the door of the furnace, or, which is better, by means of tubes placed in the chimney or flues, by which it is heated previous to its entrance; because, as before stated, the admission of cold air must have a prejudicial effect. It is evident from this, that the combustion must proceed almost entirely by air supplied on the surface. Indeed, when properly applied, the supply of air through the ash-pit is almost reduced to nothing; and accordingly the fire can be kept in good condition, even though the ash-pit is closed in front.

In addition to the consumption of smoke, the application of steam in this way is attended with another advantage; the saving of fuel, which is certainly a great recommendation in its favour. Different statements have been given of the amount of saving effected; but if we can place reliance in the reports given by the patentee, it seems to be considerable.

Watt, from his numerous experiments, concluded, that for each pound of coal (English caking) consumed, rather more than eight pounds of water could be evaporated, thus requiring about eight pounds of coal for the cubic foot of water. Since his time, however, improvements in the construction of furnaces have been introduced, which have increased the amount of evaporation.

Mr. Watt's maximum result with Wallsend caking coal, conducted with the greatest care, was 8.9 pound of water to the pound of coal. The lowest was 5.93, the mean 7.4.

Mr. Parkes of Warwick states the ordinary result with one pound of caking coal to be 7.5 pounds, evaporated from 212°, but by his improved method in the furnaces of other persons, it was 8.7, while by a similar mode with his own furnace, it was 10.32 pounds.

With one of the Cornish boilers, coated and covered with the greatest care, and in all other respects rendered as perfect as possible, the result was 11.8 pounds, and a similar result was more recently obtained by Henwood.

The following is a tabular view of the results of Watt and others, shewing the quantity of water evaporated, and the quantity of fuel required for the horse-power of an engine:

| EXPERIMENTS | lbs. water to 1 lb. of English coal | lbs. English coal to cubic foot water, or horse-power per hour | Cubic feet of water to 84 lb. English coal | Cubic feet of water to 112 lb. English coal | |-------------|-----------------------------------|-------------------------------------------------|------------------------------------------|------------------------------------------| | Watt's average | 7.4 | 8.4 | 10. | 13.33 | | United Mines Loan (Ed. Phil Jour., July 1839) | 9.58 | 6.23 | 12.93 | 17.24 | | Parkes of Warwick's method | 10.32 | 6.03 | 13.9 | 18.5 | | Huel Towan (Ed. Phil. Jour., July 1839) | 10.55 | 5.9 | 14.23 | 18.97 | | Mr. Henwood's experiment | 11.87 | 5.32 | 15.8 | 21. |

By the use of the Ivison patent, when properly applied, the amount of evaporation becomes much greater, as has been certified by repeated trials, by scientific men of eminence, as the following table shews. The experiments were conducted with Scottish coal; and considering the heat given out by it, as compared to English caking coal, to be as three to four, the results will shew the amount of evaporation, or, which is the same thing, the quantity of fuel used in comparison with what is given in the preceding table.

| EXPERIMENTS | lbs. water to 1 lb. of Scottish coal | lbs. water to 1 lb. of English coal | lbs. English coal to cubic foot water, or horse-power per hour | Cubic feet of water to 84 lb. English coal | Cubic feet of water to 112 lb. English coal | |-------------|------------------------------------|-----------------------------------|-------------------------------------------------|------------------------------------------|------------------------------------------| | Average of 8 experiments, certified April 13, 1839 | 11.41 | 15.21 | 4.09 | 20.5 | 27.4 | | Average of 3 experiments, certified May 10, 1839 | 13.94 | 18.58 | 3.35 | 25. | 33.46 | | Average of 12 workings, certified July 1839 | 13.25 | 17.66 | 3.52 | 23.91 | 31.78 | | Maximum result of do. | 14.72 | 19.62 | 3.12 | 26.47 | 35.3 | | Average of the four preceding lines | 13.43 | 17.96 | 3.46 | 24.02 | 32.3 |

From the above tables it appears, that the greatest quantity of water evaporated by the pound of English coal is 11.37; the maximum result by Ivison's process is, using Scottish coal, 14.72; the average 13.43; which, if we consider the English to the Scottish as four to three, gives 17.9, or equal to only about 3.46 of English coal to the cubic foot of water or horse power of an engine, certainly by far the largest and most economical result on record.

That this mode of consuming smoke is accompanied with saving of fuel, is also proved by comparative trials made with and without the use of steam, on the same furnace, as is shewn by the following table: | Date | Time | Period of day | Pounds of Coal used | |------------|----------|---------------|--------------------| | Sept. 25, 1838 | 5 h. 15 m. | P.M. | 560 | | | | | | | | | | | | | | | |

| Date | Time | Period of day | Pounds of Coal used | |------------|----------|---------------|--------------------| | Sept. 25, 1838 | 5 h. 15 m. | A.M. | 812 | | | | | | | | | | | | | | | |

Average: 558

For the same time, then, and for the same amount of work done, the consumpt of fuel, as shewn by the above table, is in the ratio of 812 to 540, or as 100 to 66, thus indicating a saving of about 33 per cent.

By the admission of steam into the furnace in this way of consuming smoke, the draught is prodigiously increased; hence the absolute necessity, when applied to furnaces having tall chimneys, of resorting to means to counteract it. It is remarkable that the damper in common use has comparatively little or no effect. The means found to answer best are the admission of cold air into the chimney, by apertures, which can be varied in size, according to circumstances; or when this is found not to be sufficient, to have covers to the ash-pit, by which the draught up through the bars may be prevented; for it is only when the draught is properly checked, and a due quantity of air admitted above the fuel, that none flows into the ash-pit. In furnaces to be erected for the purpose, the draught can be sufficiently checked, by making the chimney much narrower and shorter than those now in use. In fact, were it not for the necessity of having a vent to carry off the gaseous products, the chimney might be dispensed with, as the steam of itself creates sufficient draught to carry in air for the combustion. Hence one very great advantage, in addition to the many others, to be derived from this mode; as, independently of economy in fuel, there must be an immense saving in the outlay in constructing furnaces; the application of the steam-tube with its distributor costing very little.

Different opinions are entertained with regard to the rationale of this process. Some suppose that the steam merely acts mechanically, by carrying along with it a sufficient quantity of air to maintain the combustion; and that that air flowing over the fuel, not only carries on its consumption there, but, acting on the smoke, causes its combustion also, by which of course there is an additional supply of heat. Besides, as the upper part of the furnace is free from smoke, the radiation from the fire must have a more powerful effect on the bottom of the boiler, and those parts of it exposed in the flues being also free from soot, will more easily transmit the heat. If, however, we place reliance on the results of the trials stated above, and the names of the scientific gentlemen by whom they were conducted is a proof of their accuracy, it is difficult to conceive how so large a result should be obtained, were there no further action than the consumption of the fuel. It is generally admitted, that were the whole of the heat from coal undergoing combustion rendered available, about fourteen pounds of water would be evaporated on an average by each pound of coal. But this cannot be done, because part of the heat must necessarily pass off by the chimney; but even with this waste, the quantity of water evaporated by Scottish coal was in some trials beyond fourteen pounds. This has given rise to the idea, that the steam, in addition to its mechanical action of carrying air into the upper part of the furnace, acts also chemically, that in fact it is decomposed and consumed. It has been long known, that when steam is passed through tubes stuffed with incandescent carbon, it is decomposed, and resolved into gaseous products. In a paper published by Dr. Fyfe in the Edinburgh Phil. Jour-

nal, 1837, it is shewn, that these gaseous products are not carbonic acid and carburetted hydrogen, as at one time supposed, but carbonic oxid and pure hydrogen. Now a similar decomposition may be effected on the steam in the Ivison process; for when it is brought into contact with the carbonaceous matter of the smoke, as given off at a high temperature from the coal on the charring-plate, it may be decomposed, and give, by its action on the smoke, carbonic oxid and hydrogen; and these being freely supplied with air, will be burned, and by their combustion give out an additional supply of heat, and it is well known that the heat given forth by hydrogen is very great. If this explanation be the correct one, (and it is strengthened by the fact, that when the coal is thrown far back in the furnace, so as to be beyond the influence of the steam, it is not consumed), then the smoke is not burned by the direct action of the air, but by first getting oxygen from the steam, so as to form carbonic oxid, and which oxid then receives oxygen from the atmosphere. An additional quantity of oxygen is thus required; and hence most probably the additional amount of heat evolved by the process.

SMOKE-Silver. Lands were held in some places by the payment of the sum of sixpence yearly to the sheriff, called smoke-silver (Par. 4 Edw. VI.) Smoke-silver and smoke-penny are to be paid to the ministers of divers parishes as a modus in lieu of tithe-wood; and in some manors formerly belonging to religious houses, there is still paid, as appended to the said manors, the ancient Peter-pence, by the name of Smoke money. (Twisd. Hist. Vindicat., 77.)